Hybrid lead/ lithium install

I am planning to install a lithium bank for my service battery. Currently there are 2 AGM 150Ah each. I will be removing one AGM and will add a lithium bank in parallel with the remaining AGM.

FWIW, I have a “hybrid” system on my boat. Each bank is (3) lithium 230 aH in parallel with (1) 230 aH AGM on each side of the boat which can be chosen via an A/B switch. The internal lithium BMS is happy with the charging system as is the AGM. No additional switching is involved. When the lithium reaches full charge, the internal BMS’s shuts down the charge, but the AGM keeps the load on the alternator so no spikes. Also, the AGM will stay topped off by the lithiums since they float at above 13 volts. Also, if the lithiums shut down from deep discharge, the AGM will carry on so no black outs. This turned out to be a very successful drop in set up, providing 1,500 amp hrs of trouble free power for three years now.

There is plenty of evidence that the hybrid system works.  The only issue I have seen is the reluctance of some insurers to accept the arrangement.

32Mike - no, when the LiFePO4 batteries reach the low point voltage in their discharge curve, the AGM is still sitting happy at 12.8v and starts to take some of the load. Off you push the LiFePO4 battery’s BMS all the way to cut-off the AGM is still at a relatively happy point in its own discharge curve. So you get graceful failure (or handoff) and can still discharge the AGM as normal. On the other hand while the Lithium takes the load the AGM is unstressed and takes a little power to keep it full. 

Ok, yeah, I just googled the discharge curves of the different batteries. So, basically, the lithium low voltage cutoff number is not that dead for the AGM.