Victron energy BMV-700 Manual page 13

The service life of batteries depends on many factors. Battery life may be
shortened by under-charging, over-charging, excessively deep
discharges, excessive charge or discharge current, and high ambient
temperature. By monitoring the battery with an advanced battery monitor,
important feedback is given to the user so that remedial measures can be
taken when necessary. Doing this, which extends battery life, the BMV
will quickly pay for itself.
3.3 How does the BMV work?
The main function of the BMV is to follow and indicate the state-of-charge
of a battery, in particular to prevent unexpected total discharge.
The BMV continuously measures the current flow in and out of the
battery. Integration of this current over time (which, if the current is a
fixed amount of Amps, boils down to multiplying current and time) gives
the net amount of Ah added or removed.
For example: a discharge current of 10A during 2 hours will take 10 x 2 =
20Ah from the battery.
To complicate matters, the effective capacity of a battery depends 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 be 'pumped' into the battery than can be retrieved during
the next discharge. In other words: the charge efficiency is less than
100%.
3.3.1 About battery capacity and the rate of discharge
The capacity of a battery is rated in ampere-hours (Ah). For example, a
lead acid battery that can deliver a current of 5A during 20 hours is rated
= 100Ah (5 x 20 = 100).
at C
20
When the same 100Ah battery is discharged completely in two hours, it
may only give C = 56Ah (because of the higher rate of discharge).
2
The BMV takes this phenomenon into account with Peukert's formula: see
section 5.1.
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