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TECHNICAL BACKGROUND

n

Peukert’s Law

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

n

t

Cp

= battery capacity available with the given discharge current

I

= the discharge current level

log T2 - logT1

n

= the Peukert exponent =

log I1 - log I2

T

= 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

current levels.

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

battery.

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The harmful effects of ripple voltage

on batteries

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

pack.

n

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.

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