Any thoughts on a backup AGM parallel connection to a lifepo4 battery bank?

[GVSolar]

I am in the process of creating a large bank of four TechDirect/BigBattery pre-built 24v BYD units (3kw usable capacity each). Three of the units are already "awakened" (three charge/discharge cycles) and connected in parallel. They are receiving their charge from (2) Schneider Conext MPPT 60-150s (DC coupled 3000w) and a 1500w string of REC/Enphase M-215s (AC Coupled) thru a Conext SW4024 inverter. System monitoring is thru a Conext SCP/Combox.

In order to commission the system I first connected two FullRiver DC-115 AGM (115ah) batteries in series (24v), and connected them to the inverter/chargers for LFP programming (replacing my previous bank of Trojan L16s). After setting the charge parameters for LFP I added the parallel string of 3 TD/BYDs to the 24v AGM. This parallel LFP/AGM bank is performing as expected - and I am now installing a separate fused disconnect switch to bring the AGM battery in or out of the main bank of LFP as needed (adding a fourth TD/BYD unit this week).

I originally thought of this primarily as a maintenance issue (programming/reserve capacity) but after some reading on the subject the notion of the AGM battery acting as a island of last resort in case of a BMS disconnect (overcharge/low temperature, etc.) began to appeal to me.

Thoughts on implementing this in my totally off-grid system would be very much appreciated.

Some reading on other LFP/AGM parallel banks:

Parallel lead acid and lithium circa 2018 - Class B Forums

Are we going to start seeing parallel lead acid & lfp battery setups now with the drop-in lithium batteries? There's already a commercial option for this: https://www.bos-ag.com/products/lith...ens

www.classbforum.com

Battery Mixing integrating Lithium Lifepo4 AGM or Gel

Ok, I know Im out on a limb here with the general consensus. What I’m looking for here is a technical explanation as to why not, or how to, run a mix of AGM/Gell and Lithium The attached shows my current setup, and my thinking of how I will integrate lithium. So firstly some information: My...

forums.sailboatowners.com

Lithium battery systems | Nordkyn Design

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[grizzzman]

There a lot of naysayers on this subject. But very little "I have done this" from them. I'm doing this with FLA CG2. For the most part, your AGM bank will just be trickle charged day and night. The surprise for me was load sharing under heavy load.

 

[upnorthandpersonal]

I'm planning on doing this, but with LTO cells instead of lead acid since it can handle much lower temperatures. This would primarily be used to keep the LiFePO4 pack warm, but would also be used to keep certain critical systems running when the main LiFePO4 pack gets disconnected.

 

[grizzzman]

I'm planning on doing this, but with LTO cells instead of lead acid since it can handle much lower temperatures. This would primarily be used to keep the LiFePO4 pack warm, but would also be used to keep certain critical systems running when the main LiFePO4 pack gets disconnected.

I'm planning on doing this, but with LTO cells instead of lead acid since it can handle much lower temperatures. This would primarily be used to keep the LiFePO4 pack warm, but would also be used to keep certain critical systems running when the main LiFePO4 pack gets disconnected.

I wonder how big a battery you would need to pull that off?

 

[upnorthandpersonal]

I wonder how big a battery you would need to pull that off?

I'll probably start with something in the 1 to 2 kWh range. It takes surprisingly little to keep the LiFePO4 pack warm when it's enclosed and insulated. The critical parts I want to keep running are also not power hungry, so this size should do fine.

 

[DW SD]

I will do hybrid AGM + LFP.
This is a good forum post which validates also. https://www.canalworld.net/forums/index.php?/topic/99667-another-lithium-battery-thread/