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I have a home in a countryside where grid power cuts off very often (3-5 times per week) for a few hours, especially in a winter time.

I came up an idea to make a backup system that will automatically or manually turns on when grid power goes out. (Currently I'm using portable power generator.)

I don't have electrical engineering degree but I know basic concepts and I've build and rebuilt a few EV bikes an scooters a few years ago. (I've software engineer background.)

The goal is to have 7-10 kWh energy backup system that can provide at least 220 V, 16 A. I prefer to have 220 V, 32 A power to run entire home appliances on it.

1. Battery: For batteries I am going to use EV or HEV battery Li-ion battery cells. (Ford fusion, Chevrolet volt ... ) because they are relatively cheap in my area. I can build 48 V, 96 V, or 220 V battery packs myself.

2. Charging: Currently I can accept charging the batteries with grid power (220 V) when it is available.

3. Using the power: As I said, 16 A - 32 A will be enough to run my house off grid. It will be good If I would have auto switcher between grid/backup system but I can do it manually.

My Idea:

I have an idea to buy used or broken APC UPS and modify it for my needs and change battery with my bigger battery pack.

The question is how deeply I have to modify the ups system to use/charge bigger battery pack? Will it be able to provide power of a long period of time? If so, which model UPS should I look up?

If using the UPS system is a bad idea, I have to purchase separately battery chargers, control module, inverter and etc... In this case can you suggest any components?

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  • \$\begingroup\$ Sounds like you want to connect a large (20 kWhr) battery system that is random, and not necessary compatible, to a large (4-7 kW) UPS. The only answer, given your experience, would be to find both and directly ask here if the pair you find are compatible with each other and with your goals. \$\endgroup\$
    – periblepsis
    Commented Oct 2 at 12:36
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    \$\begingroup\$ "How do I design the whole project" kind of questions are too broad for this Q&A site. Also, product/shopping recommendations are explicitly off-topic. electronics.stackexchange.com/help/dont-ask electronics.stackexchange.com/help/on-topic \$\endgroup\$
    – Lundin
    Commented Oct 2 at 12:42
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    \$\begingroup\$ you could make it safer and simpler by getting a hybrid charger/inverter and forget about using a duinky UPS to charge a huge battery pack. \$\endgroup\$
    – dandavis
    Commented Oct 2 at 16:27
  • \$\begingroup\$ How big is the APC? \$\endgroup\$
    – MicroservicesOnDDD
    Commented Oct 14 at 21:42

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I don't know of all the models available on the market, but for 16 A at 220 V you need at least a 4 kW inverter in your UPS, and at least a 7-8 kW one for 32 A.
I have relatively plenty of experience in electronics, but even I would rather not be modifying a UPS if it's too much hassle.
Sometimes the cheapest solution is the momentarily more expensive one if it fits your needs.
If you start looking for the right UPS to modify, then checking its circuitry out, learning about it, trying to modify it, only to realize you have wasted your time, nerves and money, you will start regretting having started the headache-inducing adventure.

Since we are typically discouraged here from suggesting specific manufacturers/devices/models, and it would take me some time to find the right one for you, I would suggest you start looking at specifications of various models on the Internet until you come across one that suits your needs.
You seem to have a good enough understanding of what's needed to get it done.

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