I was also told to disconnect the battery on our aircraft after it went flat. The maintenance guys never really gave an explanation why, other than something on the hardware was responsible.
We looked at buying a spare battery but they are very expensive from Eurocopter, I could purchase the exact same battery from an aviation battery supplier here in the UK for £1200 LESS than from EC!.
Technically you should be a qualified engineer to change the battery as it is bolted to the aircraft .
Invested in a ground power unit and only ever used it on one occasion when volts were slightly low