Carl wrote:
Hi Nigel,

When you have batteries arranged this way you need to be very careful
that they charge and discharge in balance. Otherwise you end up with
voltage variations between the pairs that leads to early failure (I
know - it seems odd). Your positive lead should connect first to one
pair and the negative first to the other pair.

Here's a diagram of correct wiring from the high end german battery
supplier - Sonnenschein.

http://www.sonnenschein.org/Really%20FAQ's.htm

Carl

That is a common but unfounded myth.

With appropriate jumpers between the parallel batteries, the voltage
difference caused by cables entering/exiting from the same side are
less than 0.001 volts which is far less than internal battery
incongruities and will have zero effect on the battery life.

If the two batteries were separated by a considerable distance, say 10
feet, and the jumpers between them were smaller than the starter motor
cables, in that case one battery would tend to be used slightly more
than the other but only during heavy loads. As soon as the load is
removed, the batteries will equalize within a few seconds.

Andina Marie