Victron energy BMV-712 Smart Manuel page 16

3.3.2 About charge efficiency (CEF)
The charge efficiency of a lead acid battery is almost 100 % as long as no
gas generation takes place. Gassing means that part of the charge
current is not transformed into chemical energy, which is stored in the
plates of the battery, but is used to decompose water into 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 easily 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 battery type, age and usage.
The BMV takes this phenomenon into account with the charge efficiency
factor: see section 4.2.2, setting number 06.
3.4 Several battery state of charge display options
The BMV can display both the Amp-hours removed ('consumed Amp-
hours' readout, compensated for charge efficiency only) and the actual
state of charge in percent ('state of charge' readout, compensated for
charge efficiency and Peukert efficiency). Reading the state of charge is
the best way to monitor the battery.
The BMV also estimates how long the battery can support the present
load: the 'time-to-go' readout. This is the actual time left until the battery is
discharged to the discharge floor. The factory discharge floor setting is 50
% (see 4.2.2, setting number 16).
If the load is fluctuating heavily it is best not to rely on this reading too
much since it is a momentary readout and must be used as a guideline
only. We always encourage the use of the state of charge readout for
accurate battery monitoring. The battery state of charge indicator (see
chapter 7 "Display") scales between the configured discharge floor and
100 % state of charge and reflects the effective state of charge.
3.5 History data
The BMV stores events which can be used at a later date to evaluate
usage patterns and battery health.
Select the history data menu by pressing ENTER when in normal mode
(see section 4.3).
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