There is no diodes on them brian. It is a straight wire from one battery to the other. Same thing on a tractor trailer with more batts. I have seen up to 4 batts in series. I not sure what you mean by cross charge. If a battery is 12.6volts and the other is 12.4 volts you start the car or truck it will draw more from the higher volt system. After the vehicle is running it charges the lowest one first and then brings both up equally. If you want a pic I can get one off brother in laws dodge :D
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For car/truck batteries, the cells would have to be exactly the same voltage. Otherwise current would flow from the higher voltage battery to the other. This "charging" current would only be limited by the gauge of the wire and the battery capacity.
CHC, you say there is a wire between them. Do you mean in a series chain? Is it a 24/36v starting system?