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101 renewable - how to recharge and equalize deep cycle battery

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Last Updated
7th of January, 2020

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Charging a deep cycle battery goes on for quite a while following through four charging phases: bulk charging, absorption charging, equalization and float charging.

The bulk charging stage is where the charging current flows constantly into the battery and the battery voltage step by step will increase in value. You can give the battery whatever current will accept but not to exceed 20% of the ampere-hour manufacturer rating value and the battery will not overheat.

The absorption charging phase is where the charging voltage is constant and current delivered to the battery decreases in value step by step. The current will continue to flow into the battery until the battery is fully charged. As an indicator that the battery is fully charged the charging current drops in value until it reaches 1% ampere-hour capacity of the battery or even less. For example, the final charging current for a 100 ampere-hour battery is 1.0 amp or less.

The equalizing charging phase is an optional phase (it depends on the battery charger manufacturer type) and it represents a controlled 5% overcharge. In this way, we aim to equalize and balances the voltage respective the specific gravity in each cell having as direct effect an increase in the charging voltage. The equalizing charging phase is reversing the build-up of chemical effects like stratification, where acid concentration is greater on the bottom of the battery.

The equalizing charging phase also helps remove sulfate crystals that might have built upon the plates. The frequency recommendation varies by battery 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).

The equalizing charging phase, will fully recharge the battery and increase the charging voltage to the manufacturer's recommendations if you cannot find one the increased value through equalization must be considered 5% on top of the battery nominal voltage, which is either 12V DC + 0.6V DC or 24V DC + 1.2V DC or 48V DC + 2.4V DC. Heavy gassing should start occurring (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 floating charging phase is a total optional phase and the main role is the general maintenance of the battery. During the floating charging phase, the charging voltage is reduced, held constant indefinitely to maintain a fully charged battery.

The following diagrams depict multi-stage charging algorithms for three different types of Deep Cycle Battery

High-Frequency Deep Cycle Battery (Golf Cart):~


Standard Deep Cycle Battery (Lead-Antimony/Antimony):~


Absorbed Glass Mat Deep Cycle Battery (AGM):~

Absorbed Glass

Gel (Cell) Deep Cycle Battery:~

In the case of Gel type of Deep Cycle Battery, it is very important to use the battery manufacturer's charging recommendations whenever possible to obtain optimum performance and an extended life cycle.

Flooded Deep Cycle Battery:~

In the case of Flooded Deep Cycle Battery, we have to use an external constant charging current, which will be set not to deliver more than 12% of the RC (Reserve Capacity) rating of the battery and we have to monitor the state-of-charge constantly. It is a very good idea to use timers that will cut-off the charging phase and will help prevent overcharging the battery.

For discharged Flooded Deep Cycle Battery, the following table gives you the recommended battery charging rates and the time associated with that:~

Reserve Capacity
(RC) Rating
Slow Charge

Fast Charge

80 Minutes or less [32-ampere hours or less] 15 Hours @ 3amps 5 Hours @ 10amps
80 to 125 Minutes [32 to 50-ampere hours] 21 Hours @ 4amps 7.5 Hours @ 10amps
125 to 170 Minutes [50 to 68-ampere hours] 22 Hours @ 5amps 10 Hours @ 10 amps
170 to 250 Minutes [68 to 100-ampere hours] 23 Hours @ 6amps 7.5 Hours @ 20amps
Above 250 Minutes [over 100-ampere hours] 24 Hours @ 10amps 6 Hours @ 40amps

The method is to slowly recharge the battery at 70° F (21.1° 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 unit that characterizes the charge or discharge of battery over a period of time. 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 recommendations and temperature.

More expensive three-stage microprocessor controlled chargers type are available and they will automatically provide bulk, absorption, and float charging stages. A four-stage charger will provide an equalizing charging phase in addition to the bulk, absorption, and float charging phases.

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 shown below with the electrolyte at 70° F (21.1°  C.

During the charging process of a Deep Cycle battery, we encounter the following voltages: Float Voltage; Charging Voltage; Equalizing Voltage.

Following table lists orientation voltage values for a different type of Deep Cycle Batteries:

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

In the case of flooded battery type, we have to compensate for electrolyte temperature (the electrolyte temperature has a negative temperature compensation coefficient). In order to obtain this compensation we have to adjust the charging voltage with .0028 (2.8 millivolts) up to .0033 (3.3 millivolts) volts/per cell/ per degree F. For example, if the temperature is 30° F (-1.1° C), then we increase the charging voltage to 15.19 volts for a wet low maintenance battery. If 100° F (43.3° C), then we 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 type of chargers on any battery that is sulfated or deeply discharged. The electrolyte should never bubble violently while recharging the service voltage because high currents only create heat and excess of explosive gasses.

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