DC to DC charger between 2 lfp battery banks

[motu42]

Hi,
I'm trying find a way to add another battery bank to my current 12V 200Ah LPF bank which is proving undersized.


The problem is that adding more LFP batteries in parallel is not good, especially since the existing bank has been running for 18 months, and the new bank would likely have quite different params. By all accounts thats a good way to get out of balance etc.


The other option is to fully replace the existing bank with a larger one, and try to sell off the old bank. Possible, but expensive, since I expect the price for used lfp's is well below new, and they are only 18 months old.


So I have been looking at DC to DC chargers. In an RV its recommended to use one to get the house battery charged from the alternator/start battery. Seems an accepted solution.


So assume I keep my existing bank (A), and I add a new bank (B), which could be quite different size etc. I move all solar charging to (B) and connect (B) to (A) via a 30A DC/DC charger. I continue to run all load from (A). Nothing but the charger runs from (B), lets assume proper fusing etc is in place. (I dont charge my lfp bank from the engines.)



I use a voltage sensor to turn the charger on when (B) is over 13v, and off otherwise.



So simple cases:
The sun shines, (B) raises to 13V and charging begins, transferring 30A to (A). (A) fills to 14.4, charging stops, (B) fills to 14.4.


As house loads drain (A), charging continues, transferring charge from (B) to (A) until (B) drops to 13V. Then (A) start to drop.


More complex:


My wife uses the air fryer (!) drawing 1500W through the inverter from (A), lets say 150A. The charger starts, transfers only 30A and (A) depletes quickly. 20Min later the air fryer is done, (B) continues to charge (A) until its back to 14.4 (or (B) is depleted.)


What have I missed here? It seems that I can have two unequal battery banks joined this way and avoid the imbalance problems. The disadvantage is only that I can transfer at best 30A between them (which is enough). Plus if one dies, I have a spare that can become (A) with some reworked cabling. And I can replace the banks as required.



Are there failure modes Ive missed? Other problems?


Thanks
Rob
Foxglove

[Ballsnall]

Just parallel them, they will be perfectly fine with your as new 18 month old batteries. Do a balance of course, then forget and enjoy.

[s/v Jedi]

Agree, you can parallel the two batteries. You do need a fuse, battery switch and battery monitor for each. If you have a Cerbo GX and want a view of total battery capacity left etc. then simply connect the shunts from each battery to the BAT terminal of a SmartShunt and go from there, then use the SmartShunt as the data source in the GX device.

Now you can keep an eye on each battery, disconnect one etc.

The way this works: the voltage stays the same over a large SOC area. If one battery has a lower SOC then at some point it’s voltage goes lower and thus it delivers less amps with the other battery making up. Same with charging, the battery that is full first goes to higher voltage but has to wait as the other battery takes all the amps at a lower voltage. So they balance automatically at both knees of the SOC graph.
While all that works, you can view this in the separate monitors. It works very well. You can consider these “power packs” that you switch on/off the power bus.

It is recommended to do a full charge initially and then every couple weeks or so.