Replacing UPS lead acid batt w LiFePO4

[dc443]

Yo what if we have a smart LFP BMS that isnt trash, and does something intelligent with SLA deployments. I mean LFPs are also supposed to be kept topped up at 100% and it does not degrade them as badly. I think the key here could be that the BMS ought to be built and programmed to readily accept input trickle current and dissipate it comfortably without overheating itself constantly or some BS like that. I replaced my SLAs in my crappy little UPS unit recently. I think the problem is the cost. They're heavy as hell but still dirt cheap.

Hopefully there are enough people who will take "junk" UPSs and give them a second lease on life by just battery swapping but... Yeah im gonna look for LFP cells next time around

 

[RCinFLA]

The LFP cathode is the most rugged cathode material of all lithium-ion chemistries. LFP cells tolerate maintaining full charge much better than other Li-Ion chemistries.

There are three primary factors in maintaining full charge that causes degradation.

In order of severity of lithium-ion cell degradation:
1) Positive cathode lattice structural weakness due to lithium-ions being removed from cathode lattice.
2) Negative graphite anode expansion stress (by about 11-13%) due to being stuffed with lithium-ions.
3) higher cell voltage potential accelerating the decomposition of electrolyte.

Cathode degradation

The cathode lattice is a three-dimensional array. The issue for most non-LFP lithium-ion battery cathodes is support in the 'z' axis of the lattice. In many lithium-ion cathodes the lithium is the primary 'z' axis support so when fully charged and most of the lithium has left the cathode, the lattice layers of cathode can collapse permanently damaging the lattice, so it does not re-accept lithium ions when discharging cell.

In LFP cathodes, the iron provides a vertical 'z' axis support so there is less damage when full charging removes most of the lithium from the positive cathode lattice.

Negative graphite anode expansion

The roughly 12% volume expansion of graphite anode when fully charged puts stress fractures in the Solid Electrolyte Interface (SEI) protective shell around the graphite.

The SEI layer is to retard electrons in graphite anode from escaping into the electrolyte which will cause the electrolyte to breakdown. It is initially grown by manufacturing process and is continually repaired and grows in thickness during cell usage. Fractures in SEI layer will be repaired on subsequent recharge cycles, but this consumes some of the free lithium in cell reducing cell capacity. It also thickens the SEI layer which increases cell impedance. It is the primary reason for capacity fade and increase in cell impedance over use cycles of battery. At higher discharge current the rise in cell impedance may be the limiting use factor causing too much cell voltage slump.

Electrolyte decomposition

Electrolyte decomposition happens when electrons escape the electrodes into the electrolyte. The graphite anode SEI protective layer retards this but it is not 100% effective, particularly when cracks in SEI shell happen.

The greater the cell voltage, the more likely electrons will escape the electrodes. Overcharging causes the electron escape from graphite anode to drastically increase. Electrolyte is fairly stable between 0.1 and 4.3v cell voltage at 25 degs C. The degradation rate increases at greater cell temperature. These numbers are not a hard line and is just the points when degradation greatly accelerates. Since all lithium-ion chemistries use pretty much the same electrolyte materials, cathode chemistries that provide higher cell voltage have less high-end voltage safety margin on electrolyte degradation.

For a LFP cell, 3.43v is the number for being fully charged when a cell has no current flow and has reached equilibrium rested condition. Since very small cell current creates a few mV of overpotential the number is often stated as 3.45vdc. Some electrode additives and minor contaminates of electrode material can cause this number to have some small variance of a few mV's.

 

[nifty-stuff.com]

Hopefully there are enough people who will take "junk" UPSs and give them a second lease on life by just battery swapping but... Yeah im gonna look for LFP cells next time around

All this discussion got me in the mood to search craigslist for UPS, so I'm spending a few minutes on Thanksgiving to pick up this unit for $25 (in the listing he even said "could use new SLA batteries":

 

[Delmar]

All this discussion got me in the mood to search craigslist for UPS, so I'm spending a few minutes on Thanksgiving to pick up this unit for $25 (in the listing he even said "could use new SLA batteries":

I think we have created a monster with this thread. Please everyone research the model number of the UPS before throwing expensive batteries at it. The BX1500LCD is a stepped (non-sine) wave UPS. Go back and read the OP, and the printing on the underside of the UPS.

Additionally the H-bridge is designed to overheat just as the standard battery runs out of power. Adding expanded batteries will not extend the runtime. Even if you run at a fraction of the rated power, the internal timer may ignore the battery voltage and shutdown the UPS anyway.

As stated in post #12, I recommend the APC “XL” models that are bulletproof, pure sine and rated for continuous operation. Easy to identify by the external battery connector.

 

[nifty-stuff.com]

Thanks for the feedback @Delmar !

I also have a CyberPower 825AVR and have tested the charge voltage on one of my small LFP batteries. At "rest" (float?) the voltage reads 13.54v. I've had the battery hooked up to it for about a year and it's worked well without any noticeable problems.

The XS 1500 is definitely a different animal completely.

As it relates to float-voltage, etc. that are within the "safe" ranges of the LFP BMS, but also "not recommended" with a higher float value, etc:

I'm curious, when people talk about this being "bad" for LFP batteries, is there a way to quantify this damage / risk? For example, does this take a healthy battery with 3,000 cycle-life and reduce it to 2,000, 1,000, or 500? Meaning, it would be great to be able to quantify the "damage / risk" because in some conditions it might be "worth it", to run in that "less-than-ideal" situation, no?

I'm definitely a noob with this stuff and REALLY love as much data as possible!

 

[Delmar]

I'm definitely a noob with this stuff and REALLY love as much data as possible!

Experience gained = equipment ruined. You purchased an inexpensive UPS to learn from before purchasing a more expensive and quality one. Just make sure you don’t destroy the LFP or anything powered by the UPS.

CyberPower 825AVR also is Simulated Sine Wave

I already ruined one nice pure-sine APC 3000XL by overvoltage from solar panels, that is now a cadaver for the operating one. And yup I did learn...

 

[nifty-stuff.com]

Just make sure you don’t destroy the LFP or anything powered by the UPS.

I'd be shocked (see what I did there? ) if plugging an LFP battery with it's own BMS into a UPS would do any harm to anything downstream. I mean, I could be totally wrong, but I imagine the BMS and/or protections in the UPS (assuming both are working as designed) would kick in before things got bad enough to cause problems.

I figure worst-case (again, assuming BMS and UPS work as designed) is that the LFP battery lifespan is truncated, no?

 

[nifty-stuff.com]

All this discussion got me in the mood to search craigslist for UPS, so I'm spending a few minutes on Thanksgiving to pick up this unit for $25 (in the listing he even said "could use new SLA batteries":

View attachment 121631

I'm powering this APC Back-UPS XS1500 with two "FULLRIVER 60-CELL LIFEPO4 PACK 18650 12.8V 22500MAH 288WH" batteries.

The UPS keeps the LFP batteries at 26.5 volts.

I did a test-run with it running my PC setup.

The draw was 70 watts average

It ran for 130 minutes until I stopped the experiment when the battery voltage started dropping relatively quickly and got down to 24.8 (before the BMS kicked-in).

So, nothing blew-up (yet). LOL!

 

[ejfluhr]

Did you change the float voltage to 26.5v or is that its default? I have an APC Smart UPS that floats at 27.3v. From the info I've found, that could be a tad too high for 8s LiFePO4. 26.5v would be much more ideal.

 

[Supervstech]

I’m preparing to drop a pair of 7Ah LFP’s into an XS 1000 and use it for a tankless water heater backup device…
I will post results once it’s setup.

 

[nifty-stuff.com]

Did you change the float voltage to 26.5v or is that its default? I have an APC Smart UPS that floats at 27.3v. From the info I've found, that could be a tad too high for 8s LiFePO4. 26.5v would be much more ideal.

I think I just got "lucky"???

When I first hooked the two batteries up in series, I put a meter on them and watched them VERY VERY closely over the next few hours to see what the charge-voltage settled-in at. I don't think I've ever seen them get higher than 26.5 V.

(Since I know UPS's will usually be set to charge SLA higher, I was feeling a little nervous that my meter might be off, so I just double-checked with a multi-meter and got 26.4v)

 

[ejfluhr]

I bought 2 used K2 Energy K2B24V10EB from Battery Hookup to transplant in my APC SMT1000. I've connected one up and run a battery longevity test that lasted for about 30min with 270W load. Next I need some safe, permanent installation of the batteries into the case. 2 batteries fit with a bit of room to spare, but I need to anchor them to prevent movement. Once that is done and it's all working, I'm going to try lowering the float slightly below 27v for better longevity.

 

[Scoodood]

Hi,

I have been following your discussion, but I am still unsure if I should replace my UPS battery with the LiFePO4 battery. Some of you talked about floating voltage, maximum draw, ...etc. But I am not sure what to look for in the product specification. Can someone please help me out?

I am using CyberPower UPS CST135XLU, and its battery is also dying. The batter dimension listed on the CyberPower webpage is incorrect. I measured mine, and they matched the following alternatives, which are very close to 5.9(L) x 2.6(W) x 3.9(H):

1. TEMGOT LiFePO4 12V/7Ah
2. Nermak LiFePO4 12V/10Ah Deep Cycle
3. CyberPower RB1270XC2 Lead Acid 12V/7Ah

My UPS is connected to a 27" iMac, a Win11 Intel i7 PC, and a router. The UPS had about 5 minutes of juice when the power was out, which was good enough for me. After 4 years of use, the battery is finally dead.

I know for sure option 3 will work, but does anyone know if option 1 or 2 would be a better choice? Or should I just buy a new UPS?
If you have better choices than the above list, please suggest them.

Thanks

 

[PeSCaLo]

Hi,

I have been following your discussion, but I am still unsure if I should replace my UPS battery with the LiFePO4 battery. Some of you talked about floating voltage, maximum draw, ...etc. But I am not sure what to look for in the product specification. Can someone please help me out?

I am using CyberPower UPS CST135XLU, and its battery is also dying. The batter dimension listed on the CyberPower webpage is incorrect. I measured mine, and they matched the following alternatives, which are very close to 5.9(L) x 2.6(W) x 3.9(H):

1. TEMGOT LiFePO4 12V/7Ah
2. Nermak LiFePO4 12V/10Ah Deep Cycle
3. CyberPower RB1270XC2 Lead Acid 12V/7Ah

My UPS is connected to a 27" iMac, a Win11 Intel i7 PC, and a router. The UPS had about 5 minutes of juice when the power was out, which was good enough for me. After 4 years of use, the battery is finally dead.

I know for sure option 3 will work, but does anyone know if option 1 or 2 would be a better choice? Or should I just buy a new UPS?
If you have better choices than the above list, please suggest them.

Thanks

Hello, the lifepo4 batteries you mentioned does not provide the 300-500 watt that you require for your 2 computers.

 

[Scoodood]

Hello, the lifepo4 batteries you mentioned does not provide the 300-500 watt that you require for your 2 computers.

Just wondering, how do you make sure a battery can handle 300-500 watts from their spec sheet?

 

[PeSCaLo]

Just wondering, how do you make sure a battery can handle 300-500 watts from their spec sheet?

Look for Maximum discharge. In watts or Amps.

Amps need to multiply by voltage= 12.8v in your case

 

[Scoodood]

Look for Maximum discharge. In watts or Amps.

Amps need to multiply by voltage= 12.8v in your case

Hi @nbtesh,

Please correct me if I am wrong. Based on the datasheet of the UPS and its battery, the UPS uses two 12V / 7Ah lead acid batteries. Theoretically, these batteries can deliver 24V/7Ah = 168Wh (in series) or 12V/14Ah = 168Wh (in parallel). Besides, the UPS can handle a max load
I am unsure if the batteries are connected in series or parallel, but I learned my calculation from here.

Question #1
Theoretically, at an 810W max load, the required current is
810W / 12V = 67.5A (battery in parallel)
810W / 24V = 33.75A (battery in series)

Then the battery should be able to hold
7Ah / 67.5A = 6.2 minutes (battery in parallel)
7Ah / 33.75 = 12 minutes (battery in series)
Am I right?

Question #2
If the above assumption is correct, then the lead acid battery should be able to handle 2 PCs that draw less than 500W, right?

Question #3
If questions 1 and 2 are correct, why can't the LifePO4 battery handle a similar load (<500W)?

Question #4
The Nermak LiFePO4 12V/10Ah battery has a Continue Discharge Current = 10A and Peak Discharge Current = 20A. What does it mean? How does it apply to the calculation in question #1?

Thanks

 

[PeSCaLo]

Question #1
Theoretically, at an 810W max load, the required current is
810W / 12V = 67.5A (battery in parallel)
810W / 24V = 33.75A (battery in series)

Then the battery should be able to hold
7Ah / 67.5A = 6.2 minutes (battery in parallel)
7Ah / 33.75 = 12 minutes (battery in series)
Am I right?

The max capacity of your UPS is 810 watts , the max capacity of your lead acid sealed batteries is 500 W each, so it’s more than enough. usually that UPS uses two batteries connected is series for 24 V please check the technical sheet for your UPS to confirm.

Question #2
If the above assumption is correct, then the lead acid battery should be able to handle 2 PCs that draw less than 500W, right?

correct, ups max draw is 810watt.

Question #3
If questions 1 and 2 are correct, why can't the LifePO4 battery handle a similar load (<500W)?

those lifepo4 batteries you sent are low discharge. <200watt

Question #4
The Nermak LiFePO4 12V/10Ah battery has a Continue Discharge Current = 10A and Peak Discharge Current = 20A. What does it mean? How does it apply to the calculation in question #1?

discharge=10A*12.8= 128watt each.
peak discharge =20A*12.8=256watt each for how many seconds?

 

[Supervstech]

Just wondering, how do you make sure a battery can handle 300-500 watts from their spec sheet?

Most data sheets in battery sales page will list continuous discharge spec, and max discharge spec with a duration, like 30 seconds or so.
A 7Ah LFP battery is going to have around a 7A continuous discharge spec, and around 14A max discharge for a few seconds…

The SLA battery can output around 40A continuous, and 80A max…
So, you would need 8 or so LFP 7Ah to meet the 12V discharge spec… or 4 ish to meet the 24V spec.

 

[Scoodood]

Hi @nbtesh, thanks for your reply again.

discharge=10A*12.8= 128watt each.
peak discharge =20A*12.8=256watt each for how many seconds?

The Nermak datasheet says the battery can only handle 20A for less than 5 seconds.

Question #5
What would happen if someone pushed beyond that limit by accident?

The max capacity of your UPS is 810 watts , the max capacity of your lead acid sealed batteries is 500 W each, so it’s more than enough. usually that UPS uses two batteries connected is series for 24 V please check the technical sheet for your UPS to confirm.

500W means that the battery can discharge at 500W / 12V = 42A. @Supervstech mentioned that a typical SLA could discharge from 40A to 80A max. I think I have begun to see it now. Besides, the UPS datasheet mentioned that the battery should not exceed 24V / 9Ah for safety reasons (see snapshot below). It didn't explicitly say anything about connection type. Still, I believe you are correct that they should be in series because our previous calculation showed that the high voltage design also makes better sense (longer runtime and smaller wire requirement).



# Question 6
The UPS datasheet mentioned that the replacement battery should not exceed 24V / 9Ah. I thought a bigger ampere-hour (Ah) battery meant a bigger capacity (or longer runtime). I thought the UPS should have some built-in regulator to limit the voltage and current level. What kind of energy hazard does it mean?