Can Lead Acid & Lithium Battery Banks be Paralleled to 1 Inverter?

[Steve_S]

I've read through this thread quickly. I run Hybrid LFP & Big Lead (Rolls Surerete deep cycle) and t works with quirks. I HAD considered used EV packs but the chemistries are too far out of range from life with FLA and after a few months of research went LFP because it is very close to FLA profiles.

Even with LFP & FLA, there are compromises but they can be worked around. I must say, that even so they are close, you have to be careful and watchful because the Low Volt disconnection for FLA versus LFP and of course you should never take FLA below 50% DOD.

For a Twist that no-one mentions. When you have Big LFP cells and multiple packs within an LFP Bank they will level up between themselves (if properly configured). Having a Big FLA bank sitting on side, can actually act as a buffer and provide added voltage which levels up the rest. BUT THE CATCH, You need to be able to either manually or automatically disconnect the FLA bank from the LFP bank when the FLA reaches LVD point or other situations requiring disconnection. There s tech out there than can do this ad even be programmed... it's NOT cheap ! Myself, because I am in attendance all the time, I use a BlueSea 9001e as shown below, NOTE they also have models with AFD (Alternator Field Disconnect) for anyone charging via alternator. BTW: A Side effect benefit if you have LFP cells that wander (most do have divergence) the FLA steadies that somewhat by having that constant current from them and if there is a balancer doing its job (not a passive balancer). I can say, it's nice to look at multiple LFP packs of differing capacities and see all the cells in all packs with 10-12mv difference.

e-Series Selector Battery Switch - Blue Sea Systems

350 Amps continuous rating for inboard gasoline or diesel engines.

www.bluesea.com

 

[snoobler]

Ugh. Revisit.

Everything in the context of a daily-cycled off-grid PV system or possibly a UPS-type backup system.

14S Lithium 3.7 + 24S FLA/AGM/GEL (lead) in parallel. 3.7 battery has a robust BMS.

Something I didn't consider: what happens between absorption and rest and options in between.

I missed the fact that once the lead drops to about 54.4, it will require very little current to keep it at that level - about 0.1A/kWh (rough estimate).

A charge to 55.2V would make this workable.

55.2 is a common "standby" float voltage where lead batteries will get fully charged over longer periods of time. Having a parallel bank absorption and float voltages set to 55.2 would actually behave more tolerably. This would push the 3.7 chemistry to 3.94V/cell, which is around 70% SoC.

Assuming this is being floated by a PV system for several hours per day, Once charging terminates, the lead wouldn't pull that much from the 3.7 bank to keep it floated. The 3.7 would do 99% of the discharge work above 51.2V, but it would slowly transition to lead. At 48.4V, the lead is at about 50% and the 3.7 is around 20% - pretty much the limit you want to go with either.

These voltages are approximate. One could likely find an absorption and float voltage that are workable provided one is good with limiting the 3.7V bank to about 70% SoC (which is awesome for cycle life) and experience only somewhat accelerated lead cycle life degradation (likely no degradation in a standby application). One would need to approach it carefully and experimentally to find what works best for their combination of particular brands/chemistries and usage patterns.