Batteries are somewhat curious things in that the amount you can take out of them depends on how quickly you do it and I'm indebted to Ample Power (USA) for showing that Peukert's Law is well (if inversely) demonstrated by pouring beer into glasses!

If you pour it in quickly, only a small amount of liquid is transferred: the rest is foam. The slower you pour the beer, the more beer and the less foam enters the glass. You may feel you need to test this a few times. (Hic – 'scuse me!)

Peukert's actual equation is InT=C where I is discharge current, 'n' is a value related to the battery's construction, T is the duration of discharge, and C is the capacity removed as a result of that discharge.

In practice 'n' varies, but 1.2 is common. The lower the 'Peukert exponent' the better able is the battery to supply heavy current.

Peukert's Equation

Almost from Plante's invention of lead-acid batteries (in 1860), it has been observed that the true capacity of a battery is related to its rate of discharge (the faster the rate of discharge, the less of its nominal amp/hour capacity that can be delivered. The effect is a result of the battery's internal resistance which, in turn, is dependent on the battery's type and method of construction.

The phenomenon was extensively studied and quantified by a researcher named Peukert. In 1897, Peukart developed an equation that describes the effect. It is has subsequently been very thoroughly established that Peukart's law is accurate within +/- 0.5%-1.0%.

Peukart's equation is:

InT=C

In this equation:

‘n' as an exponent related to battery construction.

‘I' is the discharge current (in amps)

‘T' is the duration of discharge (in minutes)

‘C' is the capacity removed as a result of that discharge.

If ‘n' were to be 1, then 100 A/h is always 100 A/h regardless of the rate of discharge. In practice batteries don't work like this – ‘n' is always greater than ‘1'.

Most batteries have an exponent of about 1.2. The lower the exponent, the better the battery is able to supply high currents.

Battery makers (but rarely resellers) will be able to advise the Peukart exponent for their products. They are likely to be surprised by the request, but any technically-knowledgable battery person should be able to supply the information, particularly as the effect is of considerable significance with electrically-powered vehicles.

Using Peukert's Law enables us to establish the battery size required for any large current draw. It also and usefully shows the effect of various rates of current draw on existing batteries.

In particular Peukart's Law demonstrates the difficulty of assessing remaining battery charge even by measuring incoming and outgoing current without correcting for the Peukart effect.

Thus a battery with an exponent of 1.2 will, if discharged at 2.0 A/h actually be depleted by 2.3 A/h (13% higher). If discharged at 20 A/h it will be depleted by 36.4 A/h (45% higher).

The formula can also be used to establish desirable battery capacity for different rates of current draw. Thus to draw 100 A/h from a typical battery (with an exponent of 1.2) we need a battery of 251 A/h.

A correction for the Peukart effect should really be incorporated in battery energy monitors. But as far as I can establish (several battery monitor vendors responded to my query with ‘Peukart' – never heard of him' – delivered as if their lack of trade knowledge was somehow my fault!) the only battery monitors that include Peukart correction are those made by Ample Power.

Peukart's Table

Exponential amps are tabulated for currents with different exponents of ‘n'.

The lowest Peukart exponents (typically 1.1) are exhibited by gel call and AGM (Absorbed Glass Mat) batteries. Peukart's exponents for some (but mostly US) batteries can also be found on the web.

A useful Peukart calculator can be found at www.gizmology@net/batteries.htm.

Another useful site is: www.geocities.com/CapeCanaveral/Lab/8679battery.html

For detailed information on batteries and caravan/motorhome electrics generally refer to my book ‘Motorhome Electrics – and Caravans Too!'. Full details (plus a great deal of further useful information) is on my website: www.caravanandmotorhomebooks.com

This information has been prepared by express permission from Collyn Rivers, Caravan and Motorhome Books, PO Box 356 Church Point NSW 2105 (email collynr@bigpond.com). It is copyright to Caravan and Motorhome Books, but be reproduced providing due acknowledgment is included.