I have used this rechargeable Li-Ion battery with success:
I installed one in an LGB 2090 Diesel and it ran for 5 hours before going flat. The battery pack has an internal fuse (probably a polyswitch) and under/over charge protection built in. It's very compact and lightweight (unlike SLA batteries). It will deliver approx. 11-12 volts. I also tested it to see how many amps it will deliver before the breaker shuts it down - I had 4 single motor LGB locos running off mine pulling approx 1.7 amps, so it will easily handle any single or double motored LGB loco. The breaker actually tripped if I tried to start all 4 locos at once, so I'm guessing the limit is around 2 amps. The only downside is the supplied charger is only 350 mA, so it takes a long time to recharge. For $26 including free air-mail shipping, it's a deal.
The vendor also sells other size LI-Ions- from 1800 up to 9000 mAh. There are also eBay merchants with similar batteries.