Sump pump battery backup

Question

I currently have a sump pump in my basement. I recently lost power during a rain storm and had to pump water out by hand until power was restored so I am looking to install a battery backup for the sump pump. I already have a 110 amp hour marine deep cycle battery which powers a 60 foot pound electric trolling motor for at least 4 hours so I believe that a sump pump should be able to be powered by this deep cycle battery.

I was looking at the sump pump and saw that it should take 920 watts when it is operating and I calculated that by multiplying 8 amps at 115 volts which is what the cap on the top of the sump pump says. I was looking at power inverters and I was wondering would a 1000 watt power inverter be enough power? I heard that when sump pumps first turn on they use more power than their continuous operating level.

EDIT- I was planning on keeping the sump pump plugged in to an outlet unless we lose power. If we did lose power, I was planning on unplugging the sump pump from the outlet and plugging it into the inverter that was being powered by the marine deep cycle battery.

Answer 1

The best plan I saw suggested in another question was to get a second 12V based pump. Place (or adjust) it so its start sensor is higher than your AC-powered pump, and also connect the battery to a battery maintainer. You get several benefits:

1. If the power goes out, the second pump will kick in when the water reaches it (which should be within about an inch of when the first pump would've started)
2. If the first pump dies, the second pump will still operate independently
3. If the inflow exceeds the capacity of the first pump, the second pump will augment the pumping capacity when the water reaches it (so you'd have both pumps running together).

Answer 2

There are a few aspects to consider.

Run time

Look at the history of how long power stays out when it goes out. Some causes can be corrected within a few minutes. If a crew needs to be dispatched to fix a downed power line, it could easily be hours. If there are many power lines down, it can be a lot longer than that.

Your 110 AH battery should power the pump for around 3 hours until it ages, given that the pump cycles on and off. If you're in an area that often gets serious storms, you might want a bigger battery, a second battery, or even a small generator.

Availability

But what if you're not at home to switch the power? Tyson's suggestion of adding a battery powered pump will, at a minimum, buy you some some time to get there and switch things if necessary.

Redundant pumps are a good idea, anyway, since sump pumps do fail independent of a power outage (pump or float switch; I've had to replace the float switch on ours about every 4 yrs.).

Inverter

If you go the inverter route, get one with a lot more spare capacity; 1000 watts is enough for the pump while it's running, but the pump takes very high current to start, which could pop the inverter's breaker; then you have no source of power. A battery-powered pump is better for this reason (in addition to Tyson's point that the proper charging circuit is typically built-in).

Answer 3

I had a commercial battery back-up unit over 20 years ago. It powered a separate DC pump. The point I would add is during an extended outage, I pulled a battery out of a car and switched it with the back-up battery. Inconvenient , but rarely needed.