Bad for BMS to [often] hit HVD and disconnect charger?

[Alfalfameister]

A friend didn't top balance his cells. I would have lent him my benchtop power supply, but he didn't know I had one, and now too much hassle for him to disassemble pack (rather tight location and all that).

He did connect them in parallel for two days (but reading in this forum, that didn't really do a lot of good).

Anyway, he has a Smart Daly BMS, and he set HVD to 3.60v and LVD to 3.0 (or 2.9, I think).

His charger is set to 55v only. It's all fine and dandy and charges fine, but once one cell hits 3.5, it shoots up to 3.6v within a few minutes (the rest are still in the 3.37 to 3.39v range).

BMS cuts off charging, and the 3.6v goes down to 3.5v within a few minutes, then charging starts over (3.500v voltage "release" - BMS turns off charging when one cell hits 3.600v, and resumes charging only when that cells gets back to 3.500v)

This happens (well, when he gets to fully charge) several times (about 4-5 times), then finally the 3.6v takes an hour to go back down to 3.500v (then it may resume charging, but within minutes hits 3.6v, and again takes a long time to drop to 3.500v).

I know the BMS tries to balance the cells, but it cannot keep up the "burning off the extra charge" with the charge rate (and the balancing function turns off anyway when the BMS disconnects the charger - only balances while charging).

Questions:

1. Is this bad for the BMS (hitting the HVD of one cell and disconnecting the charger), esp when it does this several times per day?

2. Will his cells eventually get balanced? (even if it takes months?)

He is happy with the performance of the battery pack (EVE 280Ah 16S config), and doesn't need to eke out ALL the performance, but he is worried if it's bad for the BMS to be hitting the HVD and disconnecting the charger.

One other thing: he once tried changing the charging voltage to 54v, and that one cell never hit 3.6v (it hit 3.5v, and the others slowly were catching up - I guess the balancer, when the charger was only charging at less than 1A, could keep up). But he said 54v charge rate not going to work for him, as when he hits about 60% SOC, charging slows down too much.

 

[BarkingSpider]

Any serial lithium pack is like a chain, its as good as its weakest cell. Assuming they are all at the same initial stage of charge, no amount of balancing can get back capacity that was not there in the first place.

Work out what is the maximum voltage the pack can take before the voltage of the weakest cell shoots up to 4v and set your charge voltage to that number. That will avoid the disconnections.

If you have a quality BMS, it should balance your cells for you as they drift over time.

 

[cinergi]

It's not bad for the BMS and it will eventually balance. Temporarily reducing the charge voltage so that particular cell doesn't go over 3.6 will speed up the balancing process (since it won't disconnect the battery for long periods of time). Depending on the balance current it could definitely take a long time -- so the question is, when does this battery need to go into service? Because obviously you can't have the battery continually disconnecting if you need it

 

[Alfalfameister]

It's not bad for the BMS and it will eventually balance. Temporarily reducing the charge voltage so that particular cell doesn't go over 3.6 will speed up the balancing process (since it won't disconnect the battery for long periods of time). Depending on the balance current it could definitely take a long time -- so the question is, when does this battery need to go into service? Because obviously you can't have the battery continually disconnecting if you need it

The Daly BMS only disconnects the charging portion (one can still discharge), so basically, my friend will just not get to fully charge the battery until it balances out. The battery is already in service, but, instead of, say, charging to 280Ah, he probably gets to charge to only 250Ah (because of that one cell; the 250Ah is pure guess, as I don't know what SOC he's at).

At about 30% left (again, pure guess), or about 80Ah left, the pack stops discharging. He is grid-tied, so it's more for saving money than actual necessity (basically, extra solar power during the day is stored).

I believe his disconnect is at 50v (and my guess is that there's still about 80Ah or 30% left). So, instead of a theoretical 200Ah usable (from 80Ah to 280Ah), he only has, say 170Ah usable.

Or something to that effect. Basically, and my figures above are just guesses, the bottomline is, he's leaving something on the table. At least until his cells eventually balance, even if it takes a while.

He said that if he has time one of these days, he will follow that advice of temporarily reducing the charge voltage, and he might have to do that several times.

 

[cinergi]

Ah, separate port BMS. Cool .. yeah then you're OK. It might take a lifetime to balance out, though. 280Ah cells are huge

 

[cinergi]

BTW, if it does take that long, I might consider balancing it out faster because you're stressing out that single cell more than the others, which will cause it to degrade faster and mismatch.

 

[Dzl]

The Daly BMS only disconnects the charging portion (one can still discharge), so basically, my friend will just not get to fully charge the battery until [ IF ] it balances out.

Bear in mind the Daly Smart BMS has a maximum balance current of just 30 mA

To put that in perspective, your friends pack is 280,000 mAh.

 

[Alfalfameister]

Ah, separate port BMS. Cool .. yeah then you're OK. It might take a lifetime to balance out

Bear in mind the Daly Smart BMS has a maximum balance current of just 30 mA

To put that in perspective, your friends pack is 280,000 mAh.

Hahaha.. . Yeah, I know. I told him he can borrow my benchtop power supply, but says his pack is too big and heavy and not easily accessible after he put it in there. Ah, well...

 

[Marinepower]

Hahaha.. . Yeah, I know. I told him he can borrow my benchtop power supply, but says his pack is too big and heavy and not easily accessible after he put it in there. Ah, well...

My understanding is that the charge rate is also a factor in giving a BMS an opportunity to balance a cell. So if you charge a 280h pack at 40 amps for eg, a passive balancer does not have time to retard the high cells. But you discharge to the voltage the BMS commences balancing and then slowly charge at 1 amp, there will be a lot more balancing achieved. So perhaps fine a way to charge more slowly for a time (like cover all the solar panels but one? or use the bench top power supply if it can cut its charge at a set voltage )

 

[Alfalfameister]

So perhaps fine a way to charge more slowly for a time (like cover all the solar panels but one? or use the bench top power supply if it can cut its charge at a set voltage )

Lol. That would be too difficult; but, yes, he can lower the amps going in simply by either lowering the charge voltage or limiting the charge (he can do that with his charger, but rather coarse adjustments - maybe in about 1A increments).

Thanks!

 

[ericgrand]

He can also discharge this single cell to the same level than the other with a load applied only on this cell. This will speed up the process.
Balancing at 30 mA is really for minimum adjustement on a well balance pack...
This is the drawback of LFP cells, the voltage does not gives a good indication of the SOC, unless you go over or below the knees of the charging/discharging curve.
A good idea of the total unbalance would be to check which cell is triggering low voltage cut-off and what is the voltage of the other...
It may be that a lot of cells are in fact unbalanced...

 

[Steve_S]

BTDT and doing it again due to hard Thrash Testing on packs. (only for those willing to take the RISK)

Eve's can handle 1C charge/discharge but do not like it. Preferred rate is 0.5C max or 140A. The recommended charge rate is 0.2C to hi of 0.5C.

When the cells get to this state, there is NO easy fix. Sorry but there it is. No amount of passive/active balancing can or will correct that.
He will have to ID the miscreants that wander off and the low cells.

Disconnect BMS Wire harness BEFORE disconnecting BMS leads or busbars from cells, one oopsie & POOF ! Magic Smoke (cause of 75% BMS failures).

Starting with the Lo Cells, push 3.50V @ high amps 30A+ but do not exceed 140A, until the amps accepted drops to 10A.
Repeat for each lo cell... yes PITA... Make sure the connection to cell is GOOD, otherwise the heat, resistance will come to haunt you.
Once all the LO cells are dealt with do same for the two Fast Chargers... BUT at a lower 10A rate to 3.60V till they reduce to low amps 1,2A is good if you can get there.
Now the bigger PITA... IF you want to take them up to 3.65V, do it in incremental steps minding the amps and allowing for REST between taking charge to allow the cells to settle.
* Even once you have them all topped at 3.60 and accepting <2A they will settle and more likely to around 3.50-3.55.
The operational / function "zone" 3.1 to 3.4V per cell which is where the curve is and most of the power resides.
At this point, the cells can be set in parallel using 14G wire and allowed to sit & have all cells level between themselves. *NOTE* Do NOT attempt to pull a load off that battery while it is this way. Safety measure of disconnecting the (+) is PRUDENT !

To download a larger clearer version:
General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V | DIY Solar Power Forum (diysolarforum.com)

Hope it helps, Good Luck
Steve

 

[AussieSim]

Hahaha.. . Yeah, I know. I told him he can borrow my benchtop power supply, but says his pack is too big and heavy and not easily accessible after he put it in there. Ah, well...

He could bleed off charge from the highest cell. I didn't have a suitable lightglobe handy, but a 2 or 3 pin PC 12V fan plus arduino breadboard connecting wires does the trick.