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I have the following equipment and want to know what I need to make this a functional battery backup for a sump pump.

I understand the missing parts are an inverter, a fuse box and a fuse. I referred to this article from Eaton on how to select and install an Inverter-Charger for sump pumps, which has lots of useful information about my exact application. The question I have is, since I already have a charger: can I just buy a sufficient inverter (say 1500 W pure sine wave inverter, to be safe) and use my existing charger? Or does this application require a charger-inverter combination device?

Laying it out more specifically:

  1. If I have this 12 V, 100 Ah battery, then I hook up a DC-to-AC inverter to it, then on top of that I connect the smart charger to each terminal and plug that into 120 VAC household outlet,
  2. then turned all the devices on 24/7 and plugged a 120 VAC load into the inverter,
  3. would I have a backup system that charges and provides power from the grid when that's available but automatically stops charging and starts powering the inverter from the battery when grid power goes out? If not, why or what is missing? Is there a controller in the combination charger-inverter that's missing in this hypothetical setup?
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  • \$\begingroup\$ Is the charger of a type suitable for lithium batteries of type you showed. If nothing is explicitly stated about them being compatible then I would be very, very cautious about connecting them. \$\endgroup\$
    – Andy aka
    Dec 22, 2022 at 16:50
  • \$\begingroup\$ Yes, I was careful to select a charger suitable for the LiFePO4 battery, and I have used the charger a handful of times already to charge or top-off the battery. It has trickle charge capability and all kinds of 'smart charging' bells and whistles for various battery types including lithium. \$\endgroup\$
    – cr0
    Dec 22, 2022 at 16:52
  • \$\begingroup\$ Not what you asked but a search term for further reading for what you are describing is an online UPS. \$\endgroup\$
    – winny
    Dec 22, 2022 at 17:23
  • \$\begingroup\$ Thanks @winny, I am aware of UPS backup systems and actually tried one already for a sump pump, but the one I tried was insufficient for the pump. \$\endgroup\$
    – cr0
    Dec 22, 2022 at 18:14

1 Answer 1

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Connect a fully-charged battery to the charger through an ammeter or DVM with sufficient current range, and unplug the charger to simulate a power failure. If there's no back-current, then it should be safe to simply connect battery, charger and inverter in parallel. The pump should run full-time from the inverter; i.e., it would be an always-on UPS.

Caveats:

  • How much current does the sump-pump draw? If more than a few amps, expect battery run-time and longevity to be shorter than the 1.2 kW-h storage would indicate.
  • Be sure the charger can keep up with the current draw, lest it blow an internal fuse and no longer charge after the motor cycles on.
  • Be sure that the inverter is properly grounded and has a GFCI, since you're working with mains voltage in a wet location!
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