Victron energy BMV-600S Manuel page 9

To complicate matters, the effective capacity of a battery is dependent
on the rate of discharge and, to a lesser extent, on temperature.
And to make things even more complicated: when charging a battery
more Ah has to "pumped' into the battery than can be retrieved during
the next discharge. In other words: the charge efficiency is less than
100 %.
About battery capacity and the rate of discharge:
The capacity of a battery is rated in Amp-hours (Ah). For example, a
battery that can deliver a current of 5 Amps during 20 hours is rated at
C = 100 Ah (5 x 20 = 100).
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When the same 100 Ah battery is discharged completely in two hours,
it may only give C = 56 Ah (because of the higher rate of discharge).
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The BMV takes this phenomenon into account with Peukert's formula:
see section 4.3.4.
About charge efficiency:
The charge efficiency is almost 100 % as long as no gas generation
takes place. Gassing means that part of the charging current is not
transformed into chemical energy that is stored in the plates of the
battery, but used to decompose water in oxygen and hydrogen gas
(highly explosive!). The "Amp-hours" stored in the plates can be
retrieved during the next discharge whereas the "Amp-hours" used to
decompose water are lost.
Gassing can easely be observed in flooded batteries. Please note that
the "oxygen only" end of charge phase of sealed (VRLA) gel and AGM
batteries also results in a reduced charge efficiency.
A charge efficiency of 95 % means that 10 Ah must be transferred to
the battery to get 9.5 Ah actually stored in the battery. The charge
efficiency of a battery depends on the battery type, age and usage.
The BMV takes this phenomenon into account with the charge
efficiency factor: see section 4.3.4.
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