I did a virtually identical thing with a cheap 12v drill over a year ago. That cured the 'dead battery' problem, but still left me with the problem of preventing it. The drill has a cheap powerpack type charger, similar to an old phone charger, not really suitable for extended charging. Plus, I thing leaving something on charge 24/7 does it no favours. So I bought some cheap £1 timer sockets (B&Q I think) and now it lives permanantly plugged in but gets 10 minutes of charge a day to keep it topped up unless it gets used, in which case I give it a longer charge. result after a year is a permanatly charged battery with no loss of capacity, and a lot cheaper than a specialist charger.