12Volts
If you were going to use a Lipo it would need to be a 3S pack which has a nominal voltage of 11.1. (Unless the 4500 uses a different back up system). I have the TCW back up boxes for my AFS 5500T's. I've had them a while and discovered they were seriously dead.
I bought replacement packs directly from TCW. I used one of my RC chargers to charge and cycle the packs a couple of times. Because of the location I mounted the boxes I decided to mount the new packs on the outside of the boxes for accessibility. They are mounted to the bottom of the boxes with industrial strength Velcro.
I opened the boxes, drilled a hole and brought the power leads and temp probe leads outside of the box. I modified the packs for a charge lead and ran wire forward to the panel area. I now have connectors under the panel to charge the packs individually. This will not affect the internal charging once the plane is up and running, but meanwhile, I can keep the packs healthy.
As Mike said, the biggest issue with a Lipo is having the appropriate charge system. I understand the newer TCW units have Lipos so a replacement would be easier and cheaper with a pack from RC sources.