brian5,

I've asked the SAME question: why do some battery manufacturers not list the Ah value?

The problem in understanding Ah arises when we speak about a battery of a given Ah capacity. If we have a battery rated at 100 Ah, that battery can supply 5 Amps of current for 20 hours. That same battery can't supply 100 Amps for 1 hour, however. In fact, it can only supply 100 Amps for about 1/2 an hour. Why?

The true capacity of a battery is dependent on the rate of discharge. The faster the rate of discharge, the less total Ah capacity can be delivered. This phenomenon was described mathematically back in 1897 by a researcher named Peukert. He formulated the equation:

I(exponent n)T=C

In Peukert's equation, the letter I is the discharge current, letter n is a value related to battery construction, letter T is the duration of discharge, and the letter C is the capacity removed as a result of that discharge. If exponent n is equal to one, then we have the familiar circumstance where 1 Amp for 100 hours is equal to 100 Ah. (I = 1, n = 1, T = 100, so C = 100 Ah.) But, exponent n is never equal to 1, even in the best of batteries. Exponent n has normal values of 1.05 to 2, with about 1.2 being a common value. Lets use n = 1.2 in Peukert's equation with I = 100 Amps. We now find that C = 251 Ah. In other words, if we want to draw 100 Amps for 1 hour, we need a battery of 251 Ah, assuming the battery has a Peukert's exponent n = 1.2. Suppose we have an exponent of 1.1. For 100 Amps, C now equals 159 Ah considerably lower than 251 Ah. As mentioned, exponent n is related to battery construction. The lower the value, the better the battery will supply high currents.

Note that for low values of current, the value of n doesn't have much impact on capacity C. As currents increase, however, the effect of n is significant.

What this demonstrates is the need to measure Ah using Peukert's equation if we really want to stop guessing about battery capacity.

For a battery with an exponent of 1.2, a 2 Amp draw for an hour actually removes 2.3 Amp hours, or about 13% more than a linear measurement indicates. A 20 Amp draw for an hour results in a depletion of 36.4 Ah ...a whopping 45% more than a linear measurement would show!

How accurate is Peukert's equation? Recent tests indicate that errors are in the range of 0.5-1%.

Given that these cars are designed by German engineers, it does not surprise me in the least that they want to make sure that the battery they specify must have the correct Ah value ...

In America it is much more effective to market the battery with Cold Cranking Capacity (CCA), because more must be better, right? However, the CCA value is only part of the equation and says nothing about the draw on the battery by our power hungry cars!

In order to list the Ah value on a battery, it must actually be measured, at least on a sample basis, on the production line ... who's got time for that?

Hope this helps.

__________________
PAX5
BMW CCA #20645