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