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 EqualizingCharge 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 (or 30 points).  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 cardeful 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 athttp://www.batterytender.com/index2.html by drilling down through Charging Tutorials & Charging Basics.  The following are multi-stage charging algorithms from Deltran (Battery Tender) for three different types of  -------  DeepCycle Battery

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:

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° F (21.1°  C):

Float Voltage
ChargingVoltage (Charging Voltage Definition)
Equalizing Voltage

To compensate for electrolyte temperature, which has a negative temperature compensation coefficient, adjust the charging voltage .0028 (2.8 millivolts) to .0033 (3.3 millivolts) volts/cell/degree F.  For example, if the temperature is 30° F (-1.1° C), then increase the charging voltage to 15.19 volts for a wet low maintenance battery.  If 100° F (43.3° C), then decrease the charging voltage to 13.81 volts. If left unattended, cheap, unregulated trickle or manual battery chargers can overcharge your battery because they can decompose the water out of the electrolyte.  Avoid using fast, high rate, or boost chargers on any battery that is sulfated or deeply discharged.  The electrolyte shouldnever bubble violently while recharging because high currents only create heat and excess explosive gasses.