I've replaced the batteries with 2 identical numax batteries that I've had at home for 3 years(periodically charging them up approx every 3 months)...
I originally changed them 3 years ago because I wrongly assumed that I had a battery issue(fault code 3 flashes kept coming up on heater)but my battery voltage was nowhere near as low as this time....it was a fuse issue,swapped it out and everything's been fine and dandy for 3 years now....