There are up to four phases of battery charging-bulk, absorption, equalization and float. The bulk stage is where the charger current is constant and the battery voltage increases. You can give the battery whatever current it will accept not to exceed 20% of the ampere-hour rating and this will not cause overheating. The absorption phase is where the charger voltage is constant and current decreases until the battery is fully charged. This normally occurs when the charging current drops off to 1% or less of the ampere-hour capacity of the battery. For example, the ending current for a 100 ampere-hour battery is 1.0 amp or less.

The optional equalizing phase is a controlled 5% overcharge, which equalizes and balances the voltage and specific gravity in each cell, the effect of increasing the charge voltage. Equalizing reverses the build-up of chemical effects like stratification, where acid concentration is greater in the bottom of the battery. It also helps remove sulfate crystals that might have built up on the plates. The frequency recommendation varies by manufacturer from once a month to once a year, from 10 to 100 deep cycles, or when a specific gravity difference between cells reaches .03. To equalize, fully recharge the battery; next, increase the charging voltage to the manufacturer's recommendations (if you cannot find one, ADD 5%). Heavy gassing should start occurring (be VERY CAREFUL about safety precautions). Take specific gravity readings in each cell once per hour. Equalization has occurred once the specific gravity values no longer rise during the gassing stage.

The optional float phase is where the charge voltage is reduced, held constant and used indefinitely to maintain a fully charged battery. Please refer to Section 9 for more information about storing batteries and float charging them.

An excellent and easy to understand tutorial on battery charging basics can be found at by drilling down through Charging>Tutorials>Charging Basics. The following are multi-stage charging algorithms from Deltran (Battery Tender) for three different types of car batteries:

Standard Deep Cycle (Lead-Antimony/Antimony)

[Source: Deltran]

Absorbed Glass Mat Deep Cycle (AGM)

[Source: Deltran]

Gel Cell Deep Cycle

[Source: Deltran]

It is important to use the battery manufacturer's charging recommendations whenever possible for optimum performance and life. In addition to the earlier cautions, here are some more words of caution:

6.1. NEVER, NEVER disconnect a battery cable from a vehicle with the engine running because the battery acts like a filter for the electrical system. Unfiltered [pulsating DC] electricity can damage expensive electronic components, e.g., emissions computer, radio, charging system, etc. Turn off all electrical switches and components, turn off the ignition and then disconnect the battery.

6.2. For non-sealed batteries, check the electrolyte level. Make sure it is covering the plates, and it is not frozen BEFORE starting to recharge.

6.3. Do NOT add distilled water if the electrolyte is covering the top of the plates because during the recharging process, it will warm and expand. After recharging has been completed, RECHECK the level.

6.4. Reinstall the vent caps BEFORE recharging, recharge ONLY in well-ventilated areas, and wear protective eye ware. Do NOT smoke or cause sparks or flames while the battery is being recharged because batteries give off explosive gasses.

6.5. If your battery is an AGM or a sealed flooded type, do NOT recharge with current ABOVE 12% of the battery's RC rating (or 20% of the ampere-hour rating). Gel cells should be charged over a 20-hour period and never over the manufacturer's recommended level or over 14.1 VDC.

6.6. Follow the battery and charger manufacturer's procedures for connecting and disconnecting cables and other steps to minimize the possibility of an explosion or incorrectly charging the battery. You should turn the charger OFF before connecting or disconnecting cables to a battery. Do not wiggle the cable clamps while the battery is recharging, because a spark might occur, and this might cause an explosion. Good ventilation or a fan is recommended to disperse the gasses created by the recharging process.

6.7. If a battery becomes hot, over 110 o F (43.3 o C), or violent gassing or spewing of electrolyte occurs, turn the charger off temporarily or reduce the charging rate. This will also prevent 'thermal runaway" that can occur with VRLA batteries.

6.8. Insure that charging with the battery in the car with an external MANUAL charger will not damage the vehicle's electrical system with high voltages. If this is even a remote possibility, then disconnect the vehicle's battery cables from the battery BEFORE connecting the charger.

6.9. If you are recharging gel cell batteries, a manufacturer's charging voltages can be very critical. Sometimes, you might need special recharging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to recharge gel cell and AGM batteries because of their charging profiles; using them will shorten battery life or cause "thermal runaway". Match the charger (or charger's setting) for the battery type you are recharging or floating.

Use an external constant current charger, which is set not to deliver more than 12% of the RC rating of the battery and monitor the state-of-charge. Timers that will cut-off the charger will help prevent overcharging the battery. For discharged batteries, the following table lists the recommended battery charging rates and times:

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

[Source: BCI]

The BEST method is to SLOWLY recharge the battery at 70o F (21.1o C) over a 10 to 20 hour period (C/10 to C/20) using an external constant voltage (or tapered current charger) because the acid has more time to penetrate the plates and there is less mechanical stress on the plates. C-rate is a measurement of the charge or discharge of battery overtime. It is expressed as the Capacity of the battery divided by the number of hours to recharge or discharge the battery. For example, assume that the ampere-hour capacity of the battery is 220, then it would take 11 hours to recharge or discharge the battery using a C/20 rate. A constant voltage or "automatic" charger applies regulated voltage at approximately 13.8 to 16 volts, based on the manufacturer's recommendations and temperature. A 10 amp constant voltage charger will cost between $30 and $60 at an auto parts store is suitable for most simple recharging or charging applications.

More expensive three stage microprocessor controlled chargers are available that will automatically provide bulk, absorption and float charging. A four-stage charger will provide an equalizing charge in addition to the bulk, absorption and float charging. An excellent automatic constant voltage battery charger is a 15-volt regulated power supply adjusted to the manufacturer's recommendations or, if not available, to voltage ranges below with the electrolyte at 70 o F (21.1o C):

Battery Type Charging Voltage Float Voltage Equalizing Voltage
Wet Low Maintenance 14.4 13.2 15.1
Wet Maintenance Free 14.8 13.4 15.5
Sealed & VRLA 14.4 13.2 15.1
AGM 14.4 13.6 15.5
Gel Cell 14.1 13.2 N/A
Wet Deep Cycle 14.5 13.2 15.8
Charging Voltages

To compensate for electrolyte temperature, which has a negative temperature compensation coefficient, adjust the charging voltage .0028 (2.8 millivolts) volts/cell/degree F. For example, if 30o F (-1.1o C), then INCREASE the charging voltage to 15.07 volts for a wet, low maintenance battery. If 100 o F (37.8 o C), then DECREASE the charging voltage to 13.90 volts.

If left unattended, a cheap, unregulated trickle battery charger can overcharge your battery because they can "boil off" the electrolyte. Do NOT use fast, high rate, or boost chargers on any battery that is sulfated or deeply discharged. The electrolyte should NEVER bubble violently while recharging because high currents only create heat and excess explosive gasses.

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