<font color=blue>I think they make battery packs (or something like that) for sump pumps, as it's common for people to want an emergency backup </font color=blue>
Hopefully, I can save someone some money. I have one of these sitting on a shelf gathering cobwebs. Assuming they haven't changed recently, here's what they consist of...
1. A large plastic box that holds a car battery.
2. On top of the battery box is a smaller plastic box that contains the electronics to keep the battery charged. This must be plugged into a regular 115v outlet via a 'wall wart'.
3. Other cables lead out of the battery box to the 12v sump pump.
4. The 12v sump pump is clamped on the discharge pipe of the 115v sump pump so that when the water rises above the regular sump pump, the 12v can come on.
5. The discharge hose from the 12v sump pump has to either connect to the same pipe the regular sump pump connects to or have it's own pipe to the outside. If it connects to the same pipe, you have to install a check valve to ensure the 12v sump pump doesn't just pump the water back into the same hole.
Both sump pumps (electric and the 12v) have to fit in the same sump hole. You have to have a shelf nearby for the battery box. You have to have an outlet nearby that can handle both the 12v wall wart and the cord from the electric sump pump. Plus all the plumbing.
Add to this the substantially reduced pumping capability of the 12v pump and, in my experience, you have an expensive lesson learned. In my case twice, because I installed the first and second generation of these contraptions (purchased from Sears).
The single advantage these devices have is they will come on automatically and pump until the battery dies. They'll work great if the power is out for some other reason than a rain storm, i.e., you don't have a lot of water to be worried about.
I now use a generator.