Victron energy BMV-600S Manuel page 6

1.3 How does the BMV work?
The capacity of a battery is rated in Amp hours (Ah). For example, a
battery that can deliver a current of 5 Amps for a period of 20 hours is
rated at 100 Ah (5 * 20 = 100). The BMV continuously measures the
net current flow into or out of the battery. This way it can calculate the
amount of energy removed from or added to the battery. But since
battery age, discharge current and temperature all influence the
battery's capacity; you cannot rely simply on an Amp hours reading.
When the same 100Ah battery is discharged completely in two hours, it
may only give you 56Ah (because of the higher rate of discharge).
As you can see the battery's capacity is almost halved. This
phenomenon is called Peukert efficiency (see chapter 2.3.2). Also,
when the temperature of the battery is low, its capacity is decreased
even more. This is why simple Amp hour counters or Voltmeters give
you far from an accurate state-of-charge indication.
The BMV can display both the Amp hours removed (not compensated)
and the actual state-of-charge (compensated by Peukert efficiency and
charge efficiency). Reading the state-of-charge is the best way to read
your battery. This parameter is given in percentages, where 100.0 %
represents a fully charged battery and 0.0 % a completely flat battery.
You can compare this with a fuel-gauge in a car.
The BMV also makes an estimation of how long the battery can
support the present load (time-to-go readout). This is actually the time
left until the battery needs to be charged again. If the battery 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.
Besides the main function of the BMV, displaying the actual battery
status, this monitor offers many other features. The readout of actual
battery voltage and current, and the ability to store historic data are just
a few of many features of the BMV. These features are more
specifically explained in the corresponding chapters of this manual.
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