On the surface it seems easy to calculate how much longer a
battery will continue to supply sufficient power. One of the most
common methods is to divide battery capacity by discharge
current. In practice, however, such calculations often turn out to
be wrong. Most battery manufacturers specify battery capacity
assuming a discharge time of 20 hours.
A 100 Ah battery, for instance, is supposed to deliver 5 amps per
hour for 20 hours, during which time voltage should not drop
below 10.5 volt (1.75 V/cell) for a 12 V battery. Unfortunately,
when discharged at a current level of 100 amps, a 100 Ah
battery will deliver only 45 Ah, meaning that it can only be used
for less than 30 minutes.
This phenomenon is described in a formula – Peukert’s Law –
devised more than a century ago by the battery pioneers
Peukert (1897) and Schroder (1894). Peukert’s Law describes the
effect of different discharge values on the capacity of a battery,
i.e. that battery capacity is reduced at higher discharge rates. All
Mastervolt battery monitors take this equation into account so
you will always know the correct status of your batteries.
Peukert’s Law does not apply for Lithium Ion batteries as the
connected load will have no effect on the available capacity.
The Peukert formula for battery capacity
at a given discharge current is:
Cp = I
= battery capacity available with the given discharge current
= the discharge current level
log T2 - logT1
= the Peukert exponent =
log I1 - log I2
= discharge time in hours
I1, I2 and T1, T2 can be found by carrying out two discharge
tests. This involves draining the battery twice at two different
One high (I1) – 50 % of battery capacity, say – and one low (I2) –
around 5 %. In each of the tests, the time T1 and T2 that passes
before battery voltage has dropped to 10.5 volt is recorded.
Carrying out two discharge tests is not always simple. Often,
no large load will be available or there will be no time for a
slow discharge test. You can retrieve the data necessary for
calculating the Peukert exponent from the specifications of the
The harmful effects of ripple voltage
A battery can become prematurely defective due to the
ripple voltage produced by battery chargers. To prevent
this, the ripple voltage caused by a charger should
remain as low as possible.
The ripple voltage results in ripple current. As a rule
of thumb, the ripple current should remain below five
per cent of installed battery capacity. If navigation or
communications equipment such as GPS or VHF devices
is connected to the battery, the ripple voltage should be
no more than 100 mV (0.1 V). Any more could cause the
equipment to malfunction.
Mastervolt battery chargers are equipped with excellent
voltage regulation and the ripple voltage they produce is
always lower than 100 mV.
Another advantage of low ripple voltage is to prevent
damage to the system if, for example, a battery terminal
is not properly secured or is corroded. Thanks to its low
ripple voltage, a Mastervolt battery charger can even
supply the system without being attached to a battery
Finding the state of charge of a battery
The adjacent explanation regarding the Peukert
exponent shows that the state of charge of a battery
cannot simply be determined based on, for instance,
measuring battery voltage.
The best and most accurate manner to check the
state of charge is to use an amp hour meter (battery
monitor). An example of such a meter is the Mastervolt
MasterShunt, BTM-III or BattMan battery monitor.
In addition to the charge and discharge current, this
monitor also indicates battery voltage, the number of
amp-hours consumed, and the time remaining until the
battery bank needs recharging.
One of the things that set the Mastervolt battery monitor
apart from other suppliers is the availability of historical
data. This shows, for example, the charge/discharge
cycles of the battery, the deepest discharge, the average
discharge, and the highest and lowest measured voltage.
On page 247 you will find more information on the
benefits of the Mastervolt battery monitor.