The 300Ah 12V LFP bank in my sailboat is just a little too small for my peace of mind - and it’ll soon be 10 years old. Expanding it is not recommended as newer cells might not mix well with the old ones and the battery compartment is full anyway. Replacing it before it becomes necessary would be wasteful - it could work for many years still - but the day it fails, it’ll likely be just after we arrived in a beautiful bay in the middle of the summer holiday.
Solar is less efficient up here in Scandinavia. I have a large fold-out that I use on good days, but it’s never enough to do a full recharge. It just slows down the consumption. I don’t want an arch.
However - elsewhere on my sailboat, there are plenty of compartments that could fit another battery. And the price of another 300Ah LFP battery capacity is not so high. Especially if I could factor in BOTH the added available power AND mitigating the risk of the current bank braking down in summer, ruining several days of sailing, making do with whichever replacement batteries are available then and there etc.
But this would require a smart bidirectional DC-DC charger that could integrate with my Cerbo GX. It would be charged from the main bank whenever that is above 90% and supply charge back whenever the main bank falls below 50% - or thereabouts.
I know that a bidirectional Orion XS has been discussed several times - but as far as I know, this specific use case has not been presented in detail. As more LFP installs grow old and the need for power keeps growing, it could become a significant source of business.
You could try paralleling them. I have a large bank of mixed 12V lithium batteries connected to solar and a Multiplus in my shed, running as ESS and backup power for electronics. They’re all different ages and sizes 360ah down to 50ah, some have pouch cells, others prismatic. Each is individually fused and connected with reasonably short cables to a common busbar 1000 powerin.
I’ve only had one battery fail in this setup, and the system handled it gracefully. The BMS simply shutdown on the failed battery, (a 2P4S pouch cell, 50Ah) which had been charging and discharging unusually quickly. The system continued operating normally with just slightly reduced capacity.
When I got a warranty replacement, I discovered the original BMS was allowing cells to reach 13.8V clearly overcharging, which explained the rapid cycling and eventual failure. This kind of graceful degradation is exactly what you want in a remote sailing situation.
For your aging 300Ah bank, I’d suggest capacity testing it first. If it’s still above 80% of original capacity, paralleling with a new 300Ah bank could give you the best of both worlds doubled capacity now, plus insurance against future failure. When the old bank eventually degrades or fails, its BMS will disconnect it and you’ll still have the new 300Ah bank operational. You can add a shunt to each battery and check how much you are getting out of them.
Check with your battery manufacturer too about capacity degradation and when you should replace them.
Paralleling would essentially be the same as expanding the current bank. Cell optimization problems aside, there’s no space close enough to do that safely and efficiently.
I could also probably do it with two Orions (one in each direction) but that would be a somewhat inelegant solution and introduce new potential problems with charging in a loop if the control system (nodeRED script probably) should fail.