Battery Array has only one side getting warm to the touch
If you connect two batteries in parallel they can 'exchange' current. The side which has a sightly lower voltage starts getting current from the higher side. The additional problems is that the lower voltage may not become apparent until the batteries are under (heavy) load. 







simulate this circuit &ndash; Schematic created using CircuitLab

What you might try is to connect each battery with its own cable to one point. All cables should be the same type and length. These cables function as a series resistor. If one battery starts delivering more current then another it will cause a higher voltage drop over the cable and this 'compensates' somewhat the voltage difference. 

It is not ideal but it has the advantage of being simple!







simulate this circuit