24V LiFePO4 pack for UPS - recommendations?

[danb35]

For background, see this thread:

LiFePO4 for UPS battery replacement?

tl;dr: I'm getting annoyed with the SLA batteries in my UPSs, and thinking about replacing them with LiFePO4 packs (either preassembled or DIY), mainly for increased life span. I have two UPSs that are in need of batteries. For those who aren't familiar, UPSs typically use packs of 12V, 5-9Ah...

diysolarforum.com

tl;dr: I'm wanting to assemble a 24v LiFePO4 battery pack of around 20 Ah capacity to replace SLA batteries in a UPS.

I realize this is quite a bit lower capacity than most folks around here are interested in, but I'm not interested in powering a RV or home, but rather some network equipment. I'm getting tired of wearing out SLA battery packs on my UPS, and wanting to replace them with LiFePO4. Form factor isn't really important (i.e., I'm not expecting to fit them in the same space as the SLA batteries), though "compact" is always nice. Low-temperature protection isn't an issue--these will be indoors in a climate-controlled room. Super-rugged construction isn't particularly important for the same reason, though I'll probably build or 3D-print a battery box to keep everything together. Overcharge/overdischarge protection is important--UPSs aren't known for treating batteries well. Common port is important; the UPS would charge and load through the same connection.

At full rated output (1500 VA), and ignoring losses in the inverter, the UPS would be drawing 62.5 amps from the batteries. However, with the equipment currently connected (which I don't expect to change significantly), it's only at about 10% capacity.

Right now, I'm thinking a 8S 80A Daly BMS and maybe these cells:

98.0US $ |lifepo4 3.2v 20Ah lithium battery Lithium iron phosphate deep cycle for DIY 36V 48V 72V 40AH scooter bike motorcycle inverter|Rechargeable Batteries| - AliExpress

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But I'm open to suggestions on either part, especially the cells. What haven't I thought of?

 

[morth]

There's other detailed threads on this topic, in short, yes it can be done, but there are things to consider. One of the big ones is cooling of the inverter in the UPS. They are designed to run for as long as the SLA batteries that come in them can run and not much longer. Adding extra AH may not get you the extended runtime due to temp issues in the UPS. The charge voltage profile is not optimal for LiFePO4. Yes it will work, but there will be some tweaking needed to get the profile a bit closer to what LiFePO4 would be happy with ( and not all UPS units allow you to change these settings). Another part is the charging rate after use. Some UPS units take 6-8 hours to recharge, so not exactly fast.

I played around with doing this on a 1000VA UPS and some Headway cells and came to the conclusion that I would rather just use inverter that can run 24/7 and sort out a scheme to turn the charger on and off based on battery voltage.

 

[danb35]

One of the big ones is cooling of the inverter in the UPS.

That's a point I hadn't considered, but running it at ~10% of its rated capacity would seem likely to mitigate this concern, wouldn't it?

The charge voltage profile is not optimal for LiFePO4.

That's something I still need to check on with this unit--probing the charging connector is proving a little tricky. My big (48V) unit floats at 54.8v, which is very close to Will's recommended value (54.4v)--and I'm hoping to do the same thing with that one once I'm confident it's working with the small one.

Another part is the charging rate after use. Some UPS units take 6-8 hours to recharge, so not exactly fast.

Is there a non-obvious reason this is a problem? Obviously it'd be nice for it to charge faster (and I recognize that if I'm adding Ah, I'm lengthening charging time too), but as far as I'm concerned that's just a matter of convenience. Will it harm the cells to charge at too low of a rate?

 

[morth]

Running the UPS at lower capacity will help, the question is how for how long? The inverters in the UPS units are not designed for long run times, so once you are past a few hours, it's any one's guess as to how long it will run before it over heats. Maybe you could run indefinitely with a light load. I would want to try it to really know before I would trust it in a real power outage.

The charging aspect has 2 parts. 1. The charge rate. Charging at a low rate doesn't damage the battery, it just takes longer. In a scenario where solar charging isn't working and I have to fire up the generator, I want to charge as fast as the packs will safely allow so I don't have to run the generator for 8 hours. Part 2 is that the UPS charger isn't set up for LiFePO4. It's doesn't really stop charging, so the battery is always cooking. That's ok for SLA, but not long term with LiFePO4. I've had 2 1500VA APC units fail in charging, one resulting in venting of the SLA and creating enough heat that I needed oven mitts to throw the whole UPS outside.

 

[danb35]

I'm not looking to run for days at a time, but a few hours would be nice. Once again, though, this is making it sound like my "big" UPS (a APC 3kVA unit) would be more suitable for this project--it's designed to accept up to 200 Ah of 48V, 20Ah battery packs as add-ons, so it will take a lot more than I intend to give it. It even uses standard Anderson SB connectors for them. But I was hoping to do the proof of concept on the smaller unit, as it takes about 25% as much battery capacity (so I'm out a lot less money if I can't make it work).

But supposing the UPS itself to be a black box, and further supposing that I can't that I can't adjust its charging profile, it sounds like I need to better characterize that charging profile. Is it enough to measure voltage over time while charging, or should I be measuring current as well?

 

[danb35]

I'm waiting on a couple of parts to be able to effectively monitor voltage (and current) over time, to get a better idea of what the UPS is doing when charging its batteries. But while I'm waiting on those, any suggestions for cells in the 20Ah range? Most of the discussion I see here is for 100+Ah.

 

[danb35]

I was able to run a test yesterday, and though the data logger didn't work quite as expected, I think I got some useful data points:

• The float voltage is 26.76V. At that voltage, there's (at most) 2-3mA going into the battery, but I don't think I trust the resolution of my clamp-on ammeter that much.
• Low voltage cutout, at least during a capacity test, was 22.3V. At that time, the unit reported 4 minutes' runtime remaining, so any connected devices would already be shut down.
• When on battery, my devices draw 6A from the battery.
• Recharging the battery begins with a 430mA constant-current charge. The data logger didn't, so I'm not able to tell where it changed modes, but it finished back at the 26.76 it started with.

The limit voltages look fine--the float voltage is 3.35/cell; the low voltage cutout is 2.79V/cell. That might not charge the pack quite to 100%, but that's good for cell life, right? And the low-voltage limit is still high enough to prevent excessive discharge. Yes, completely charging the pack is going to take a couple of days at this rate, but that's OK even if not ideal.

 

[mightybara]

Hello Everyone and danb35,

I'm looking at doing the exact same thing. I have an APC UPS 3000 and I want to move away from 4 deal SLA batteries to LiFePO4 batteries as a replacement.

This is a picture of my setup in the Office.



I know, its not pretty.. But it works for a few minutes as all 4 batteries are on the way out.

Looking at putting 4 x 12v LiFePO4 units each with their own BMS built in.

Should it work?

 

[BoloMKXXVIII]

There are a few portable power stations with pass through power.
Also, have you considered LTO cells?

 

[mightybara]

There are a few portable power stations with pass through power.
Also, have you considered LTO cells?

Mind sharing an example of such?

I'm considering getting 4 of these --> https://www.aliexpress.com/item/4000242035884.html
Getting good reviews from customers...

Yes considered LTO, but its 1) Way too Large and 2) A lot more expensive than LiFePO4..

 

[BoloMKXXVIII]

Mind sharing an example of such?

SUAOKI UPS Power Station, G1000
Ecoflow Delta

 

[mightybara]

SUAOKI UPS Power Station, G1000
Ecoflow Delta

Thanks for your reply. Yes the Ecoflow Delta is a high on my wishlist.. I'd still like to make use of this 3000VA UPS though. So I'm gonna do some more research and get the 4x LiFePO4 batteries..

My goal here is to run my office when the power cuts for at most 9 hours. My office setup takes about 400W max.

Lead acid not getting me anywhere past 4 hrs... So its either LTO or LiFePO4 for it for a more usable capacity.

Regards,

 

[zorlig]

Years ago I hooked a gel 12v car battery up to one of the lower watt UPSs. It worked for a few years but when used it over heated.

I started more recently by replacing batteries on the 24v cheap UPS's, with lead acid it was limited more by the batteries than the inverter so I got better performance with lifepo4. But it was a square wave and also couldn't start the chest freezer.

Now I use a 72v online UPS, so it is built to go forever without overheating anything. It has good burst capacity, no trouble starting anything and pure sine wave.

After playing with them all I wouldn't waste much time with anything that wasn't pure sine and higher voltage.

And unless you wanted to make the batteries external, I would just stick with the standard agm and not try to squeeze a lithium battery pack inside. It's just not worth the time and benefit unless you are looking for some serious backup time from an external battery.

 

[mightybara]

Years ago I hooked a gel 12v car battery up to one of the lower watt UPSs. It worked for a few years but when used it over heated.

I started more recently by replacing batteries on the 24v cheap UPS's, with lead acid it was limited more by the batteries than the inverter so I got better performance with lifepo4. But it was a square wave and also couldn't start the chest freezer.

Now I use a 72v online UPS, so it is built to go forever without overheating anything. It has good burst capacity, no trouble starting anything and pure sine wave.

After playing with them all I wouldn't waste much time with anything that wasn't pure sine and higher voltage.

And unless you wanted to make the batteries external, I would just stick with the standard agm and not try to squeeze a lithium battery pack inside. It's just not worth the time and benefit unless you are looking for some serious backup time from an external battery.

Thanks zorlig,

I ended up with 4 x 150AH AGM FIAMM Batteries and its working excellent for my application. Surely don't need to utilize 100% of every battery, so its overcatered to manage for the watts/hours I need.

Any UPS I get HAS to be Pure Sine Wave... nothing else... older APC units are easy to modify!... Looking at getting a 1500VA for another part of the house and run 4 AGM batteries to it as well.

Regards