Possible method of using LiFePo4 bank as primary and old SLA AGM bank as secondary?

[cbskies]

Hey Guys, first off I LOVE this forum, I have learned a ton here and I greatly appreciate the free sharing of knowledge and resources made available here by you all, many thanks.

I am building a new LiFePo4 32 cell 48v system for my friends off grid cabin using EVE LF280K Grade A Cells, 280ah. His Cabin is currently outfitted with a 500ah AGM 48v system which has served him well over the past 8-9 years and I believe this existing battery bank has quite a bit of life left in it as he has done a good job of keeping cycles minimal on it. It seems a shame to just completely put his old bank out of service so I was wondering about maybe using it as a secondary bank, I have done some research on this forum and looks like such a system is possible and people are doing so.

I came up with an idea that I'd be very curious to bounce off you guys to see what you think, if its a terrible idea, let me have it!

So his AGMs are 8 12v connected in 4s2p, I had the idea of buying two 2v AGM 300ah batteries and adding one to each of his AGM strings thus giving the AGM bank a little ~2v booster. I'm pretty decent with arduino, raspberry pi, programming, relays and such so i'm fairly confident I could set up the following scenario, if it would make sense.

The new LiFePo4 bank will act as the primary bank and if the system is charging in any way whatsoever the Lipo bank takes precedence and will charge first.
Once the Lipo bank is fully charged the AGM bank will be setup as a 'dump load' with a separate charge controller and the panels will begin to charge it.
With the 2v booster add-on added to the AGM bank the voltage ranges of the two systems will be more in parallel. When discharging once the Lipo bank reaches aproximately 90% dod the voltage of the two banks should match at which point relays would make contact and the two banks start discharging in parallel.
My guess is that the Lipo bank, with it's flatter discharge curve would somehow slowly slurp off the AGMs?
Once the voltage of the two banks in parallel reaches approximately 51.6v the AGMs should be approximately 55% dod and the Lipos approximately 20% dod and the system would call for the generator to fire up at which point relays open up and only the Lipos begin charging and the process repeats.

If such a set up is possible I would expect that the generator would only be required during the longest of winter storms, considering his normal usage and the large capacity of the two banks combined.

Here is a voltage chart I made based off information online which shows what the voltage of the two banks would be during discharge. I think anyway.
The 2v AGM has a cycle voltage range of 2.35v-2.4v

Capacity	LiFePo4 Bank Voltage	AGM Voltage w/ 2v Add-on	Voltage of 2v AGM alone	Standard AGM 48v
100%	54.4	53.33	2.4	50.92
90%	53.6	52.875	2.395	50.48
80%	53.2	52.39	2.39	50.00
70%	52.8	51.865	2.385	49.48
60%	52.4	51.33	2.38	48.95
50%	52.0	50.775	2.375	48.40
40%	51.6	50.21	2.37	47.84
30%	51.2	49.605	2.365	47.24
20%	50.0	49.0	2.36	46.64
10%	48.0	48.395	2.355	46.04
0%	40.0	47.79	2.35	45.44

I'm not sure if the voltage range on the 2v booster AGM is 0-100% or 50-100% dod so my math may be a bit off with that column but you get the idea.

Would a setup like this be reasonable? Please let me know your thoughts and if there is something I am missing.

Thanks again!

 

[sunshine_eggo]

IMHO, not viable. 2V would need to be exactly the same REAL capacity as the 48V cells.

Peak charge voltages of LFP and AGM are essentially identical. To get the 25S AGM fully charged, you'll need to 61.7V or 3.85V/LFP cell - bad.

If you can charge for near 24 hours at a time, you could get the 25S AGM fully charged at 57.5V, and that's at 3.59 for the LFP, but no one recommends holding LFP at 3.59V for 24 hours.

The more common scenario in paralleling is 24S 48V AGM and 16S 51.2V LFP in parallel. The AGM will do very little before the LFP drops to ~52V, but they will start to take progressively more of the load and prevent the LFP from discharging to zero.

LFP has cycle life.
AGM has float life.

The 48V/51.2V parallel seems to maximize both chemistries strong points and only uses the AGM capacity if necessary. Your proposed solution complicates charging and shortens AGM life with only a minor benefit to LFP cycle life if any.

Make sure you fuse each battery. AGM cells typically fail in short, and you don't want the LFP dumping gobs of current into the AGM.

 

[cbskies]

Thanks for your thoughts. I'm not quite sure what you mean by the 25S AGM. The AGM bank would be 2 sets of 4 12v with one 2v. I was wondering about the Amp hour mismatch. The existing 12v batteries are 250ah the 2v is 300ah, I figured it would be ok if the 2v booster was greater than the rest. Could totally be wrong on that.

You make a good point, let me refine the scenario. The two banks would only be connected in parallel once they reach the same voltage. If they are connected in parallel while charging and the voltage reaches 53.32 or 100% for the AGMs then they would be disconnected and only the LFP continues to charge. Then when the LFP is discharging once it again comes in the same range as the AGM they would be reconnected. If that makes sense.

 

[cbskies]

So basically the extra 2v for the AGM is not necessary and only complicates things with no real added benefit.

Sorry now I realize what you mean by the AGM 24s and 25s, im learning.

 

[cbskies]

Thanks again @sunshine_eggo for your response. After studying it a bit further I understand now much better what you are saying.

 

[sunshine_eggo]

Thanks for your thoughts. I'm not quite sure what you mean by the 25S AGM. The AGM bank would be 2 sets of 4 12v with one 2v.

12V has 6 cells.

4 * 12V = 24 cells.

24 cells + 1 cell = 25 cells = 25S

I was wondering about the Amp hour mismatch. The existing 12v batteries are 250ah the 2v is 300ah, I figured it would be ok if the 2v booster was greater than the rest. Could totally be wrong on that.

It would help hammer the 12v cells.

You make a good point, let me refine the scenario. The two banks would only be connected in parallel once they reach the same voltage. If they are connected in parallel while charging and the voltage reaches 53.32 or 100% for the AGMs then they would be disconnected and only the LFP continues to charge.

You don't understand AGM or LFP charging. Charts with voltages are for RESTING voltages. Both chemistries must be charged at elevated voltage to take on a full charge. For AGM, those are the resting votlages 10-24 hours AFTER ALL current has been removed - no charging or discharging of any kind. For LFP, their voltage tends to settle to a useful level in 15-30 minutes.

If you charge a 25S 50V battery to *only* 53.32V, you will destroy it. Lead acid of all flavors must be charged to 100%. They degrade when held at < 100% charge. As I mentioned above, a full charge of the AGM would take 61.7V, which would damage the LFP. As I mentioned above, a long slow 24 hour charge of the AGM @ 57.5V would work.

You would need to work it the other way, i.e., disconnect the LFP and continue to charge the AGM.

 

[cbskies]

Gotcha, that makes a lot of sense. I really appreciate your help. Glad I consulted the forum!

 

[Quattrohead]

It is one of those things that is worth experimenting with if you know what you are doing. Sometimes parallel batteries of different types get along well and sometimes they don't.