I am planning a battery upgrade for my new 4x4 which comes factory with a single 105ah AGM battery.
Due to space constraints I am planning to add around 100ah of LIFEPO4 prismatic cells to beef up the capacity.
All the loads i want to run are already wired off the AGM battery and I don't want to move all the circuits over to the lithium battery. The vehicle has a very complicated electrical system that I would like to leave as standard as possible.
So the plan is to have the lithium charging through a DC/DC while the engine is running, then when the engine is off the DC/DC will disconnect and the lithium will be used to charge the AGM as my camping loads slowly deplete it. In theory the AGM will stay at 100% SOC until the lithium is discharged. Then the lithium cuts out and I am discharging from the AGM.
I think this will work but welcome any comments, I'm a little out of my depth here.
The current plan is to use another, smaller DC/DC charger from the lithium into the AGM. Although looking at the voltage curves I wonder if I could save some money by switching to a cheaper DC/DC converter set at 12.8v, or even just a Victron Smart Battery Protect. With the battery protect only, the AGM should see a float charge of 13.8v from the full lithium, dropping down to 12.8v as it discharges. Then, I guess, they both both discharge together until the BMS cuts the lithium out at 12.0v. By which time the AGM is empty anyway and I need to run the engine or stop drawing power.
Is this reasonable? Or should I stick with a second DC/DC charger?