Is LIFEPO4 an alternative to AGMs in float service applications?

[jcc]

This is my first post. To all members of this forum, I want to say thank you, for the invaluable knowledge this forum contains.

Some background information. Currently using a 24v 2000 watts inverter/charger with 375 AH AGM batteries. Used only for backup during grid outages. The batteries are 3 years old (warranty expired). Have not tested its current capacity. But at 1 year they had 91% capacity (using bench load/battery tester). Because I live in the tropics, usually, the batteries dies soon after the warranty expires. Mainly because of the temperature (90F average). Thant means I will have to change them soon.

I have been considering LIFEPO4. But I’m not sure they are the best option for float service application. The problem is that the 99% of the information about LIFEPO4 is for cycling applications, not float service applications. The best scheme I have come with so far is using an expensive BMS that have inputs. So that it, automatically, change the charge algorithm in response to grid outages. I think this is necessary because, to preserve the battery life, the state of charge must be kept below 80%. To achieve that I think I will have to keep the cells at 3.525 (28.2v absorption for 1 or 2 seconds) when off grid and at 3.3125 when on grid. Set float to 3.3 because, if the cells are unbalanced, the BMS will not be able to bleed the 60 amps that the charger provides. Keep in mind that the 3.3125 is necessary to ensure that all cells are at below 80%. This does not have to be done in cycle applications because, even if they are unbalanced, they will not be above 3.3125 for much time. Because they are being cycled. But that is not the case in a float service application. And if that happens the cells will age differently.

Because of that, (expensive battery + expensive BMS) probably my best option (cost wise) is to purchase AGMs.

Does any body know the charge algorithm used by industrial LIFEPO4 telecom battery manufacturers? I would like to know more about using LIFEO4 in float service applications. Found (2 years ago) a research paper that talks about the consequences (rapid capacity loss) of keeping the cells at high charge levels for a long periods. But have not found anything about charge algorithm, or best practices, in float service applications. Thank you very much.

 

[DLTooley]

One way to think about the comparison is that lead acid only gives you 50% of capacity while your application is estimated at only using 80%. In addition note that heat is also damaging to LFP.

Lastly do note that charging up to 100% on occasion, say in advance of a Hurricane, would be doable manually.

 

[acdoctor]

LFP enjoy partial charge and self discharge is tiny. You wouldn’t have to float them all the time. Charge to the 80% you mentioned and disconnect unit needed and check stat of charge every few months.

 

[RCinFLA]

LFP are usually fine for UPS as long as float doesn't go greater than 3.40v per cell (13.6vdc for 12v battery).

All lead-acid batteries need to be fully charged periodically or they will sulfate, reducing their capacity. This means UPS should be periodically taking battery to 14.3-14.5vdc. This is also needed by LPF to activate balancer to keep cells in balance. BMS's usually do not balance until a cell gets above 3.4v per cell (14.2v for 12v LFP battery) during charging.

You need to be careful of UPS's that came with AGM or Gel cells. They often float at 13.8v and do absorb full charge up to 14.8vdc. AGM and Gel cell lead acid batteries are made with higher sulfuric acid concentration electrolyte which gives them slightly higher charge voltages than regular lead acid batteries. These higher charge voltages are at high end for four series LFP cell 12v LFP battery.

 

[Daddy Tanuki]

I would put a small load on a timer and allow them to cycle down to 65% and then charge them up to 80% on a daily basis. everything i have uncovered either here or on other forums say that cycling is way better for them then full time float storage. Just my two cents.

 

[robby]

I would put a small load on a timer and allow them to cycle down to 65% and then charge them up to 80% on a daily basis. everything i have uncovered either here or on other forums say that cycling is way better for them then full time float storage. Just my two cents.

Yep, even if it's not on a daily basis I would discharge them at least every two week down to 40% and then bring them back up. If you only have a few Grid outages per year and AGM batteries are a lot cheaper to buy in your country then get AGM. No sense paying a premium for Lithium Iron if your only going to use them 5 times a year.

 

[time2roll]

In addition to a periodic cycle, I believe a charger that actually turns off would be best for float service. This way the program is less critical.
And yes I would choose LFP over AGM any day of the week.

 

[robby]

In addition to a periodic cycle, I believe a charger that actually turns off would be best for float service. This way the program is less critical.
And yes I would choose LFP over AGM any day of the week.

It's certainly cheaper in the long run but I was surprised when the retail outlet I dealt with said that AGM and FLA are still big sellers. She said it was because most people don't have the upfront money for LFP.
Also I suspect that they are not told about all the short comings of AGM and FLA until after they are Installed. The 50% DOD and much shorter lifespan really kills any savings.

 

[Daddy Tanuki]

Yep, even if it's not on a daily basis I would discharge them at least every two week down to 40% and then bring them back up. If you only have a few Grid outages per year and AGM batteries are a lot cheaper to buy in your country then get AGM. No sense paying a premium for Lithium Iron if your only going to use them 5 times a year.

What you think of cycling them once a month down to 50% or so? My cabin is 100% solar and as such my batteries get cycled daily.

for my bug-out rig/camper I have 8 cells identical to what I use at the cabin (I bought extras in case of a bad cell, plus so that I could "pick and choose" the best for the cabin. I was just considering these questions when this thread popped up.

the plan was to charge via solar but at a slightly lower float compared to normal LiFePo4 profile. CC/CV 13.6 with a float of 13.2 and then go ahead and set the suto scedule/timer on the A/C for two to three hours once or twice a month it takes to run the batteries to about 50% and then recharge. would once or twice a month be adequate? everything I see is either about storage, or cyclic and nothing about float.

I really want a set and forget (obviously I will watch it the first couple of months) but after that i don't want to mess with it as I have other projects to occupy my calcified brain on.

thoughts?

 

[Daddy Tanuki]

It's certainly cheaper in the long run but I was surprised when the retail outlet I dealt with said that AGM and FLA are still big sellers. She said it was because most people don't have the upfront money for LFP.
Also I suspect that they are not told about all the short comings of AGM and LFP until after they are Installed. The 50% DOD and much shorter lifespan really kills any savings.

you mean AGM and FLA?

 

[Daddy Tanuki]

In addition to a periodic cycle, I believe a charger that actually turns off would be best for float service. This way the program is less critical.
And yes I would choose LFP over AGM any day of the week.

I think that one that would shut down and then turn back on at 3.2 volts per cell would be nice.

 

[robby]

What you think of cycling them once a month down to 50% or so? My cabin is 100% solar and as such my batteries get cycled daily.

Cycling daily is good.

for my bug-out rig/camper I have 8 cells identical to what I use at the cabin (I bought extras in case of a bad cell, plus so that I could "pick and choose" the best for the cabin. I was just considering these questions when this thread popped up.

the plan was to charge via solar but at a slightly lower float compared to normal LiFePo4 profile. CC/CV 13.6 with a float of 13.2 and then go ahead and set the suto scedule/timer on the A/C for two to three hours once or twice a month it takes to run the batteries to about 50% and then recharge. would once or twice a month be adequate? everything I see is either about storage, or cyclic and nothing about float.

From one of the papers I read they said twice per month keeps the batteries from losing capacity. I only float mine for an hour every two weeks to recalibrate the Inverters SOC reading. If I was still on closed loop I would not bother with it.

I really want a set and forget (obviously I will watch it the first couple of months) but after that i don't want to mess with it as I have other projects to occupy my calcified brain on.

thoughts?

Your timer plan sounds like a good way to set and forget the problem.

 

[TampaSteve]

I just don't worry about. I use Lifepo4 in an RV. I have no choice but to float at 13.5 because I want the batteries always close to fully charged. And there is always a load of a couple amps. Sure I'm trading off some life but they'll still last for many many years.