I notice you've got both your battery-to-everywhere-else wires on the same battery pair - at the back of the picture. That means the battery pair at the front of the picture is seeing a much different electrical environment than the pair at the back. It will charge slower, discharge slower, etc. The one at the back of the picture will take more of the wear, and each pair in the string will see a different environment from any other..
With your positive and negative poles tied in a daisy-chain like that, you should take ONE of those two wires and hook it to the battery at the OTHER end of the string instead. (Do your hookups on the kiddy-corners.) That way all the batteries will see the same wiring resistance and thus the same voltages, draw the same currents, etc. Much better for system life, somewhat better for performance.
They sometimes use the same hack when wiring lamps in parallel on a bridge or long stretch of road, to keep them all the same brightness: Run the hot wire all the way to the other end of the string then hop from lamp to lamp on its way back. (When they don't wire them in series or put distribution transformers every so often, that is.)
