repurposing a UPS (APC 1500) and adding a big LiFePo4 battery to it... will it charge without issues?

[ljwobker]

My use case is something along the lines of: "I want to be able to have a KWh or two available as backup power, with a load that won't exceed more than 400W sustained with peaks of 700W for a couple of seconds." If I really really stretch my brain out, I guess I could see maybe needing to run an average load of 300W for several hours -- let's call it 8 hours which gets me to 2.5KwH (the actual capacity isn't my question unless it matters in the end - keep reading I guess)

I *think* that the old APC Smart-UPS SMC1500 that someone was kind enough to give me will meet this requirement -- the data sheet (linked below) says it'll push 900W/1400VA which is well above what I need.

I've also got a local supplier who has a bunch of used Valence batteries, in both the 12v and 24v flavors. The UPS is a 24v model so I think I could *either* 2s or 2s2p of the 12v varieties, or do 2p/4p of the 24v ones. In any case, let's assume for this discussion that I could get my hands on "several KWH of 24v battery capacity".

Now, the questions...
1) If I do end up with a bunch of capacity, will the charger that's in the UPS be able to charge these guys? I should note that I'm not asking if it can charge them QUICKLY, I'm just trying to understand if there's some threshold at which point the battery bank is "just too big" for the charger to work at all, or if there's some other non-linear behavior I need to consider. The UPS spec sheet says that the rated charge power is 73W, so needless to say this might take a LONG time to recharge, but that's not a concern. It's only a concern if the UPS won't be able to charge them at all, in which case I'd be limited to a single discharge cycle (oops).

2) are there concerns with the UPS itself running this load for an extended period of time (several hours, maybe even a day)? I know that the el-cheapo UPSs are not designed to run for hours on end, but I **think** that the SMC1500 is pretty robust and is designed to handle heavier, more sustained use... though I have no way to confirm this. Thoughts on this part would be greatly appreciated.

I also have access to a slightly different UPS, an APC SUA1500... the specs on it look very slightly higher in terms of capacity and charger, etc -- but I think they're more or less the same design at the guts.

https://www.apc.com/us/en/product/download-pdf/SMC1500
https://www.celltech.se/fileadmin/u...sbara/Valence_Modules/XP_Module_Datasheet.pdf
https://www.apc.com/us/en/product/download-pdf/SUA1500

 

[Delmar]

I cannot answer the charging question because I have yet to convert to LiPo.

Simply as an UPSverter, neither is rated for continuous operation. The better choice is the SUA that is designed with more battery capacity and runtime than the SMC, so should have better cooling. Running only 400W sustained will help. I’d plan on adding a cooling fan that blows directly at the H-bridge.

The holy grail is the bulletproof “XL-series” setup for for external batteries and rated for continuous operation. Even when they charge the cooling fan is on. They are easy to spot by the rear battery connector. Below is my SUA1000XL.

 

[HighDesertOffgrid]

I have a 1.32kwh 18650 bank connected to an APC1000 (salvaged from a dumpster). It will run a mini fridge (only tested for and hour or so). I use it as a backup to my backup running small (25w) hydroponic pumps.
It charges...slowly.
At 33% load it will probably be fine, but, as you stated, you will be using it outside it's intended purpose so you can guess the only way to know if it will work is to try it.

 

[ljwobker]

Anyone know if there are spectacular failure modes I need to worry about? "It quit working" is a lot less of a concern than "it burned the house down..."

 

[pollenface]

I'd probably use a seperate charger with my own parameters.

My UPS holds the batteries at 27.6v 365 days/year

 

[TheQuickFox]

If you have the right batteries, it will work fine. You will need LiFePO4 batteries with an advanced BMS that allows for using the batteries as drop-in replacement for traditional AGM/SLA batteries and allows for a series connection. Do not buy cheap batteries off Amazon/AliExpress etc. I used quality batteries from the French manufacturer PowerTech Europe. I can fully discharge and recharge them with my APC Smart-UPS 1500 SUA1500i.

Batteries are not cheap. But I got around 2 hours of runtime with 300 watt load. This is around 4x the runtime of SLA batteries. The battery meter of the UPS will be off because it will not expect a quadrupled battery capacity. UPS will stay on and continue to deliver power but if you use software to shut down your PC when the UPS SoC reads 10%, it will shut down your PC too early.

See the video description.

 

[davidwf]

any reason why we cant use an external Lifepo4 charger for each battery?