I have found it quite useful, when working on a motorcycle's electrical system for long periods, to plug in my 12v DC power adapter to which I have attached a pair of alligator clips (suitably marked positive and negative, as determined by my multimeter) and connected in place of the battery. Thus I can leave happily on charge my brand new battery, and never fear of "running out of juice" while diagnosing any fault...
Just a thought...
Cheers, Phil