I have an HID light I made with a Phillips 35W HID (car headlight) and I put it into a Thor-X 10,000,000 Candle Power light. I initially had a UB1250 12V 5Ah Sealed Lead-Acid Battery in it and it worked but one day the garage door opener battery died…. so the battery is now in there. It has been sitting around without a battery for awhile now and before I order another UB1250 is there anything else that you would recommend?
I have the same light… maybe not the same brand, there’s 2 battery for it… the regular Lead acid battery 12AV 9A for about $25 ( don’t know about shipping ) 
Me too. I need to get rid of it because it’s BIG and just takes up space.
I used Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking, no trouble with the slightly higher voltage, it could see as high as 16V in a car in any case.
If going for Lithium-based, 4S LiFePO4 pack (13.2V nominal, 14.4V max, 12V empty) would be as close to SLA as possible. Additionally, LiFePO4 batteries are more safe than Li-polymer.
I have found that the ballast are voltage tolerant. I power mine 4s4p with balance lead and use a Hobby charger. Batteries came from 2 recycled HP packs with matched cells. HTH
That is what I was considering doing and just using 4 series battery holders so I can remover them since I don’t yet own a lithium hobby charger (have a novak n-power power 14V 10A supply but don’t know what charger to get).
I just realized one thing I forgot to mention the ballast takes up some of the battery compartment space so the typical Thor-x Battery won’t fit…. that is why I went with the UB1250. Kind of a big thing to forget about lol
I also incorporated a battery monitor. This is the one I used but had to put a piece of duct tape to mute the brightness. Got it cheap (lucky) on ebay. Full retail was crazy.
http://www.western-robotics.com/spectrum-LiPo_hp.html
or more reasonable