Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 9

Three stages--bulk, absorption and float are normally used for wet car and motive deep cycle batteries with an optional equalizing stage. Three stages--bulk, absorption and float are normally used for AGM (Ca/Ca) and Gel Cell (Ca/Ca) VRLA car and motive deep cycle batteries. Three stages--bulk, float and equalization are normally used for wet stationary deep cycle batteries and two stages--bulk and float are normally used for VRLA stationary deep cycle batteries with an optional equalization stage is some cases.

9.1.1. The BULK stage is where the charger current is constant and the battery voltage increases, which is normally during the first 80% of the recharge. Give the battery whatever current it will accept as long as it does not exceed 25% of the 20 hour (expressed "C/20") ampere hour (AH) capacity rating, 10% of the Reserve Capacity (RC) rating, wet batteries do not exceed 125° F (51.5° C), and VRLA batteries do not exceed 100° F (37.8° C).

9.1.2. The ABSORPTION stage is where the charger voltage, depending on the battery type, is constant between 14.1 VDC and 14.8 VDC at 80° F (26.7° C) and the current decreases until the battery is fully charged, which is typically the last 20% of the recharge. For wet batteries, gassing (making a bubbling sound) usually starts at 80% to 90% of a full charge and is normal. A full charge typically occurs when the charging current drops off to 2% (C/50) or less of the AH capacity of the battery and each cell of a wet battery is moderately gassing equally. For example, end current for a 50 AH (C/20) battery is approximately 1.0 amp (1000 milliamps) or less. If the battery will not "hold" a charge, the current does not drop after the estimated recharge time, and a wet battery is hot (above 125° F (51.5° C)), then the battery may have some permanent sulfation. (Please refer to Section 16 for more information about sulfation and how to remove it.) Manual two-stage chargers that have a bulk and absorption stage must be turned off when the battery is fully charged to prevent overcharging.

9.1.3. The optional FLOAT stage is where the charge voltage, depending on the battery type, is reduced to between 13.0 VDC and 13.8 VDC at 80° F (26.7° C), held constant. It can be used indefinitely to maintain a fully charged battery to overcome the natural self-discharge of the battery. The current is reduced to approximately 1% (C/100) or less. Three-stage "smart" chargers usually have the bulk, absorption and float stages. (Please refer to Section 13 for more information about storing batteries and continuous float charging.)

9.1.4. The optional EQUALIZING stage is a controlled 5% to 10% absorption overcharge to equalize and balance the voltage and specific gravity in each cell. Equalizing reverses the build-up of the chemical effects like electrolyte stratification where acid concentration is greater at the bottom of the battery. It also helps remove sulfate crystals that might have built up on the surface or in the pores of the plates. The recommended frequency varies by motive deep cycle battery manufacturers from once a month to once a year. For stationary deep cycle batteries, some short daily (30 minutes or less) equalizations have proven to be beneficial and not require the longer equalization cycles. They are not as hard on a wet battery because they do not produce as much gas or heat the battery. You should equalize wet batteries when one or more of the following occur:

Some AGM (Ca/Ca) VRLA batteries, like Concorde, can be equalized under certain conditions, but carefully follow the battery manufacturer's recommended procedures or you will damage the battery.

To equalize, check that the electrolyte is covering the plates in each cell and fully recharge the battery. Then increase the charging voltage to the battery manufacturer's recommendation, or if not available, add 5% to 10% to the absorption charging voltage. Heavy gassing should start occurring in each cell. Do not allow the wet battery to get above 125° F (51.5° C) or a VRLA battery above 100° F (37.8° C). Take Specific Gravity readings in each cell once per hour. Stop equalizing when the Specific Gravity values no longer rise during the gassing phase and when every cell is gassing evenly. Insure that the plates are covered with electrolyte at all times, and add distilled, deionized or demineralized water if required, but do not overfill. Only equalize if the battery manufacturer recommends it. Four-stage "smart" chargers typically have the bulk, absorption, float and equalization stages.

An excellent and easy to understand tutorial on battery charging basics can be found at http://batterytender.com/battery_basics.php. The following graphs are examples of charging algorithms used by Deltran [Battery Tender] for power sport, car and deep cycle batteries:

It is extremely important to use the battery manufacturer's recommended temperature compensated charging voltages and procedures whenever possible for optimum battery capacity, maintenance and service life. A good rule is not to use a charger (or charging setting) for batteries that is greater than 25% of the AH (C/20) capacity or 10% of the RC rating of the battery or battery bank being charged. For example, if the battery has RC of 100 minutes, do not use charger that will exceed 10 amps. The exception is when you are charging a large AGM (Ca/Ca) deep cycle battery bank. Due of the higher acceptance rate of the AGM (Ca/Ca) batteries a larger alternator to 33% of the capacity of the battery bank being charged can be used. Using a smaller alternator, unless temperature protected, can be damaged. The exception is not to use a charger (or charging setting) for SLA batteries that is greater than 10% of the AH (C/20) capacity.

9.2.1. Help prevent blindness and always wear glasses when working around a car or deep cycle battery in the unlikely event that it might explode.

9.2.2. Use the battery manufacturer's charging recommendations and temperature compensated voltages whenever possible for optimum capacity, maintenance and service life. MATCH the charger (or charger's setting) for the battery type you are recharging (or maintaining) and insure the charging voltages are compatible. Except for Gel Cell (Ca/Ca) VRLA batteries, a small overcharge is slightly better than an undercharge. Overcharging Gel Cell (Ca/Ca) VRLA batteries can cause voids between the plates and loss of capacity can result.

9.2.3. Lead-acid batteries should always be recharged within 24 hours after they have been used and the sooner the better. Before recharging, check the electrolyte and insure that it is not frozen and that it covers the plates at all times to prevent sulfation and to reduce the possibility of an internal explosion. Do not recharge frozen batteries because you will damage them. Allow them to thaw out first.

9.2.4. After recharging, recheck the electrolyte levels after the battery has cooled, top off with distilled, deionized or demineralized water as required, but do not overfill. (Please refer to Section 3.1. for more information about filling batteries.)

9.2.5. Reinstall the vent caps on wet (flooded) batteries before recharging and recharge ONLY in well-ventilated areas because explosive and toxic stibine or arsine gasses can be produced during the absorption stage. Insure the vent caps are not clogged. Do NOT expose lead-acid batteries to a lit cigarette, sparks or flames because they produce flammable gasses and could explode.

9.2.6. Follow the charger manufacturers' procedures for connecting and disconnecting cables. Connect the positive (+) lead of the charger to the positive (+) terminal post of the battery to be charged and the negative (-) lead of the charger to the negative (-) terminal post. Operate in a manner to minimize the possibility of an explosion or incorrectly charging the battery. You should always turn the charger OFF or unplug it before connecting or disconnecting cables to a battery. Do not wiggle the cable clamps while the battery is recharging, because a spark might cause an explosion. Good ventilation or a fan is recommended to disperse the gas created by the recharging process for wet batteries. As a safety feature, some chargers are designed not operate unless the battery has a partial charge or if the leads are reversed.

9.2.7. If a wet battery becomes hot, over 125° F (51.5° C), or if it violently gasses or spews electrolyte, turn the charger off temporarily or reduce the charging rate. This will also prevent "thermal runaway" that can occur with AGM (Ca/Ca) and Gel Cell (Ca/Ca) VRLA batteries if the battery temperature is over 100°F (37.8° C). If an air cooled alternator becomes too hot during the bulk charging phase, stop and let it cool down or use an alternator temperature sensing voltage regulator, like a Balmar, or a water cooled alternator, Bosch for example.

9.2.8. Insure that charging the battery with an external charger will not damage the electrical system or appliances with high voltages. If this is even a remote possibility, then disconnect the grounded battery cable from the battery before connecting the charger to the battery.

9.2.9. If you are recharging Gel Cell (Ca/Ca) VRLA batteries, the battery manufacturer's charging voltages are very critical. You might need special charging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to properly recharge Gel Cell (Ca/Ca) or AGM (Ca/Ca) VRLA batteries because of their higher voltages or charging profiles. Overcharging Gel Cell (Ca/Ca) and AGM (Ca/Ca) batteries will significantly shorten battery service life or cause "thermal runaway" if the battery temperature is over 100°F (37.8° C).

9.2.10. If a battery is charged with a manual or defective charger and all the electrolyte is "boiled" out, some batteries can cause a FIRE or produce DEADLY CO (Carbon Monoxide) or other gasses.

9.2.12. Never disconnect a car battery cable from a vehicle with the engine running, because the battery acts like a filter for the electrical system. Unfiltered (pulsating DC) electricity sometimes exceeding 40 volts is produced by the alternator and can damage expensive electronic and electrical components such as emissions computer, audio system, charging system, alarm system, etc.

9.2.13. Alternators are not designed to recharge dead (or flat) batteries and the stator can be burned or diodes go bad.

9.2.14. Wet battery gassing usually starts at 80% of a full charge during the absorption stage. A full charge normally occurs when the charging current drops off below 2% (C/50) of the AH capacity and the battery is moderately gassing (bubbling). For example, the end current for a good 50 AH (C/20) battery is approximately 1.0 amp (1000 milliamps) or less depending on the battery type.

9.2.16. Recharge battery banks the same way you discharged them. For example, if you discharged two or more fully charged and identical batteries connected together such that all the batteries discharged the same, i.e., the same State-of-Charge (SoC) readings on all of the batteries, you should recharge them connected the same. If you discharged two or more fully charged and identical batteries not connected together such that the batteries discharged differently, i.e., different State-of-Charge readings on each of the batteries or banks, you should recharge them separately. When the batteries are connected together in a bank(s), it is a question for keeping the discharges and charges balanced; otherwise, you will undercharge or overcharge one or more of the batteries or banks. Over time, undercharging will reduce capacity due to the accumulation of sulfation. The total time to recharge the batteries or banks together or individually is about the same because you have to replace the amp hours consumed.

9.2.17. Do not recharge batteries directly from a gas or diesel powered generator that does not have regulated DC voltage and most do not. A better approach to recharging batteries is to power a "smart" battery charger with the generator so the batteries are not overcharged or undercharged.

Basically, there are three battery charger configurations--single bank, multi-bank and multi-station. A single bank charger is one that is designed to provide a single voltage to recharge a single battery or bank of batteries. It is by far the most widely used configuration. A multi-bank charger provides single voltages to multiple banks of batteries by using an internal isolator. This type of charge can also act as a single bank charger and commonly used to recharge unbalanced two, three or four 12-volt batteries in series to power a motor. A multi-station charger is a used to recharge more than one battery at the same time. It is functionally two or more single bank chargers in the same case.

Unless the charging system or charger has adjustable voltage settings, there is no one system that can recharge all battery types. For example, if the absorption charge voltage is set for a Low Maintenance (Sb/Ca) or AGM (Ca/Ca) VRLA battery at 14.4 VDC, the system would undercharge most wet Standard (Sb/Sb) or wet "Maintenance Free" (Ca/Ca) and overcharge some Gel Cell (Ca/Ca) VRLA starting batteries. This would reduce the battery's service life. Unless a charger is temperature compensating, it is assumed by the manufacturer to operate at 77° F (25° C). Some chargers are equipped with an electronic switch that senses battery voltage at some predetermined level before the charger will operate. For deeply discharged batteries, this gives the appearance that they can not be recharged. Please see the charger manufacturer's operator manual for instructions on how to override this "soft start" feature. A good quality charger used on a cheap battery is better than a bad quality charger used on a good battery.

A vehicle charging system is made up of three components, an alternator (or DC generator), voltage regulator and a battery. Usually when a vehicle is jump started, it is NOT driven long enough to fully recharge the battery. The length of time to fully recharge the battery depends on the amount of discharge, the amount of surplus current that is diverted to the battery, how long the engine is run, engine speed, and ambient temperature. An alternator is sized by the vehicle manufacturer to carry the maximum accessory load and to maintain a battery and NOT to recharge a dead battery. For example, if 300 amps were consumed for two seconds to start a car from a fully charged battery, it will take an 80 amp charging system approximately 7.5 seconds to replace the .167 amp hours of power used. If 25 amps are available to recharge the battery, it will take 24 seconds and 10 minutes at one amp. With a dead 120 minute RC (60 amp hour) battery, it would take approximately 90 minutes at 80 amps, 4.8 hours at 25 amps, or 120 hours at one amp to fully charge (100% State-of-Charge) it.

More information can be found in Section 5 or Dan Landiss' Car Batteries Are Not 12 Volts on http://www.landiss.com/battery.htm about vehicle charging systems. Some battery experts believe that some vehicle charging systems undercharge starting batteries and that the batteries should be periodically recharged with an AC "shore" powered battery charger to optimize their service life by removing accumulated lead-sulfate or electrolyte stratification.

If you have added after-market lights, winches, audio amplifiers, two-way radios or other high powered accessories to your vehicle and engage in stop-and-go driving, the vehicle's charging system might not produce enough current or voltage to keep your battery fully charged. You might need to increase the capacity of the charging system. If you are also recharging deep cycle battery banks, please see the caution in Section 9.2.7. above. Ideally the combined load of all the accessories should be less than 75% of the charging system's maximum output, so that at least 25% is available to recharge the battery.

A manual constant current charger chargers the battery at a constant current rate and the battery voltage will increase as the State-of-Charge rises. If you use an external constant current charger, set it to deliver NO more than the lesser of 1% of the CCA, 12% of the RC rating, or 25% of the C/20 rated AH Capacity of the wet battery and also carefully monitor the current flowing into the battery. C-rate is a measurement of the charge or discharge of battery over time. It is expressed as the Capacity of the battery divided by the number of hours to recharge or discharge the battery. For example, a 48 amp hour battery would have a charging or discharging rate of 4.8 amps for ten hours. With manual chargers, you need to determine how many amp hours have to be replaced and determine the amount of charging time based on the constant current output of your charger. Manual constant current chargers will overcharge a battery if not turned off when the battery is fully charged. Some constant current chargers have a timer that can turn off the charger and help prevent it from overcharging the battery. These types of chargers are not recommend to recharge a VRLA battery because the absorption voltages are critical, especially for Gel Cell (Ca/Ca) VRLA batteries.

For fully discharged wet batteries, the following table lists the recommended battery charging rates and times using a constant current charger:

Reserve Capacity (RC) Rating	Slow Charge (RECOMMENDED)	Fast Charge
80 Minutes or less [32 ampere hours or less]	15 Hours @ 3 amps	5 Hours @ 10 amps
80 to 125 Minutes [32 to 50 ampere hours]	21 Hours @ 4 amps	7.5 Hours @ 10 amps
125 to 170 Minutes [50 to 68 ampere hours]	22 Hours @ 5 amps	10 Hours @ 10 amps
170 to 250 Minutes [68 to 100 ampere hours]	23 Hours @ 6 amps	7.5 Hours @ 20 amps
Above 250 Minutes [over 100 ampere hours]	24 Hours @ 10 amps	6 Hours @ 40 amps

A manual two-stage (bulk and absorption) constant voltage charger applies a regulated voltage to the battery at a constant level during the absorption stage. The current drops to below 2% (C/50) of the battery's capacity at the battery manufacturer's recommended absorption voltage when it approaches 100% State-of-Charge; then the charger needs to be manually turned off. The recommended charging method using a constant voltage charger is to slowly recharge the battery using a charger sized to recharge the battery over a ten-hour period (C/10). To prevent damage to a fully discharged battery, the current should be less than 1% of the CCA (Cold Cranking Amps) rating during the first 30 minutes of charge. The charger (or DC power supply) should be adjusted to the battery manufacturer's absorption voltage recommendations without the battery connected before charging. Typical battery charging voltages are in the table below with the electrolyte temperature at 80° F (26.7° C), but the battery manufacturer's temperature compensated charging voltages and procedures should always be used, if available. A manual constant voltage charger (or DC power supply) could overcharge and damage a battery if not turned off when the battery is fully charged.

Battery Type Ca=Calcium Sb=Antimony	Absorption Charging Voltages	Float Charging Voltages	Equalizing Charging Voltages
Wet Standard (Sb/Sb) Deep Cycle	14.5	13.2	15.5
Wet Low Maintenance (Sb/Ca)	14.4	13.2	15.8
Wet "Maintenance Free" (Ca/Ca)	14.8	13.2	15.8
AGM (Flat Plate) VRLA	14.3	13.6	15.6*
AGM (Spiral Wound) VRLA	14.6	13.6	Not Applicable
Gel Cell (Ca/Ca) VRLA	14.1 or 14.4*	13.8 or 13.2*	Not Applicable

If the external or "shore powered" charger is NOT temperature compensating, you should adjust the charging voltage using the battery manufacturer's recommended temperature compensation voltages. If not available, then use the values from the table below to correct for the temperature of the electrolyte in the battery. If the electrolyte temperature can not be measured and the battery has not been recently moved from a warmer or colder location, charged or discharged, the ambient air temperature can be used. For example, if the electrolyte temperature is 20° F (-6.7° C), then increase the charging voltage to 15.408 volts for a wet Low Maintenance (Sb/Ca) battery if the normal absorption charging voltage is 14.4 at 80° F. If 100° F (43.3° C), then decrease the absorption charging voltage to 14.064 volts for the same battery.

Electrolyte Temperature Degrees F	Electrolyte Temperature Degrees C	Add to Charger's Output Voltage
160°	71.1°	-1.344
150°	65.6°	-1.176
140°	60.0°	-1.008
130°	54.4°	-.840
120°	48.9°	-.672
110°	43.3°	-.504
100°	37.8°	-.336
90°	32.2°	-.168
80°	26.7°	0
70°	21.1°	+.168
60°	15.6°	+.336
50°	10°	+.504
40°	4.4°	+.672
30°	-1.1°	+.840
20°	-6.7°	+1.008
10°	-12.2°	+1.176
0°	-17.8°	+1.344

The taper current chargers have no controlled current and voltage and are dependent upon the internal resistance of the battery. The current starts high and tapers off as the voltage increases when the battery approaches 100% State-of-Charge (SoC). With a taper charger, a high current (up to C/2), can be only applied to non-sealed wet batteries for 30 minutes maximum or until the battery heats up to 125° F (51.7° C). The current is then regulated downward by the battery until the charge state reaches 100% where it is at a minimum (2% or less) at the battery manufacturer's recommended absorption voltage level. A better approach to recharge the battery with a taper charger is to size the charger to recharge the battery over a minimum of a ten-hour period (C/10). This technique allows the acid more time to penetrate the plates and there is less mechanical stress on the plates. Manual taper current chargers will overcharge a battery if not turned off when the battery is fully charged and are not recommend to recharge VRLA batteries.

The next step up is an "automatic" two-stage (bulk and absorption) charger that will stop charging when the battery has approached a full charge by turning off at some predetermined current, voltage cut-off point, time, or combination of current, voltage or time. If the battery manufacturer's recommended absorption voltage is used, there is less chance of under or overcharging a battery than with a manual charger. A four to 10-amp automatic starting battery charger will cost approximately $50 (US) and is suitable for most simple car battery charging applications with battery capacities up to 100 amp hours (C/20). Some automatic two-stage chargers, like the Dual Pro, Ctek XS 800, etc. will turn back on and recharge the battery when the voltage drops to a predetermined point (normally 90%-95% SoC). Some also have features like selection of battery type; temperature compensation, which is critical if recharging occurs in temperatures other than 80° F (26.7° C); do not produce sparks when the clamps are connected; will not turn on if the polarity is reversed; or will help prevent VRLA battery "thermal runaway".

The best chargers for wet and some AGM (Ca/Ca) starting and motive deep cycle batteries are four-stage "smart" microprocessor-controlled temperature compensating chargers. They will automatically switch between bulk, absorption, float, and equalizing charging and some have adjustable voltage set points or selection for the different battery types, automatic temperature compensation, or features found in automatic two-stage chargers. The best chargers for Gel Cell (Ca/Ca) or AGM (Ca/Ca) VRLA batteries are the less expensive three-stage temperature compensating versions that have bulk, absorption and float charging capability (or settings) especially designed for VRLA batteries. They will also help prevent VRLA battery "thermal runaway". When continuously connected, the microprocessor based "smart" chargers can continuously charge a battery and keep it fully charged indefinitely. Some one-half to two-amp three-stage versions cost less than $50 (US), for example Battery Tender Plus, BatteryMINDer, etc., are ideal for for maintaining starting and deep cycle batteries (less than 100 AH) that are used less than once per week. Good application examples are for power sport vehicles (ATVs, Jet skis, motorcycles, snowmobiles, etc.), RVs, caravans, farm and lawn tractors, and antique vehicles and vehicles in storage.

There are basically two types of float chargers. The first type is used to float or maintain wet or VRLA car or motive deep cycle batteries that have been fully charged. The second type is used to float charge or maintain wet or VRLA stationary deep cycle batteries.

If you are using wet or VRLA batteries in starting or motive deep cycle applications and already have a two stage charger, then a voltage-regulated "float" charger, power supply or battery maintainer set at approximately 13.2 VDC can be continuously used after the battery has been fully charged. An example is a Vector VEC080, costing less than $30 (US). Float chargers will maintain batteries at a 100% State-of-Charge with a C/100 rate to offset the battery's internal self-discharge and prevent them from sulfating. Batteries that have the same plate chemistry (battery type) can be connected in parallel to a float charger after they have been fully charged and the charger's current output capacity is greater than 1% of total amp hour capacity of the batteries connected to it.

If you are using wet or VRLA deep cycle batteries in stationary applications, then use a float charger at approximately 13.8 VDC that is sized to carry the maximum load plus an extra 10% or more depending on how fast you want to recharge the batteries.

A trickle charger is typically a cheap, unregulated voltage (C/100) charger used to maintain a battery after it has been fully charged typically costing less than $20 (US). Do NOT use these types of chargers because they can easily overcharge and destroy the wet battery by "boiling" the electrolyte out and dry out the battery or undercharge it. If you have to use a trickle charger, using it on a timer is highly recommended.

High rate fast, boost or starting assist chargers (or settings) are high rate chargers that are designed to provide high current for up to 15 seconds to start your engine when the battery is discharged. These types of chargers (or boost settings) to recharge your battery are NOT RECOMMENDED because they can easily overcharge and destroy it with excessive current or voltage. If one is used, please do it with extreme caution in a well ventilated area and adhere to the charger manufacturer's recommended procedures.

Vehicle generators capable of producing Direct Current were used up until the 1950's to recharge car batteries. They were replaced by alternators because generators were not as reliable because of their mechanical voltage regulation, expense to manufacturer, and added weight. Most portable "generators" used today are alternators to produce AC voltage. Some have rectifying diodes to produce "DC" voltage to replace batteries for 12-volt loads. These portable DC generators can provide a bulk (up to 80% State-of-Charge) or equalizing charge to a battery that has been disconnected from the it's load. But check the output voltage across the battery terminals, recharge with care, and monitor the process because they typically do not have voltage regulation, can easily under or overcharge the battery and destroy it. You will require 14.1 to 14.8 VDC depending of the battery type to fully recharge your batteries at 80 degrees F. Generators without voltage regulators are NOT RECOMMENDED for absorption or float charging. A better approach would be to use a "smart" external "shore power" battery charger, like a Vector, plugged into the AC output of a portable generator to provide the voltage regulation to properly recharge the house batteries.

Inverter/Charger is an AC to DC battery charger with a built-in DC to AC converter popularly known as an "inverter" that is battery powered when AC or "shore power" in not available. Some manufacturers of inverter/chargers are Mastervolt, Newmar, Parallax, Progressive Dynamics, TrippLite, Victron, Xantrex, and others. When selecting an inverter/charger be sure that the charger matches the battery type you are trying to charge and will produce the battery manufacturer's recommended temperature compensated charging voltages. Some inverter-charger combinations are float chargers for stationary batteries and will only produce a maximum of approximately 13.8 VDC, so your need to periodically give the batteries an absorption or equalizing charge to extend their overall service lives.

When a battery is discharged, more power has to be replaced due to loss. However, some of the power is converted to heat and lost due to the resistance in the cables, connectors and elements within the battery. For most batteries that are discharged less than 20% of their full capacity, an estimate of time is the amp hours to be replaced divided by the current output of the charger. For example, a 40 amp hour battery with a 5% discharge would require approximately two amps hours to be replaced. Using a five-amp charger, it would take approximately 24 minutes (2 amp hours/5 amps x 60 minutes) to recharge the battery. A 10-amp charger would take approximately half the time or 12 minutes. For batteries that are discharged more than 20% of their full capacity, an estimate of time is twice the amp hours to be replaced divided by the current output of the charger. For example, a 40 amp hour battery with a 95% discharge would require approximately 38 amp hours to be replaced. Using five-amp charger, it would take approximately 15.2 hours recharge the battery. A 10-amp charger would take approximately half the time.

In descending order of accuracy and depending on the battery type, one or more of the following three methods is normally used to determine if a battery is fully charged:

After the battery has cooled to room temperature, recheck the electrolyte levels. The plates must be covered at all times to prevent an internal explosion or sulfation. If the battery will not "hold" a charge, the charging current does not drop below 2% (C/50), and is warm or hot, then it might have some permanent sulfation. Please refer to Section 16 for more information about sulfation and how to remove it.

Normally, overcharging will consume more water from a wet battery than normal and the electrolyte levels will be low. Other signs of overcharging wet batteries are a "rotten egg" odor, violent gassing, spewing of electrolyte, black "tide-marks" on the inside walls of the cells, or black deposits on the bottoms of the filler caps. Other signs of overcharging are lumpy brown sediment or muddy red or brown electrolyte. Signs of overcharging a AGM (Ca/Ca) or Gel Cell (Ca/Ca) VRLA or SLA battery are a hissing sound, loss of capacity or overheating. If overcharging occurs, test the charging voltages.

The following are some tips for consumers on buying battery chargers for car and deep cycle lead-acid batteries. Please see Section 7.1 for definitions of the battery types. An excellent and easy to understand tutorial on Battery Charging Basics can be found at http://www.batterytender.com/.

9.7.1. Always wear glasses when working around a battery in the unlikely event that it might explode.

9.7.2. MATCH the charger's output voltages to the battery type and manufacturer's recommended absorption, float and equalization (if required) charging voltage requirements. A mismatch can easily overcharge or undercharge the battery. Some charger manufacturers state that their chargers are able to recharge all or most battery types. There are differences in the charging voltages and profiles (algorithms) for each battery type, so one charger setting can NOT possibly fit all types of batteries because of the differences in plate chemistries and alloys used. If the documentation that came with the battery or charger or the manufacturer's Web site does not state voltages, contact one of their Customer Service representatives and ask. If you do not charge your batteries at 80° F (26.7° C), temperature compensation needs to occur on the charging voltages to properly recharge the battery. A recent study has shown that cell equalization will significantly increase the life of wet (or flooded) Standard (Sb/Sb), Low Maintenance (Sb/Ca), "Maintenance Free" (Ca/Ca) batteries. Equalization is NOT recommended for Gel Cell (Ca/Ca) and most AGM (Ca/Ca) VRLA batteries.

9.7.3. Size the charger based on the discharge amount and how fast you need to use the batteries again. Slow recharging is recommended, so chargers that are sized 10% of the capacity of wet, AGM (Ca/Ca) or Gel Cell (Ca/Ca) VRLA batteries should be used. Fast or "boost" charging batteries can kill batteries because they can warp the battery's plates. Do not exceed the battery manufacturer's charging current or voltage limitations. For most car batteries, a charger output of four to 10 amps should be sufficient and for motorcycle and power sports batteries, one to two amps. For more information on charger sizing, please see Chris Gibson's article on http://www.smartgauge.co.uk/chargesize.html.

9.7.4. Determine special features you want, for example, "smart" microprocessor controlled, "automatic shut off" (two stage), automatic temperature compensation, "soft start" (no sparking when leads are connected), portability, waterproofing, indicators, ammeter, lead polarity reversal protection, short circuit protection, high temperature protection, etc.

9.7.5. Determine the total cost of ownership. Shop online on the Internet by using search engines, like http://www.google.com or http://www.yahoo.com to find the best prices. A charger is a long term investment and a good charger used on a cheap battery is much better choice than a bad charger used on a good battery.

9.7.6. If you have a two-stage charger, use a float charger (or battery maintainer). After the battery has been fully charged with a two stage charger or the vehicle's charging system, you can continuously maintain the full charge with a voltage regulated, one-half to two-amp float charger matched to your battery type while the battery is not being used. This will prevent sulfation from occurring while the battery is not being used. Cheap, unregulated "trickle" chargers can overcharge and destroy your battery.

9.7.7. If you use a one-half to two-amp "smart" charger, like a Battery Tender Plus, to recharge a larger battery, you might have to periodically "reset" the charger every six hours, by unplugging it and plugging it back in. Some small output "smart" chargers have fixed timers that will switch the absorption mode to float mode, thus not allowing sufficient time for a large capacity battery to be completely recharged. This fixed timer is used to keep a sulfated battery from boiling dry.

Opportunity charging is recharging in between the normal daily charging cycle. An example is a electric fork lift truck battery being recharged when not in use during the workday and during meal breaks. Some experts will argue that a deep cycle battery should be sized so that the average Depth-of-Discharge (DoD) should not fall below 50% (or 80% depending of the plate chemistry). And the battery should be charged only once per day because each charge cycle removes a microscopic layer from the entire grid and eventually the upper portion of the grid can not carry the current. That is one of the main reasons that grid design and composition is extremely important in lead-acid batteries for a long service life. Other experts will argue that opportunity charging significants lowers the average DoD and causes multiple, shallower cycles per day, which is better than a higher average DoD and a single deep cycle per day with a lower average DoD. The answer to this question probably lies somewhere in the middle. You will need to compare the effects of lower average DoD and multiple cycles vs. greater DoD and one cycle by using the battery manufacturer's data to determine the break even point. Generally, opportunity charging is good, especially when the average DoD is between 20% and 50% and you can fully recharge battery at least once during a 24 hour period when being used and once per week while it is not in use to prevent sulfation.

When a wet (flooded) Low Maintenance (Sb/Ca) battery reaches the absorption stage which is approximately 14.4 VDC at 80° F (26.7° C) or 80% State-of-Charge during a charge, it will start to gas (bubble) and is a normal part of the charging process. Gassing is the electrolysis of water into two parts Hydrogen gas and one part Oxygen gas and can be explosive. The gas bubbles given off by the plates will help to mix the electrolyte as they rise to the surface. This will help to prevent electrolyte stratification. Electrolyte stratification is acid concentration that is greater at the bottom of a battery than at the top, especially within batteries with more than 100 amp hours capacity. Normal charging should produce moderate amount of even gassing of all cells, which is good. Overcharging a battery or rapidly charging with high voltage will produce heavy gassing, heat, consume excessive quantities of water, accelerate positive grid corrosion, warp the plates, and is NOT recommended. Ventilation is required for all lead-acid batteries and good ventilation is mandatory for wet batteries to dissipate the explosive and toxic gasses produced during charging.

An AC-DC Converter (or AC-DC Power Supply) is used to convert 120 or 240 VAC power to filtered 12 to 13.8 volt DC power to run DC appliances while connected to "shore power" instead of running on "house" battery power. Converters are normally voltage regulated to provide a constant supply of DC power and if the voltage is high enough, partially recharge a battery. To fully recharge a battery, you will need 14.1 to 14.8 volts at 80 degrees F, depending on the battery type. If you use a converter or converter/inverter, then you should fully charge your batteries at least once per week. While connected to shore power, a better solution is to temporarily separate the house load from the house batteries, use the converter to run your house load, and use a "smart" charger to recharge and maintain your house batteries.

A manual battery charger is designed to recharge a battery and typically produces higher voltages. An automatic or "smart" battery charger is designed to stop charging when a preset current, voltage or time is achieved or to produce different voltages, depending on which charging cycle it is in. Battery chargers typically do not have the degree of filtering that a converter or power supply has.

Charge controllers and voltage regulators are devices used to control the level or levels, in the case of three and four stage units, of DC voltage from a source of power to the battery or batteries. Typically charge controllers are used to control the output of solar panels and voltage regulators for DC generators or alternators.

Discharging, like charging, depends on a number of factors such as the initial State-of-Charge, average Depth-of-Discharge, condition and capacity of the battery, load and temperature. To determine the amount of discharge time (T) for a fully charged battery at 80° F (26.7° C), the simple formula is ampere-hour rating (C) divided by the average load in amps (I) or T = C / I is often used. So, 100-ampere hour battery with an average 5-amp load should last approximately 20 hours (100 AH / 5 amps). The total number of amps that are produced when a fully charged battery is discharged over a 20 hour (C/20) period and to 10.5 volts is the most commonly used specification for expressing the capacity of deep cycle batteries used in most RV and Marine applications; however, five (C/5) or six hour (C/6) for Golf Carts or eight hour (C/8) rates for RV/Marine batteries might be more realistic.

For example, if a deep cycle battery's capacity is rated at 100 ampere hours (AH) at the 20 hour (C/20) rate, it will produce approximately 83 AH at the eight hour (C/8) rate, 63 A in two hours (C/2), and 55 AH in one hour (C/1). This is due to the Peukert Effect.

Repeatedly discharging a wet Low Maintenance deep cycle battery below 20% State-of-Charge (approximately 12.0 volts) or shallow discharges of less than 10% can significantly reduce the number of life cycles. Please see the graph on average Depth-of-Discharge in Section 11.3. New batteries often require a precondition or "break-in" period of up to 30 cycles before they will produce their rated ampere hour capacity. The capacity is reduced over time as the active material flakes (sheds) off the plates and some of the pours fill with hard sulfate.

To reduce the amount of time that your charging system is running, only recharge the battery to 80% State-of-Charge level at an amp hour rate not exceeding the number of amp hours that need to be replaced or C/4 (25% of the AH Capacity), whichever is less. For example, if 50 amp hours has been consumed from a 100 amp hour battery, then you do not want to recharge it at rate any greater than 25 amps in one hour. At a 25 amp charging rate, it should take approximately two hours to get to a 80% State-of-Charge. Please note that it will take almost the same amount of time, at a reduced current, to recharge the battery the remaining 20% to bring it to 100% State-of-Charge as it took to recharge it originally from the 50% to the 80% level. If the battery is recharged to the 80% State-of-Charge level, it should be recharged to 100% at least every 10^th cycle or once per week, whichever occurs first.

Using AGM (Ca/Ca) VRLA batteries also will reduce the amount of recharging time because they have a higher acceptance rate than wet lead-acid batteries. (Please see Section 7.1.4 for more information and AGM batteries.)

Battery manufacturers set the concentration of the sulfuric acid in the electrolyte of a fully charged wet battery to optimize the capacity, service life, water consumption, use in float applications, high discharge rate capability, battery size and self discharge rate. When the Specific Gravity is increased on purpose or by additives, the following attributes are increased: capacity, service life, water consumption, high discharge rate capability, and self discharge rate. When you decrease the Specific Gravity, the reverse occurs. You might ask why increasing the Specific Gravity on wet starting and motive deep cycle batteries is not a good thing? The answer is that it also accelerates the corrosion of the positive plate grids and connecting straps and you could have a premature battery failure, thus effecting overall service life, but clearly there might be some short term gains at the expense of increased watering.

Normally, unless there is a spill, battery acid should never be added to a battery. If the temperature compensated Specific Gravity reading needs to be increased in a cell for whatever reason, remove a small amount of some the existing electrolyte and replace it with fresh battery acid with a 1.300 Specific Gravity. Repeat the process until the cell matches the Specific Gravity readings of the rest of the cells or, if the battery is fully charged, the manufacturer's temperature compensated recommended value for a fully charged cell. If the temperature compensated Specific Gravity reading needs to be decreased in a cell for whatever reason, remove a small amount of some the existing electrolyte and replace it with distilled, deionized or demineralized water. Repeat the process until the cell matches the Specific Gravity readings of the rest of the cells or, if the battery is fully charged, the manufacturer's temperature compensated recommended value for a fully charged cell. Some typical Specific Gravity readings at 80° F (26.7° C) for full charged cells are:

SLA or Sealed Lead-Acid batteries are part of the VRLA Battery family and are normally under 50 amp hours in capacity. Most of the SLA batteries in use today are AGM (Ca/Ca) because they are less expensive, but there are a few Gel Cell (Ca/Ca) SLA batteries in service. For fast recharging, you should limit the current to 30% of the amp hour capacity of the battery and use an absorption voltage of 2.45 VDC/cell or 14.7 VDC for a 12-volt battery. When the charging current has dropped to .01 times of the amp hour capacity, then the battery is fully charged and the fast charging voltage should removed or reduced to a float charging voltage of 2.25 volts/cell or 13.5 VDC for a 12-volt battery.

9.16. How Do I Recharge Unevenly Discharged or Non-identical Batteries at the Same Time?

If discharging the batteries unevenly or using non-identical batteries has to occur, then use an isolated multi-bank charger, single bank charge with an external diode isolator (adjusted for the voltage loss), or combiner to recharge the batteries at the same time.

9. HOW DO I CHARGE (OR EQUALIZE) MY BATTERY?