Replacing UPS lead acid batt w LiFePO4

[Scoodood]

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.

Now I get it. Although this SLA battery datasheet didn't mention the current discharge, 40A - 80A discharge is the standard range for an SLA battery. On the other hand, an LFP battery is typically capped around 7A. Everything makes sense now. Thanks a lot, @Supervstech

 

[afro-solar]

There has been some discussion in the past about putting in a LiFePO4 battery in a consumer grade UPS. There were no real conclusions except everyone sort of agreed that it would work, but no one has actually done it.

Well, I'm going to do it tomorrow. I have a small APC UPS that has a dying lead acid battery. I've ordered a Chins 12v 8Ah LiFePO4 battery that will be delivered tomorrow. The current battery is kind of an odd size at 6" x 3.75" x 2". It's that 2" which is kind of odd. It rather thin at 2". But when I opened up the APC UPS, there was a spacer inside that is about 1/2" thick. So I just used some tools and cut off the plastic spacer so that it should fit the Chins battery. Chins specs says 5.96 x 3.72 x 2.55 inches. Everything else should be plug and play. See attached pictures. I'll update this thread after I install it tomorrow.

View attachment 81714View attachment 81715View attachment 81716View attachment 81717

It should work fine. The main issue is having a voltage balancer if the UPS needs 24V.

 

[afro-solar]

What's the discharge rating of the battery? They're usually 1C, and that's only 8A. I would be concerned that 8A can't power the UPS or at least not if anything is connected to it.

With a potential output power of 257W, it could easily pull 20A.

For UPS's, the capacity tends to be between 7Ah - 9Ah. Not all 12V batteries are the same in terms of capacity.

 

[afro-solar]

LOL Yep. I have a label maker just for that!
I was reading up the specs for this UPS. It says that it charges the battery at 2 watts. It also says that when installing the UPS for the first time to let it charge the battery for 16 hours. So I assume the original lead acid battery is somewhere around 32 Whr. I should be getting at least 2x the backup time with the LiFePO4 battery in case of a black out. I'm pretty happy about that. The new battery is happily settling in the UPS at 13.3V right now. The manual does not say anything about an absorb cycle. I have no idea if it will do it or not. If I had to guess, probably not. This is a cheapo UPS and I don't expect it has anything that would help prolong its life.

It will take forever to charge, but also forever to discharge.

 

[afro-solar]

Nice.

To be clear, you do need a BMS. Cells will drift even when being held at a constant voltage as they all self-discharge at different rates. You need the BMS to compensate unless your cells are truly perfectly matched.

Unless you are building the pack yourself, all Li-Ion batteries will ship with a BMS as standard. What would be optional is Bluetooth monitoring.

 

[sunshine_eggo]

For UPS's, the capacity tends to be between 7Ah - 9Ah. Not all 12V batteries are the same in terms of capacity.

Not relevant. Lead acid batteries can deliver well over 1C charge rates as there is no BMS limiting the current. A LFP battery that has a discharge limit will not deliver more than this amount for a significant period of time because the BMS will cut it to 0A.

 

[afro-solar]

Sounds Great.
I make my UPS Batteries without BMS. Since the UPS acts as a BMS, controls the charging max and min voltages. >10 working without a glich..

This is fine for LA, not so much for Li-Ion packs.

 

[afro-solar]

I just completed my upgrade to 304 AH batteries. I used a 100A BMS just in case, although the max load is about 50A. Under normal conditions, a BMS may never be needed...but you never know...and it can't hurt (except my checkbook.)

For Li-Ion, you always need a BMS, mainly because the inverter/UPS is unable to accurately determine cell or pack voltage.

 

[afro-solar]

Exactly what I meant.
To prevent the 2V you mentioned: Use High Quality, Reputable Cells. Top Balance in parallel. Then Connect in Series.
Process is everything

I wouldn't chance it. Use a BMS with Li-Ion cells, if for nothing else, to prevent a fire.

 

[afro-solar]

I keep on wondering why APC and other main stream UPS makers have not adopted new designs centered around using LFP batteries. Most of my UPS systems are older 1500W APC units, so putting in 4x LFP batteries is not going to work.

UPS vendors make their money from the UPS circuitry, not the battery. So they have no incentive to focus on battery longevity. That is a problem left to the user once the UPS has left the shop shelf.

 

[afro-solar]

I was thinking that it could be revenue from battery replacement but I really wonder how many people use OEM battery replacements. I have never purchased a single one in 20+ years! Several cheaper brands last just as long if you get them factory fresh. As for the price difference, the gap is really not that big between LFP and AGM.

Most people would call the OEM for battery replacements. The people on this forum are not most people.

 

[afro-solar]

I work for a large high tech company where there are many of these APC SmartUPSs around. The SLA batteries all last about 3 years and then fail. They bulge out so that they are hard to remove. I have see a trend where people just give up and throw the entire UPS away...and do not get a replacement because it is too much of a hassle (that is how I got mine!) I would think APC would redesign these to use LiFePO4 batteries and tout them to corporate customers that want long term reliability.

Not worth the hassle with their brand name. They make money anyway.

 

[afro-solar]

ooooh. I would love to do this.
I've have been thinking about it for months and finally decided to search here if it was possible.
I've got 3 UPS' and don't get enough minutes during blackouts so I've been thinking of extending or replacing with LIFEPO4 cells and bms.
Is there a DIY smart UPS/BMS board or even something that can be managed with a raspberry pi ?

I have several UPS's I'd like to change to Li-Ion, but they all require 2x 12V batteries, which makes it difficult to design a voltage balancer as part of the drop-in replacement.

 

[afro-solar]

Quoted from:

The Weak Link - The BMS​

"We now have a few years of experience with lithium-ion batteries, and what is becoming clear is that while the LiFePO4 cells hold up very well, that is not the case with the Battery Management System (BMS). Overall the number of prematurely failed batteries is small, but with 10,000+ batteries sold it is clear that in 99% of cases it is the BMS that fails, turning the battery into an expensive piece of gender-neutral-cave decoration!
While we very much advocate for using batteries with a build-in BMS (without one the battery would be unsafe and likely fail very quickly!), manufacturers struggle to make their BMS as bullet-proof as it should and needs to be. Surge currents due to the input capacitors of large inverters, motors, and air-conditioners can and at times will kill the BMS, rendering the battery useless.
At least one well-known battery manufacturer is now enforcing their warranty conditions to the letter, and that requires the use of an external current limiter when their batteries are used with large inverters (“large” being defined as 3,500 Watt and up). This leads to the ironical situation where the BMS is there to protect the battery, and now a current limiter gets connected to protect the BMS. What will be next to protect the current limiter…
Seeing how the BMS has become the weak link, manufacturers should really work (hard) on hardening that. Nothing is 100% bomb-proof, but there certainly is room for improvement! Another solution could be to acknowledge that the BMS is the weak link and make it so it can be replaced without too much effort, for example a gasketed lid on the battery that is removable with a few screws, and a BMS that has connectors and bolted lugs, so a repair shop can swap the board. It makes no sense to throw away a battery where 90% of the cost is in the cells, and 10% in the BMS, just because the BMS failed."

http://[URL]https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

[/URL]

This is where OEM's make the mark... the BMS. Both how rugged the BMS is, as well as the software to run it. Otherwise, the cells are pretty much the same.

 

[afro-solar]

Not relevant. Lead acid batteries can deliver well over 1C charge rates as there is no BMS limiting the current. A LFP battery that has a discharge limit will not deliver more than this amount for a significant period of time because the BMS will cut it to 0A.

I'm not talking about charge/discharge current. I'm talking about capacity.

 

[gnuorder]

I recommended my friend buy some 12v LFP batteries and I got 2 as well thinking they were drop in replacements because they were advertised as such. I didn't think much about it until I had a power outage and they shut down in seconds. That's when I realized the built in inverter was only rated for 10amps. I have one working as it only is used for a router, cable modem and switch, probably under 100 watts. The others I tried cut off with any load. From what I gather reading this thread that there is no drop in replacement yet and I would have to build a pack or have one built with a beefy 50+ amp BMS? That's not a problem for me but it might be for my friend. Is there a site that makes packs for this purpose? Regarding the BMS and trickle charge voltage, wouldn't the BMS cut off charging when the cells top out? Could we use a beefy diode or a few in parallel to keep the UPS from charging and use a proper LFP charger?

 

[ejfluhr]

The whole LFP-in-UPS is a tough challenge; there are ways to solve it but I haven't seen any "drop-in" solutions.

I've got an APC SmartUPS that can -barely- fit inside the case 2 x K2 Energy K2B24V10EB 8s LiFePO4 (so "24v native" rather than series 12v batteries), widely available on the used market. These have a theoretical output of 24A, so 2x=48A @ 24V which falls short of 1000W. I'm not running that draw which is OK with me as I'm just looking to insulate my NAS units from brownouts. MY APC "float" voltage is 27.2v, theoretically can be adjusted lower for better battery life but I haven't done that yet (I have the control cable though).

 

[DIYrich]

I recommended my friend buy some 12v LFP batteries and I got 2 as well thinking they were drop in replacements because they were advertised as such. I didn't think much about it until I had a power outage and they shut down in seconds. That's when I realized the built in inverter was only rated for 10amps. I have one working as it only is used for a router, cable modem and switch, probably under 100 watts. The others I tried cut off with any load. From what I gather reading this thread that there is no drop in replacement yet and I would have to build a pack or have one built with a beefy 50+ amp BMS? That's not a problem for me but it might be for my friend. Is there a site that makes packs for this purpose? Regarding the BMS and trickle charge voltage, wouldn't the BMS cut off charging when the cells top out? Could we use a beefy diode or a few in parallel to keep the UPS from charging and use a proper LFP charger?

Would this work? Ask the seller about charging them.

I have over 12 of these ~500Wh 12V lifepo4 batteries

The most important thing to know is they're only 20A max discharge (240 Watts) These were used in hospital medical carts I've tested them to get the exact capacity remaining I have about 12 of U1-45 (45Ah originally) that still have 40 to 41Ah left (90%) Asking $200 each Various others...

diysolarforum.com

 

[DIYrich]

I know having batteries and UPS are redundant, but there are times when the Sol-Ark cuts power and reboots. I was originally thinking of getting 12v and 5v regulators to run the modem/router/wifi, then this thread. I have a bunch of old APC's with worn out batteries.

Any thoughts on a 48v to 12v regulator/converter (30 amp or larger), and wiring that into the battery terminals of the APC?

This also solves the problem when I am playing with the batteries. The APC automatically switches between AC and DC power.

 

[Supervstech]

I know having batteries and UPS are redundant, but there are times when the Sol-Ark cuts power and reboots. I was originally thinking of getting 12v and 5v regulators to run the modem/router/wifi, then this thread. I have a bunch of old APC's with worn out batteries.

Any thoughts on a 48v to 12v regulator/converter (30 amp or larger), and wiring that into the battery terminals of the APC?

This also solves the problem when I am playing with the batteries. The APC automatically switches between AC and DC power.

I don’t think the buck converter will play nice with being charged…