I can’t say why lead-acid batteries don’t require balancing internally.
But batteries in series will always stray out of balance (slowly). To minimize stray, you want to match leads from both batteries as well as practical (match resistance).
A little frustrated after upgrading to 24 volts Growatt 3000
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But batteries in series will always stray out of balance (slowly). To minimize stray, you want to match leads from both batteries as well as practical (match resistance).
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12 and 24 volt batteries are 6 and 12 cells in series.
I also thought that's why larger battery banks are large individual cells in series rather than just a bunch of batteries in parralell.
In looking at Tesla, I don't think they have a way of balancing all those little individual cells that are in series.
timselectric
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They actually do. This is part of the BMS's job.
But putting BMS's in series causes imbalance. Because each BMS balances it own cells. Independent of the other BMS/cells in the series string. So the cells under control of one BMS drift away from the cells under control of the other BMS.
The only way to keep all cells in a series string balanced. Is to manage them with a single BMS.
A 24V battery composed of 8 cells in series can be balanced by an 8S BMS, while a series stack of 2 4S 12V batteries will have 2 4S BMSes which will keep each pack of 4 cells balanced, but will do nothing to balance the two 4S packs against each other.
Tesla is a whole different animal - I would not be surprised to learn that they do in fact balance all of their cells internally.
But when you are building a multi-cell battery pack, it is much easier to keep cell resistances balanced / identical that when you are connecting multiple batteries in Series externally.
Cell capacity will eventually stray in any case due to different aging / degradation rates, but differing I^2R losses through different-resistance connections can easily outpace that intrinsic factor.
Lead Acid batteries have such a limited cycle-life that it’s entirely possible the battery will be shot / spent before the cells can get too far out of balance internally…
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It balances groups of 6 cells.
Ok. So if I put two lifpo4 batteries (each have separate bms, then the two batteries will become out of balance with each other?
What's the solution when that happens?
timselectric
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It's the lack of need for maintenance, that makes the BMS valuable. Placing BMS's in series puts maintenance back in the system.
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Well, ok. I guess the lead-acid equivalent would be equalization.
I have just heard and read that it's better to have la in series to minimize cell imbalance.
timselectric
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Yes
Periodically disconnecting the batteries and charging them fully, individually.
timselectric
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My bad....duh.
You’re preaching to the choir
.So is it impossible to overcharge a lead-acid cell the way you can a LiFePO4 cell?
Balancing / Equalization is trivial if cells cannot be damaged by overcharging and just cap out at some (common) maximum voltage…
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Not impossible. But they are more forgiving.
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Lead acid can be overcharged and damage the battery. First, if anything other than flooded, the electrolyte may not be able to recombinant, thus losing the water. Any battery being overcharged for EQ needs to have the temp and electrolyte level monitored and water added to keep plates covered.
High voltages with high amps will generate heat and eventually the battery plates will be damaged from excessive heat. Heat can be a good thing, it helps with the chemical reaction needed for desulphation. High voltages can blow desulphation off the plates but it can also blow good lead off a plate.
As for overcharging with higher than normal voltage and "just cap out at some common maximum voltage", if you stop charging, the battery voltage will drop back to fully charged voltage. Most of the watts put into a lead acid battery when using high voltages will be converted to heat.
Each battery has 200Ah, since I have two, total ah would be 400 (in parallel). I understand that by going in parallel I’ll have 2,560 watt-hour per battery (as per battery specs). Total would be 5,120 watt hours. Is this correct?
Good point! I feel like in the Twilight Zone ? May be I’ll keep my new Growatt and then I’ll see what to do. Thanks!
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Yes, that's correct.
Thank you, and that is the reason why I keep considering this option. More stored energy will benefit my load. The series arrangement is keeping me at 2,560 watt hour option. But also is giving me the benefit of solar energized appliances during sun hours, not bad at all.
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