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Charging batteries

Charge voltage

Mastervolt gel (2 V, 12 V) and Mastervolt AGM (6 V, 12 V)

batteries should be charged with a voltage of 14.25 V for

12 V systems and 28.5 V for 24 V systems. The absorption

phase is followed by the float phase (see 3-step+ charging

characteristic on page 242) in which the voltage is reduced

to 13.8 V for 12 V systems and 27.6 V for 24 V systems.

These figures assume a temperature of 25 °C.

For wet lead-acid batteries, the absorption voltage is

14.25 V for 12 V systems and 28.5 V for 24 V systems. The

float voltage for this type of battery is 13.25 V for 12 V and

26.5 V for 24 V systems. All of these figures are for 25 °C.

Lithium Ion batteries are charged with an absorption

voltage of 14.25 V for 12 V, and 28.5 V for 24 V systems.

The float voltage is 13.5 V for 12 V and 27 V for 24 V


Charge current

A rule of thumb for gel and AGM batteries states that the

minimum charging current should be 15 to 25 % of the

battery capacity. During charging, you usually continue

to supply power to connected devices, and this power

consumption should be added to the 15-25 %.

This means that a 400 Ah battery bank and a connected

load of ten amperes requires a battery charger capacity of

between 70 and 90 amperes in order to charge the battery

in a reasonable time.

The maximum charging current is 50 % for a gel battery,

and 30 % for an AGM battery. Mastervolt Lithium Ion

batteries can be subjected to much higher charge currents.

However, to maximise the lifespan of the Lithium Ion

battery, Mastervolt recommends a maximum charging

current of 30 % of the capacity. For a 180 Ah battery, for

instance, this means a maximum charge current of 60



A battery charger with temperature

compensation for optimal protection

Ensuring the longest possible lifespan for gel, AGM and

Lithium Ion batteries requires a modern Mastervolt battery

charger with a three-step+ charge characteristic. These

battery chargers continuously regulate charge voltage and

charge current.

For wet gel and AGM batteries, it is recommended to have

a sensor for measuring the temperature of the battery.

This adjusts the charge voltage to the temperature of the

battery, extending its lifespan. We call this ‘temperature


Because devices such as refrigerators are always drawing

power from a battery, even while it is being charged,

Mastervolt’s temperature compensation includes a

maximum offsetting effect to protect the connected devices.

The compensation is at most 14.55 V for a 12 V system, and

29.1 V for a 24 V system.

At very high (> 50 °C) and low (<-20 °C) temperatures, wet

gel and AGM batteries may no longer be charged. Outside

of these limits, the Mastervolt battery charger will continue

to supply the connected consumers but not charge the


Adjusting the voltage to a higher or lower temperature is

not required for Lithium Ion batteries.

battery temperature in ⁰C

Temperature compensation curve

25 ⁰C reference



charging voltage in V