Seri wrote:
Anyway, it has the original lead/acid batteries that came with it, that
seem to still be holding a charge, but they're not great and lose their
charge really quickly. I've pulled one of the batteries out of the
chassis to see if there was any hope of toping it up like a car
battery, but no dice (not that I was suprised).

Does anybody know of a way of 'priming' this type of battery to make it
healthier? Is it worth me charging them up full and then discharging
them as much as possible a few times? Or should I just leave them
constantly charging?

They're scrap, the plates will be sulphated, or possibly shorting. You
can get "Bat-Aid" tablets at car shops which may make the batteries
better for a while (if you can get them in), but I think you'd be better
off just replacing them. You don't have to use the same physical sized ones.