Victron energy VBC 12-24-48V Manual page 6

1.4 How does the VE.Net Battery Controller 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 VBC 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 100 Ah battery is discharged completely in two hours, it may only
give you 56 Ah (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 4.1.2). 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 VBC 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 shown as a percentage, where 100 %
represents a fully charged battery and 0% represents a completely flat
battery. You can compare this with a fuel-gauge in a car.
The VBC also makes an estimation of how long the battery can support
the present load (time to go). This is the time remaining until the battery
needs to be charged again (50 % state of charge). Discharging the
battery below 50 % significantly reduces the service life of the battery. 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 should be used as a
guideline only. We always encourage the use of the state of charge
readout for accurate battery monitoring.
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