Expanding parallel battery pack and charging capacity

[ezwryder]

Just thinking out loud here. I’d appreciate any thoughts you might have about this: I want to expand my solar set up with additional storage and a few more panels for a faster recovery time.

I currently have 6 x 100 watt panels going into a Victron 100/30 amp charge controller. The controller charges two SOK 206 Ah heated batteries wired in parallel. When the controller maxes out at 30 Ah, the charge rate is about 15 Ah per battery. If I add a third, I would expect 10 Ah per battery.

I want to increase the recharge rate to 25 Ah per battery if I can, but I also want to avoid an overcurrent charging situation. I know there was a paper written about this and I cannot find it anywhere. I don’t recall if it had suggested remedies but I do know it clearly illustrated the potential issue.

Here’s what I think my options are.

Option 1: Swap out the existing 100/30 controller I have for a 100/50 (which I also have). My present panels max out at around 425 watts because of the charge controller 30 amp output limit. Even without adding panels, a 50 amp controller would better use all the power from the current panels, producing another 12 to 14 amps. 44 Ah into 3 batteries is 14.6 Ah, close to the present recharge rate of the two battery setup.

Option 2: Swap out the 30 amp controller for the 50 amp controller AND add two panels. The approximate max amp output of the panels would increase to 56 amps which would be capped at 50 with the 100/50 controller. That’s 16.66 amps per battery, a small increase.

Option 3: Use both the 100/30 and the 100/50 with the appropriate number of panels. If I did this I could put 80 amps into the batteries which is about 26.6 amps per battery.

I like option 3 but I am concerned about what might happen if two batteries reach the charge cut-off point before the third is fully charged. That would present a potential total of 80 amps to the third battery. I know each battery BMS has overcurrent protection but I would rather avoid having to rely on that.

Maybe this is an uncommon condition if the batteries are kept fairly well balanced and I am overthinking? Or maybe there are other ways to mitigate the potential issue?

Thanks for any thoughts you might have.

 

[FilterGuy]

I like option 3 but I am concerned about what might happen if two batteries reach the charge cut-off point before the third is fully charged. That would present a potential total of 80 amps to the third battery. I know each battery BMS has overcurrent protection but I would rather avoid having to rely on that.

That should never happen if the batteries are properly wired. (I guess it could happen if the batteries were in a different state of charge when they were first put in parallel, but after they balance out, they would track pretty well. All three will reach fully charged at the same time. Even if the cells are out of balance in one of the batteries, by the time it starts to race up, the other batteries will be mostly charged and not taking much current. Consequently, if the BMS on the battery with the run-away cell cuts out, it is unlikely the other batteries will see too much current.

One thing that may sway your decision is the availability of match panels. It seems like the sales life of panels is really short and when you go to buy more of the same you find they aren't available anymore. If you can't get more of the same panels, it is best to keep your new panels and old panels on separate arrays with separate controllers.

 

[ezwryder]

Even if the cells are out of balance in one of the batteries, by the time it starts to race up, the other batteries will be mostly charged and not taking much current.

Thanks for the reply. I found one of the original postings on this over on the Victron community site. One solution there was to set the absorption level at 14.4 (SOK limit is 14.6) so that the charger would enter the absorption phase before hitting the ceiling on any of the batteries.

I will have to check on those panels, thanks for the heads up. I do have two charge controllers I will be using.

 

[Steve_S]

Option 3 is your best option really.
As the batteries fill up to the specified voltage their internal resistance changes and the amperage they take drops as this occurs. For Example, if you are providing say 100A Charge Capacity but the batteries are only taking 20A each then that's it they won't take anymore. The Tail Current setting within the Victron SCC's is essential which for your 205AH batteries is 10.3A. Once the battery has reached 10.3A taken they are essentially 95% full and the SCC transitions to float mode (variable current) from Bulk/Absorb allowing for the last 5% topoff.

With 206AH Batteries, you can safely have 103A Charge power going into the batteries for the full 0.5C Rate that they can take.

As for the general behaviour of a bank as it fills up...
The batteries that reach full first, will take lower Amps until the battery is at set voltage and full. That allows the extra amperage to go into the other packs and as they fill up their amps taken drops as well... When one battery pack reaches full and goes into "storage mode" which is no longer taking charge the balance just goes to the other battery packs within the bank, as they fill the bank total "endamps/tailcurrent" flips to Float while tops them off until none of teh batteries are taking charge, then float service whatever existing load is being used if there is enough solar to deliver it. Meanwhile the battery packs will balance between themselves and take a slight bit of current to level up all teh packs as they balance out.

I run 6 Packs in my bank, 2x174AH, 3x280AH, 1x105AH and all of this is quite observable and actually quite interesting to watch as the bank fills. It's almost like a choreographed dance between the packs.

Notes on TailCurrent/EndAmps.
The formula is 100AH X 0.05 = 5A.
The setting for the SCC(s) is the TailCurrent/EndAmps of the "SMALLEST" battery pack in your bank of batteries.
Absorb will continue for the duration of the timer AND/OR till the EndAmps/Tailcurrent is reached at which point the SCC will transition to Float Mode to complete the last 5% of charging.
Absorb Timer: Allocate 10 Minutes per 100AH of bank capacity, so for a 400AH Bank, allocate 40 Minutes. You will most likely see TailCurrent reached before the timer expires and this is as it should be.

I use Midnite Solar Classic Charge Controllers and a Samlex EVO Inverter (set to 80A charge) for my 24V Bank.
The Classic-200 is connected to an array of 8x260W Canadian Solar Panels (4S2P) and provides 79A Charge Current.
The Classic-150 (new) will be connected to 6x395W Q-Cell Panels (2S3P)and will provide 92A Charge Current. The new controller / Array will be installed in Parallel with the C-200 as Master Controller. (They use a Paralleled Protocol and act as one system). BTW: the C-150 & Panels are from Santa so next spring installation.
* During testing, I have pushed 210A Charge into my bank and watched closely as the battery packs reached full and started to disconnect in series (due to the capacities) and absolutely no problem at all due to the IR limiting how many amps each battery would take. My BMS' are all set to allow a Max of 0.5C Rate for charging.

Hope it helps, Good Luck.

 

[ezwryder]

Option 3 is your best option really.
As the batteries fill up to the specified voltage their internal resistance changes and the amperage they take drops as this occurs. For Example, if you are providing say 100A Charge Capacity but the batteries are only taking 20A each then that's it they won't take anymore. The Tail Current setting within the Victron SCC's is essential which for your 205AH batteries is 10.3A. Once the battery has reached 10.3A taken they are essentially 95% full and the SCC transitions to float mode (variable current) from Bulk/Absorb allowing for the last 5% topoff.

Thank you for this comprehensive feedback. I feel better about what I'm planning to do!