-30 -20 -10 0 10 20 30 40 50 60
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
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
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