I currently have a sump pump in my basement that is normally powered by plugging it into an outlet. When we lose power, I have a 100 amp hour marine deep cycle lead acid battery along with a 1200 watt power inverter to power the 1/3 horsepower sump pump. The power inverter has an automatic shutoff when the voltage of the battery drops below 10.5 volts and because of that shutoff, the power inverter will start to power the pump however I can see on the display that the battery is dropping to 9 volts and then shortly after the inverter cuts the power. I was wondering how would I be able to mitigate this voltage drop in order to prolong the batteries from dropping below 10.5 volts?

sump pump nameplate

  • What gauge wires are you using, and how long are they? Mar 13, 2018 at 3:11
  • 4 gauge and 10 feet. The sump pump takes 8 amps Mar 13, 2018 at 3:25
  • 1
    What's the rated locked-rotor current on the sump pump's motor? It should be on the nameplate somewhere... Mar 13, 2018 at 3:32
  • Not sure what that is or what its measured in. I added a picture of the top of my sump pump if it is there. Mar 13, 2018 at 3:45
  • @ThreePhaseEel I don't think this is a mains voltage drop issue. It's low battery voltage. Mar 13, 2018 at 7:50

4 Answers 4


Edit: I hadn't quite realized the pump draws 8 A at 110 V, which would mean 125 A at 12 V. The wire gauge is thick enough for that (4 gauge is 20 mm2) but not by a wide margin. At these power levels, minimizing the resistance is critical, all connections have to be perfect.

To doublecheck the wiring: measure the voltage

  1. between the battery terminals
  2. on the input of the inverter
  3. on the output of the inverter

all while the pump is running. A difference between 1 and 2 indicates a wiring problem. If 3 is much lower than the rated voltage, that will cause the inverter to shut down.

Have you checked if the battery is properly charged? A voltage drop like that can indicate the battery is almost empty. When it's not providing a current, battery voltage will look okay, but voltage will drop rapidly when current is drawn.

If the pump really draws 880 W, that's a pretty big load (comparable to a starter motor), and on the edge of what a battery can supply. So that may be part of the problem.

  • As far as I know the battery is fully charged. I use a 6 amp charger to charge it completely and when nothing is connected to it, it shows that the voltage on the battery is 13 Mar 13, 2018 at 13:29
  • How long can the sump pump run before the inverter cuts out ?
    – Hobbes
    Mar 13, 2018 at 13:37
  • I would say about 3 seconds Mar 13, 2018 at 13:39
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    Have you measured the pump to see it is truly 8 Amps? It would be worthwhile seeing what it actually draws with water being pumped and how far. YMMV! Also, how old is the battery? Typically speaking, LEAD acids die off over the first couple years pretty quickly. Especially when not used regularly. Charging it (anything that causes heat) or letting it just sit, both will kill the battery, its lifespan still declines.
    – noybman
    Mar 14, 2018 at 0:11

Since you keep a 6 amp charger on the battery you may have boiled the water out of the battery it shows a full charge until a load is put on the battery then the voltage drops because there is not enough liquid to maintain the voltage/ current demand. If you can open the battery and add distilled water and this may provide some run time. Next find a quality Float charger, these do not over charge the battery like a trickle charger will and boil off the water. Last you might want to look at a larger battery, placing batteries in parallel will increase the time the system will run but if one battery is bad or has low internal resistance it will discharge the other battery unless there is an isolator. A larger deep cycle battery is what you need standard car batteries are not designed for deep cycle. Get as large deep cycle battery as you can afford (higher amp hours) golf cart batterys are also a good way to go many of these are 6v so it would take 2 in series.


Would a smaller pump suffice? That, along with Ed Beal's answer about battery health, might be all you need.

My parents used to live in a house at the bottom of a hill. Every big storm would knock out the power, produce lots of runoff, and flood the basement. Their solution was to add a 12V bilge pump to the sump, in addition to the 120V pump that was already there. The float valves were adjusted so the bilge pump would only operate when the main pump failed. The bilge pump ran off two marine batteries in parallel that were connected to a small float charger. No inverter was needed, which helped a lot with efficiency. That solution worked perfectly. It also saved them when the main pump burned out, which seemed to happen about once a year.


Top off the battery electrolyte solution with distilled water, charge the battery to 100%, and bring your deep cell battery to an automotive parts for a free battery load test to determine if your battery is bad. You can also test with a multimeter, but that's not always a good indicator of battery ability.

If your battery is bad, replace it and retest it with the pump/inverter to see if that fixes the problem. If your battery is good, purchase another battery of the same type and connect the two batteries in parallel to double the capacity. Do not connect them in series or it will double the voltage to 24VDC and destroy your inverter.

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