9
2.2 Charge Efficiency Factor (CEF)
Not all energy transferred into the battery during battery charging, is
available during discharge of the battery. The charge efficiency of a
brand new battery is approximately 90%, meaning that 10Ah must be
transferred to the battery to get 9Ah actually stored in the battery. This
efficiency figure is called Charge-Efficiency-Factor (CEF) and will
decrease with battery age. The BMV-501 can automatically calculate the
CEF of the battery.
2.3 Peukert’s exponent
As mentioned earlier in chapter 1.2 the Peukert efficiency describes how
if you discharge a battery faster than the 20hr rating, it’s Amphour
capacity decreases. The amount of battery capacity reduction is called
‘Peukert exponent’ and can be adjusted from
1.00
to
1.50
in Function
F10. 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 doesn’t care how big the discharge current
is. Of course such batteries do not exist, and an F10 setting of
1.00
is
only implemented to bypass Peukert compensation in the BMV-501.
The default setting for the Peukert exponent is 1.25, and is an
acceptable average value for most lead acid type of batteries. 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 must be provided with
your battery. The Peukert equation is stated below :
Cp = I
n
⋅
t where Peukert exponent ‘n’ =
2
log
1
log
1
log
2
log
I
I
t
t
−
−
The battery specifications needed for calculation of the Peukert
exponent, are the rated battery capacity (usually the 20hr discharge
rate
(1)
) and for example a 5hr discharge rate
(2)
. See the calculation
example below to define the Peukert exponent using these two
specifications :
5hr rating, C5 = 75Ah
→
t1 = 5hr
→
I1 = 75Ah/5hr = 15A