diy solar

diy solar

Update (~2.5yrs) - 1100ah Xuba - Sailboat

Capt,

the REC BMS is as good as they get and a 2 amp balance current is huge, even for your big bank.

What cells are you using? If they are eve 280 or equivalent and aluminium terminals, my guess is that you have some resistance developing on your terminals and it's putting you whole system slowly out of balance.

Also, you have not connected you cells in a very balanced configuration ( as mentioned above) so that is making things worse. You have not said where your balance leads are attached - but they may also be unbalancing your parallel cells, if you don't have those property connected (and terminated) in the middle of pack.

I have a 4s pack and i have had to fully dissemble and clean the terminals twice in the 3 years I have run these cells (EVE 280K without welded studs). Otherwise imbalances start to show up in the system due to busbar connections resistance. Your system is way more complicated so will need a lot more care and attention to make sure there is no unbalanced resistance in the system. A marine environment is unforgiving in this regard.

I would write to REC and have them give you advice as to how to connect up your 4p4s pack ( both the balance leads and bussbars connections and main pos and neg.).

I would then disassemble the pack, and wet-sand every terminal, bus bar and balance lead with 75 - 100% alcohol and some 1000+ grit sand paper ( the black stuff, made for metal.)

Reassemble with some anti oxidant grease. Make sure balance leads are well terminated ( crimp connection) and attached directly to the bus bar (ie, busbar on terminal, then balance lead ring on busbar, then a washer, then the nut.)

I would also take this opportunity ,while its all apart to rebalance the cells.

MP
 
@Marinepower Thanks so much. I've been doing some more experiements recently and it seems what I've shown here only appears when I try to go from ~25% to full in one sitting. If I charge them at about 60% to full the voltage difference is way less.

I'm going to re-positions the terminals today per the recommendations here, then I'll start to play around with where to put the balance terminals and clean everything!

Thanks for the help!
 
@Marinepower Thanks so much. I've been doing some more experiements recently and it seems what I've shown here only appears when I try to go from ~25% to full in one sitting. If I charge them at about 60% to full the voltage difference is way less.

I'm going to re-positions the terminals today per the recommendations here, then I'll start to play around with where to put the balance terminals and clean everything!

Thanks for the help!
Capt,

you will always see only minor voltage differences btw 10% SOC and 90% SOC, because lithium has an amazingly flat voltage profile. That same flat voltage profile however hides imbalances in your cells at those SOCs.

Also the REC BMS is an active balance BMS - not passive. It will discharge one cell and shift that charge to another. It's a gold standard BMS with a very high balance current. Most BMSs only balance when the cells are over 3.45 volts ( in your case, when the 4P cells are in aggregate at the BMS target voltage) and only when they are being charged. The REC BMS however may be different and may balance both during charge and discharge. Check your REC user manual in this regard.

Given the above - some of the voltage variation you are seeing in among you 4P packs may be balancing activity by the BMS. Is there a correlation of where you are seeing cell imblances in the 4P packs and where the balance wires are attached to the cells?

This is very solvable problem. I'm sure your cells are fine but you are going to want to get them to a place where you can charge them to full again. It's only at high state of SOC that cell imbalances show themselves and your BMS can do its job and rebalance your battery.

Very weird that you are getting fully charged at 13.7v. That to me indicates you have one 4P cell very out of balance with the others. I

MP
 
See also:

 
Got it, I agree I'm not at a bad place at all and just looking to keep optimizing to maximize the lifespan of these Xuba cells! Thanks again @Marinepower
 
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