[Aussie Mike]

I used to work a lot with DC power systems with large banks of batteries in series and in parrallel.

What Pete says is right, the batteries will ballance themselves out over time. It's only a problem if one cell fails and the voltage of one battery drops in voltage it will tend to drag on the others.

Like any battery make sure you check the electrolyte levels regularly and keep them topped up.

Cheers