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Repurpose old UPS as Inverter

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jonathan_winters

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I am building my own DIY Milk Crate, but I think this is more of a general DIY question. I hope this is the right place for it...

I'm at the point in my build where I'm ready to buy and add an inverter to my Milk Crate build, and after some searching for an inverter it occurred to me that I have a couple of old UPSs. One of them was a really nice UPS and it has a 600w output that is backed up by the internal batteries (which is exactly the size I'm looking for).

I haven't checked the voltage of the UPS' internal batteries so that could be the deal-breaker here, but if they are 12v output (or if the inverter otherwise shows that it can run from my LiFePo4 voltage), is it crazy to try harvesting the components from that UPS (everything from the UPS Batteries through to the battery-backed 120v plug outlets) to use as the inverter on my DIY Milk Crate build?

It seems to me like this should be the same as if the UPS is just running the outlets off the battery. But I've never cracked open one of those (and I'm new to this in general) so I'm hoping someone can tell me if/why this is a bad idea.
 
jonathan_winters said:
I am building my own DIY Milk Crate, but I think this is more of a general DIY question. I hope this is the right place for it...

I'm at the point in my build where I'm ready to buy and add an inverter to my Milk Crate build, and after some searching for an inverter it occurred to me that I have a couple of old UPSs. One of them was a really nice UPS and it has a 600w output that is backed up by the internal batteries (which is exactly the size I'm looking for).

I haven't checked the voltage of the UPS' internal batteries so that could be the deal-breaker here, but if they are 12v output (or if the inverter otherwise shows that it can run from my LiFePo4 voltage), is it crazy to try harvesting the components from that UPS (everything from the UPS Batteries through to the battery-backed 120v plug outlets) to use as the inverter on my DIY Milk Crate build?

It seems to me like this should be the same as if the UPS is just running the outlets off the battery. But I've never cracked open one of those (and I'm new to this in general) so I'm hoping someone can tell me if/why this is a bad idea.
Click to expand...
It's been done a few times. The battery is usually 24v nominal AGM type.
 
Yeah, should be easy enough, these low-cost UPSs invariably have one or two 7-10Ah gel batteries arranged as 12V or 24V.

LiFePO4 voltages are close enough to gel voltages to work without modification (all our el-cheapo baby UPSs have drop-in LiFePO4 packs).

One thing to watch is cooling, these units are invariably designed down to a price and have barely adequate heatsinking on the power devices. So, improve the heatsinks, add more ventilation (cut holes) and possibly add a low power fan.

Should be a good way in to green energy :)

EDIT One last thing, you will certainly want to remove or otherwise disable the UPS beeper :whistle:
 
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Thanks all! That is super helpful info. This is enough confirmation of the potential that I'm going to rip into the UPS this weekend.

My "milk crate" is actually a box I built myself - I've got computer fans to put in for airflow if the holes do not keep things cool enough, but I'll pay extra attention to temp tests while the inverter is running. (And yes I will absolutely disable the beeper! Holy crap that thing is annoying...)

The UPS is for sure a Pure Sine Wave inverter -- it a UPS that came out of a doctor's office that backed medical equipment (from 2009), so I'm very confident it will deliver clean power.
 
I've converted several APC UPSs from lead to lithium-ion. 7s7p works great as a 24v replacement :)
For example, you can see on this APC 1500 that it charges up to 27.6v. For an 8s LifePo4 that's 27.6v/8 = 3.45v/cell - perfectly OK. My discharge tests show it cuts off at ~21v. 21v/8 = 2.62v/cell and again is OK.
1664506244490.png

Also these things are not complicated, you can have the battery external in case it doesn't fit inside.

Once you have it hooked up I'd do a test and check charge / discharge voltages with you're voltmeter just to confirm you're particular model.

Here's another pic of an APC 1500 with a 7s7p and Chargery BMS + relay stuffed inside....
1664506520232.png

Here's an APC 3000 that I did with 14s12p lithium-ion and Chargery.... A 16s LifePo4 would work great - it's just double the 24v above.
1664506601197.png
 

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I have a Tripp-Lite hospital grade SMART2500XLHG UPS, 2200VA output. It can run off of my 48 V battery bank. It has 4 small 12V, 9aH SLA batteries inside or a 50A Anderson plug. I made a cable to go from my battery bank. It has a 16 amp 120 VAC output. My Dewalt circular saw would not run off the two Sol-Ark 12 K inverters (when plugged in right at power shed). When I plugged into the tripplite, that was plugged into my SA 120VAC output, it no longer tripped my inverters on a fault. Must be the filtering. They make a nice pass through inverter, if voltage is not right it takes over from the internal batteries or 48V input. It's probably not very efficient, because a high powered fan runs continuously. It's only one of many backups for me.

Edit 11/6/22: I tried my Dewalt circular saw again about 200 feet away from the power shed. It ran fine! For whatever reason the longer distance, lower voltage, helped out just like the UPS did.
 
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Big UPSs are cool. I have a large ~3kw server/rack-mount ups someone gave to me which i haven't tried to use yet, but it'll be a cool project when i get around to it.

When it comes down to it, what most DIY solar people are trying to build is basically a larger scale UPS anyway. The usual issue is that most of the UPS's the average person will come across cheaply are not very large or powerful. People who work in electronics/IT or other fields where they come across larger ones cheaply, can take advantage and make something powerful for cheaper than buying a new AIO inverter.

But if the large ones were THAT common and easy to come across in workable form, everyone would have one.. for most people buying the new AIO is much much easier and more user-friendly and easier to get forum explanation/support for.
 
Vigo said:
Big UPSs are cool. I have a large ~3kw server/rack-mount ups someone gave to me which i haven't tried to use yet, but it'll be a cool project when i get around to it.

When it comes down to it, what most DIY solar people are trying to build is basically a larger scale UPS anyway. The usual issue is that most of the UPS's the average person will come across cheaply are not very large or powerful. People who work in electronics/IT or other fields where they come across larger ones cheaply, can take advantage and make something powerful for cheaper than buying a new AIO inverter.

But if the large ones were THAT common and easy to come across in workable form, everyone would have one.. for most people buying the new AIO is much much easier and more user-friendly and easier to get forum explanation/support for.
Click to expand...
I'm with you and started off this way 4 years ago. If I could have found a whole-house UPS for $ I could afford....

But there are a couple of other issues for easy solar adoption....
1) For a 'solar system' you need a way to convert PV -> battery charging (e.g. charge controller) and of course the AOI have it built in.
2) The battery charging in an APC is pretty low amps / slow - too slow for a timely recharge if you did have to use it frequently for power outages.

But as I showed above in pics - I love these APCs.... some of mine are nearing 20years and still working!

I have an individual component based, off-grid system (with grid-assist) with several ATSs to auto-switch between inverter power and grid. I use a pair of APC 3000s to power a set of sockets thru the house to smooth the twice-daily solar/grid ATS switching for sensitive equipment such as computers/monitors, network equipment, TVs/Tivo, K-Cup(timer settings), etc...
1664549592304.png
The front one was converted to 14s12p you see above. These days, I'm working on a 14s50p (130ah@48v) external battery from my left over powerwall cells to power both :)
 
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Vigo said:
Big UPSs are cool. I have a large ~3kw server/rack-mount ups someone gave to me which i haven't tried to use yet, but it'll be a cool project when i get around to it.

When it comes down to it, what most DIY solar people are trying to build is basically a larger scale UPS anyway. The usual issue is that most of the UPS's the average person will come across cheaply are not very large or powerful. People who work in electronics/IT or other fields where they come across larger ones cheaply, can take advantage and make something powerful for cheaper than buying a new AIO inverter.

But if the large ones were THAT common and easy to come across in workable form, everyone would have one.. for most people buying the new AIO is much much easier and more user-friendly and easier to get forum explanation/support for.
Click to expand...
Bingo.
 
OffGridInTheCity said:
I'm with you and started off this way 4 years ago. If I could have found a whole-house UPS for $ I could afford....

But there are a couple of other issues for easy solar adoption....
1) For a 'solar system' you need a way to convert PV -> battery charging (e.g. charge controller) and of course the AOI have it built in.
2) The battery charging in an APC is pretty low amps / slow - too slow for a timely recharge if you did have to use it frequently for power outages.

But as I showed above in pics - I love these APCs.... some of mine are nearing 20years and still working!

I have an individual component based, off-grid system (with grid-assist) with several ATSs to auto-switch between inverter power and grid. I use a pair of APC 3000s to power a set of sockets thru the house to smooth the twice-daily solar/grid ATS switching for sensitive equipment such as computers/monitors, network equipment, TVs/Tivo, K-Cup(timer settings), etc...
View attachment 114212
The front one was converted to 14s12p you see above. These days, I'm working on a 14s50p (130ah@48v) external battery from my left over powerwall cells to power both :)
Click to expand...
yes, I dug into mine to see what kind of charger it had. It has two settings 1.4 A or 3.8 A at about 56V. Hee hee, that would take a while to charge up any battery let alone my 143kWh. It's good for up to 50,000' and 122F or 50C. It's as heavy as one of my 12K's!
 
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I've been playing around with using a lithium bank on a UPS as well. My solution to the wrong charging profile and low current is to disable charging from the UPS, and use a dedicated lithium charger instead.

Also, every UPS I have gets HOT. Like 150°f at 50% load. I added a fan to the one on my computer.
 
I'm interested in this as well. Anyone have any recommendations for a cheap ups that can run for hours without melting and using 12v?
 
bob1 said:
Less than $200 and 220 watts max.
Click to expand...
So you are looking for 220W pure sine wave UPS that runs on 12V and want to use it as inverter to run it for long period of time for less than $200, correct?
How about buying 300W 12V pure sine wave, 12V battery with the correct capacity for the run time you need which will determine the price you will want to spend, and 12V charger?
BTW, I do not think you can find 220 ~ 300W PSW UPS.
PSW 300W 12V: https://www.amazon.com/Inverter-300Watt-Adapter-Converts-Smartphones/dp/B07MM145H7/ref=sr_1_4
 
hey @offthegridinthecity - I'm seriously considering doing a very similar build/mod. I've got access to an APC 2200 and another 1500, and I think what you've done is pretty much exactly what I need... something big enough to run my small server / laptop / networking stack, about 1/3 of my motivation is to stop replacing SLAs what feels like every month, a third is to substantially increase the runtime of the system, and a third is to have a new project to noodle on. Would you be willing to share the BOM of what various gear you used to do the conversion?
 
ljwobker said:
Would you be willing to share the BOM of what various gear you used to do the conversion?
Click to expand...
Sure, but can you be more specific? Are you talking the 48v APC 3000 or the 24v APC1500 or both?

Note: These days, you might be better of just buying some LifePo4(s) on Amazon to replace the SLAs... Maybe something like this to directly put them inside - https://www.amazon.com/XZNY-LiFePO4-Rechargeable-Suitable-Emergency/dp/B09QKD4G6L/ref=sr_1_4 - but check dimensions, this was just a quick search to illustrate The lead acids are typically 12v @ 17ah as I recall.

Otherwise - to start the conversation....
- I used Chargery BMS 16T and 8T respectively for my BMSs - but you might choose something different.
- I used 18650 cells in DIY battery build.
- On the APC 3000 I rewired the Anderson connectors to have a single + / - to run to the 48v battery.
- Used a Gigivac 12v coil relay with hi amps because I had one laying around and because Chargery needs an external relay - a different BMS might not need that. Here's another relay I use with Chargery - https://www.amazon.com/gp/product/B0122Z6VZS/ref=ppx_yo_dt_b_search_asin_titleie=UTF8&psc=1
- etc...
 
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I was thinking about the -3000 as I think I have a line on an old chassis without SLA's in it already. I thought I read that you couldn't just drop in LiFePo's without the charging logic from the UPS screwing things up. Is that not the case? If it's truly drop in, I might just rig up a 2p string to get the runtime I need...
 
ljwobker said:
I was thinking about the -3000 as I think I have a line on an old chassis without SLA's in it already. I thought I read that you couldn't just drop in LiFePo's without the charging logic from the UPS screwing things up. Is that not the case? If it's truly drop in, I might just rig up a 2p string to get the runtime I need...
Click to expand...
My APC UPSs all charge to 27.6v (+/- just a bit) per 24v 'battery'. This is both the APC 1500 and the 3000s.
If you do the lifepo4 voltage math: 3.45v per lifepo4-cell * 4 cells = 13.8v per battery * 2 batteries = 27.6v. This is perfectly matched voltage wise.
Charge wise: APCs have gentle charging as in 2-3a (50-75w) on APC 1500 - e.g. 25-38w per 12v battery - which is very gentle and again I don't see a problem.
 
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