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Old 11-08-2022, 08:13 PM   #1
Silenttrouble
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Default Lithium batter voltages different in parallel.

Realized I should put this in lithium section
I'm having a stupid moment here. I'm trying to bottom balance my lithium cells so I hook them up in parallel. I left them like that for several days used to discharger to drain them down to 2.65 volts and then reinstalled them in my cart. To my surprise when I checked pack voltage it was still 50.1 which tells me I did not discharge all of them evenly or together.

So I went back and tested all cells individually some were at 2.7 others were at 3.1 and now they are way out of balance. So I hook them back up in parallel testing individual cells several of them are still at 3.1 and some are at 2.7 of course the full parallel voltage is a cell average. How long should it take a 270 amp hour 3.2 volt pack to balance? I also took the time to use my fluke and audible diode check across the full length of terminals to ensure I had good connections with the alligator clips.
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Old 11-08-2022, 09:45 PM   #2
Pat911
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Default Re: Lithium batter voltages different in parallel.

Hi Silenttrouble,

Firstly, why are you bottom balancing? the normal balancing strategy is to top balance. You may have a valid reason for bottom balancing, just asking as BMS's are mostly configure for top balance so you won't be able to use the balance function of your BMS if you bottom balance.

Now, to your question. You need to give us much more information to answer your question accurately, but depending on the initial charge of the battery and how quickly you're discharging, it could take a sh!t load of time. You do have 270Ah cells don't forget.

Let's assume your pack was initially at 50% SOC. That's 135Ah in each cell. You then parallel 16 of them up. You now have 2160Ah capacity at 3.2v. I don't know what current your discharger is, but let's assume 10 amps. That's 216 hours (or 9 days!) just to discharge them. You then have to leave them that way for a few days to a week.

If you tell me your initial SOC and discharger current I'll be able to give you a guide to how long it will take. Lastly, how are you connecting the cells in parallel? Are you using cable or bus bars?

Cheers
Pat.
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Old 11-09-2022, 06:28 AM   #3
Silenttrouble
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Default Re: Lithium batter voltages different in parallel.

Quote:
Originally Posted by Pat911 View Post
Hi Silenttrouble,

Firstly, why are you bottom balancing? the normal balancing strategy is to top balance. You may have a valid reason for bottom balancing, just asking as BMS's are mostly configure for top balance so you won't be able to use the balance function of your BMS if you bottom balance.
Pat.
Hey pat, they were already pretty low at end of season. Discharge is 2A (couldn't find a bigger one for under a couple hundred) and I took out 80AH with discharger it was still running but down at like 0.4A. After posting last night I went back out and checked to make sure current was flowing from one batt to another with ammeter.

As to why I'm bottom balancing, I my be misguided in that I knew my cells were WAY OFF. It made it near impossible to know my soc with a battery which such a flat voltage discharge curve and my bms is a zeva with tiny balancers. In my mind if I bottom balanced my cells would hit zero at nearly the same time and I would have a better idea of soc toward the end than one cell that sags out and shuts me down.

I guess my issue is the few days to a week how long does it take for them to balance, I would have though maybe not instant but not that long. But after checking with ammeter it's less than 1A from cell to cell so now I might have F*Ed up. I left them parallel for like 2 weeks before trying to discharge. I have lots of time as it gets cold I'm moving to indoor golf.

Please help with a path forward.
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Old 11-09-2022, 03:14 PM   #4
Pat911
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Default Re: Lithium batter voltages different in parallel.

Hi Silenttrouble,

2A is an awfully low discharge current for 16 270Ah cells in parallel, no matter what the SOC is. You didn't answer my question but I'll assume you've connected the cells using wire and not bus bars. If that is the case, it would explain the differences in voltages when disconnected as the wire would have had significant resistance and balancing would be ineffective. You should also connect the discharger to the positive at one end on the chain and the negative to the cell at the other end of the chain to better equalise the current between cells. If connected to a single cell at one end then that cell will discharge first and the most furthest cell last.

In your scenario, I really think you should change your strategy to top balancing. Cells are never equal and balancing puts the cells into a state where they either get to 0% SOC at the same time (bottom balancing) or 100% SOC at the same time (Top balancing), it can never be both. Even when balanced, cells will drift and it's the job of the balance function in the BMS or external balancer that will keep it in check. Most will work on a top balance strategy.

What to do from here will depend on what you have available to you and how much time you have. The best way to do it would be to connect them in parallel using bus bars, or at least VERY thick cable, keeping it as short as possible. Get a 10A (or greater) laboratory style power supply, set it to 3.65v and start charging. Since your pack is quite depleted, It will take at least 18 days to charge at 10A but I would leave it like that for at least 30 days.

What BMS do you have as you could also get it to do the top balance for you but it may take even longer than that.
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Old 11-09-2022, 04:05 PM   #5
Volt_Ampere
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Default Re: Lithium batter voltages different in parallel.

I agree with Pat911 that you should do Top Balancing - not bottom. Charge them all up to full charge and balance them there.
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Old 11-09-2022, 06:07 PM   #6
Silenttrouble
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Default Re: Lithium batter voltages different in parallel.

Quote:
Originally Posted by Pat911 View Post
Hi Silenttrouble,

.

What BMS do you have as you could also get it to do the top balance for you but it may take even longer than that.
Pat didn't mean to failed to answer. I'm using small gauge alligator clips on the posts. I think the total resistance from A to B was almost 100 ohms so I could see your point there. You suggest I top balance I was trying to solve for one sagging cell shutting me down while looking like my SOC was fine when stopped. It seems like you're not the only one suggesting that as well so maybe I just give up now put them back into series since they're all within 0.15 volts of each other and let the BMS top balance. You asked what BMS I had it's the Zeva which does have Dynamic cell balancing even if it's very slow.
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Old 11-09-2022, 06:13 PM   #7
Volt_Ampere
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Default Re: Lithium batter voltages different in parallel.

Being .15 Volts apart is a big out of balance. Good balance is .01 Volts or less!
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Old 11-09-2022, 06:56 PM   #8
Silenttrouble
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Default Re: Lithium batter voltages different in parallel.

Quote:
Originally Posted by Pat911 View Post
Hi Silenttrouble,



What BMS do you have as you could also get it to do the top balance for you but it may take even longer than that.
So if I've been doing it wrong and I want to Pivot back to top balancing given how far out of balance my cells are what is the path forward here. Charge the cells individually much closer before putting it in parallel? And then put in parallel and let the BMS do its thing for the next 5 months? One of the issues I was having beforehand was the BMS was shutting down because one cell was getting too high and charging faster than it could balance. So I lowered the target charge voltage of the charger to try to prevent that. Is that the right thought process because it won't balance if it shuts down due to a single high cell. A high sale is a shutdown not a charger stop.
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Old 11-10-2022, 01:25 AM   #9
Pat911
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Default Re: Lithium batter voltages different in parallel.

Thin wire on alligator clips, no wonder your cells are all over the place. As Volt_Ampare said, your cells are WAY out of balance now and that will have to be fixed prior to rebuilding your pack.

You really need to create a very low resistance connection between the cells to balance them as a pack. If you cannot do that, get a laboratory style power supply of at least 10 amps as already suggested and charge each one up to 3.65v. Keep charging until the current drops to 1% of the original charge current, or 100mA. Repeat 16 times. That will ensure each cell is at 100% capacity. Use a large gauge cable between the charger and the cell. Do not use thin cable with inadequate alligator clips.

If you had one sagging cell, it is most likely a bad connection between the cells. Are you using copper or aluminium bus bars to connect the cells when they're in the cart or wire links? Are the bolts torqued down to spec? Are the balance leads connected to the positive of each cell? You may have connected to the negative of the next cell with would be OK for low currents but would give a lower cell voltage when drawing higher currents due to the increased resistance due to the link.

Anyhow, first thing to do now is to balance the cells again. Do you have access to a power supply?
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Old 11-10-2022, 09:03 AM   #10
Volt_Ampere
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Default Re: Lithium batter voltages different in parallel.

I disagree that you need a "very low resistance" between parallel batteries for balancing. The current will flow until they are balanced - it will just take longer than if you have a low resistance. If they are way out of balance, having some resistance is a good thing - it prevents high currents from flowing. Most balancing circuits in BMS's only balance at a fraction of an amp (.5 or maybe 1 amp) That takes a while to balance if you need a lot of balancing. But hooking batteries in parallel with wires will pull several amps at least and you can wait as long as it takes to balance that way.
I parallel charge lipos all the time for my drones and RC planes. As many as 8 at a time. The boards I have connect all of the cells (4 cell packs) in parallel and charge through the main leads. The charger balances through the balance leads which are all in parallel as well. Relatively high currents may flow when I hook the batteries up if they are not exactly at the same State of Charge. (They are close enough) They come off the charger perfectly balanced and all exactly equally charged. Works great and saves time since I always charge them right before I go flying. I discharge any unused packs to "Storage Charge" when I get back from a day of flying so I never store them fully charged. Our golf cart batteries are just larger versions of these batteries that I have been using for 20 years.
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