I've been busy restoring a couple of c5's recently and chose to run them on a relay, without the usual Pod and Control Box set-up.
When doing this, you can utilise an original heat sensor to provide some thermal protection for the motor, but I don't think it would be an easy task to try and build in some battery protection. To help prevent over discharging the battery, I've been trying out some battery monitors to at least give the user some indication of what state the battery is in.
This is the first one I tried, purchased from Ebay;
It has a nice large red LED type display, showing the battery voltage only. It cost about £6 and takes a short while to arrive from China. I decided to fit the monitor into a "dead" pod to try and preserve some of the original look of the c5. When fitting, there are five wires to choose from which I feel makes wiring it up a little more tricky than I thought it would be. The instruction sheet is downloaded from the Ebay advert. Of the three monitors I ordered from these guys,two were found to be defective. But, I have to say, they were quickly replaced by the manufacturer with minimum fuss - so no complaints.