The first three batteries were all from the batches that suffered earlier failure and cutting out Peter, exactly as you've experienced. Battery number four is from a period in which far better results are being experienced currently, and I don't know of any failures on those.
Pairing batteries in parallel without protection can lead to failure and cross charging with a potential for damage, and that's especially true for older and mismatched batteries. If the feed wires from each battery are run through suitable Schottky diodes, they protect the batteries from damage.
Running them in series isn't practical since that doubles the voltage, and the eZee controller is limited to a maximum of 44 volts, plus or minus 0.5 volts. Any voltage over that causes a protection cutout.
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