The final parts have arrived for the conversion to lithium batteries, these are the low voltage alarms, basically they are set to alarm (quite noisily) if any of the batter cells drop below 3.3volts, they measure this across the individual cells of the battery, so will alarm if only one of the cells drops below optimum, normal cell voltage is 3.7v and peak is about 4.1v. I now have these, the 10 ah lithium battery (which as we have measured the load in the water at 5amps, allows for two hours continuous use flat out) running at 11.1 volts means we can retain the existing systems for speed control.
The lithium battery is about 1/3 the weight of a 9ah lead acid battery, so ballast will be needed, but this can now be adjusted to trim the boat for level.