Background Information - Victron energy VBC 12-24-48V Manual

4 SETUP
4.1

Background information

4.1.1 Charge efficiency factor
During battery charging, not all of the energy transferred into the battery
is available when the battery is being discharged. The charge efficiency
factor (CEF) of a brand new battery is approximately 90 %. This means
that 10 Ah must be transferred to the battery to get 9 Ah actually stored
in the battery. The CEF of a battery will decrease as the battery ages.
4.1.2 Peukert's exponent
As mentioned in chapter 1.4, the Peukert efficiency describes how,
when you discharge a battery faster than the 20 hr rating, its Ah
capacity decreases. The amount of battery capacity reduction is called
the 'Peukert exponent' and can be adjusted from 1.00 to 1.50. The
higher the Peukert exponent the faster the battery size shrinks with
increasing discharge rate. An ideal (theoretical) battery has a Peukert
exponent of 1.00 and has a fixed capacity; regardless of the size of the
discharge current. Of course such batteries do not exist, and a setting
of 1.00 in the VBC is only implemented to bypass Peukert compensation.
The default setting for the Peukert exponent is 1.25, and is an acceptable
average value for most types of lead acid battery. However for precise
battery monitoring, entering the right Peukert exponent is essential. If the
Peukert exponent is not provided with your battery, you can calculate it
by using other specifications which should be provided with your battery.
The Peukert equation is stated below:
n
where the Peukert exponent, n =
Cp = I ⋅ t
The battery specifications needed for calculation of the Peukert exponent,
are the rated battery capacity (usually the 20 hr discharge rate
example a 5 hr discharge rate
define the Peukert exponent using these two specifications.
1
Please note that the rated battery capacity can also be defined as the 10 hr or even 5 hr discharge rate.
2
The 5hr discharge rate in this example is arbitrary. Make sure that besides the C
a substantially higher discharge current.
log t −
2
log I −
1
2
. See below for an example of how to
rating (low discharge current) you choose a second rating with
20
log t
1
log I
2
1
) and for
9