Yes. (There's 2 also generators per engine, 2 in the APU, and the RAT, but clearly the idea was that the battery would be the "last" backup and able to handle braking). The APU battery isn't on Boeing's Master MEL, and the APU will shut down without it; so in a worst-case catastrophic scenario, with no APU, and a double bird strike, assuming the RAT will be of very limited to no help for braking, you don't want the main battery to smolder at the same time. Highly unlikely, but I'm sure someone is calculating probabilities
