The system I'm looking at comes with a charger, monitor and alarm. I
don't know if it monitors the battery status or not. It wouldnt be a
good system if it didn't.

Since I have a gravity drain as well, I'm more worried about the
primary sump wearing out and stopping. A battery system would run 6
hours continuous with a 75 amp-hour battery. On real bad storms my
sump only runs about once an hour for 15 seconds. That would give me
plenty of time to get a new AC sump from the hardware store.

Do you know of a water powered pump that costs under $300? I couldn't
find any.

Thanks.