Battery life

Unfortunately the BattBorg provides no method for monitoring the state of the batteries.

What you would probably want is something between the batteries and the BattBorg which can measure voltage and the Raspberry Pi can talk with it,
Then as the batteries get low they produce a lower voltage, so a low battery warning could then be as simple as checking if this voltage is below a given value.

I wonder if a simple comparator circuit might work as effectively as an A-D in this case? I mean, it's nice to have a measure of battery strength, but I find that my battery voltage stays up pretty well, and then collapses very quickly. What's more, when the battery is getting a bit flat, its voltage varies according to the motor drive, so an accurate measure would probably be unhelpful.

A variable resistor and a 741 op-amp connector to a GPIO input would give you a usable comparator for about 30p. I find that when the battery voltage (motors off) drops from the usual ~12v down to 11.5V, I've got about another ten minutes of usable motor time, or maybe an hour of idle time. Oddly (or perhaps not) I find that the first thing to fail when the battery is getting flat is the WiFi, not the motors, which gives me a rough way of telling when I need to recharge.

Actually should work fine, flash converters employ a number of comparators in parallel to do high freq sampling.

I'd rather spend time on the process of A/I programming and less designing things I can procure for £14.99 (or so). There are some nice readily available Pi cards on the market that provide an eight channel A/D.