Balancing a 4S lithium battery

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Old 07-20-20, 03:56 AM
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Balancing a 4S lithium battery

Hey everyone, a friend of mine helped me making a 4S27P lithium battery as I don't have a spot welder, the batteries were at roughly the same voltage, but when I got the battery there is a big balance issue(e.g some cells are running at 3.5V and some at 3.9V).

I have a BMS installed, but it seems like it doesn't cut the charging voltage to the batteries even when going over 4.25V which is usually the charging limit(should it even do it? I'm not sure..) I discharged it right away as I only tried that to see if I could let it charge in order to balance the cells.

Anyway, I'm thinking of getting a dedicated balance charger, something like this:
https://www.aliexpress.com/item/3281...archweb201603_

Did anyone have any experience with this? The reviews are good, and it seems like it should get the job done.
Also regarding the BMS, should I replace it? Should it cut the charging? I do have some other BMS laying around, but I don't know how good they are.
Other ideas/methods/recommendations are welcomed.

Thanks for the help
 
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Old 07-20-20, 07:55 AM
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What type lithium cells are you using?

You asked if your battery management system should be replaced. You have not told us what battery management system you are using so we can't say if yours should be replaced. My guess is "yes".

27 cells in parallel is unusual and can cause trouble. Hopefully you took care picking and matching your cells. Still, 27 cells in parallel is crazy and asking for trouble.
 
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Old 07-20-20, 08:26 AM
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You're right, sorry, should've mentioned that.
I'm using 108 NCR18650B batteries rated at an average of 3500 mAh, and they were all balanced to 3.7V before installation.
I don't know what is the used BMS(exact model), it's something a friend gave me, but I'm asking about it since it doesn't seem to stop charging at 4.25V(and I didn't want to risk getting higher than that, even reaching 4.25V felt like a really bad idea to me).

As for 27P, what trouble could it cause? And what is the difference between 27P and 5P for example in that matter?
I'm using this to power all my astronomy imaging gear in the field, that's why I need so much power.

You didn't really address the main question I had regarding the balance charger I added, are you just not familiar with that?

Thanks
 
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Old 07-20-20, 09:06 AM
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27 3'400 mAh batteries in parallel is 91'800 mAh then in a 4S configuration and you get a ridiculously s........l........o........w........ charging time with the little 50w charger you linked.

The cell you mentioned is unprotected. Is your battery management system is monitoring each cell individually?
 
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Old 07-20-20, 09:57 AM
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The charger is rated at 80W unless I'm missing something?
It's still rather slow, but I don't have any need for a fast charging.

I'm sorry that's it's so 'blurry', but It's hard to tell without the actual BMS model.
As far as I know, it should, but guessing that is as good as saying no, I kinda plan on replacing the BMS regardless, but I was only able to find for reasonable prices only BMS's with 4.28V over charge protection which sucks..
Anything better than that costs a lot of money which I'm not sure worth it.

There's also this balance charger which should be fine as well I guess? A bit more expensive
https://www.aliexpress.com/item/4000...archweb201603_

Thanks.
 
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Old 07-20-20, 12:19 PM
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I have used a two channel ISDT D2 charger for about a year and it is has been a really good charger. The only negative is they don't provide many/any cables with the charger so you may need to buy or make some cables.

I am not sure how a smart charger will act with a BMS in the loop. I normally do not have any BMS so all the charge control coming from the charger. Using both a battery management system AND a smart charger is a bit redundant. You have charge protection being provided by both which could cause an issue.

The thing that worries me is the large number of cells in parallel. I'd feel better using protected cells. You've done things right by using like cells and initially charged them to the same level. But... most packs are only paralleling a couple cells. You're doing 27 cells in parallel four times! The odds of having a cell in the pack go bad is pretty high.
 
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Old 07-20-20, 12:37 PM
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As far as I've seen(youtube videos) the BMS shouldn't really interfere the balance charger, but I can't really tell that.
As for cables, I pretty much have everything I need and have everything in order to make my own cables, so that shouldn't really be an issue I'd say.

I'm pretty much doing this as a "one time" thing, as I'd usually only want to charge my battery up to 4.1V per cell for longevity and safety. The issue is that since my battery is so incredibly unbalanced, I can't really do it right now, cause obviously if one cell is around 3.7-3.8V, the other cell is already at 4.2V and that's incredibly bad.

I really hope I won't encounter any issues in the future, but I did built the battery in a way that allows me to rather easily replace a battery or even a whole cell in case needed.

Yeah I understand it's not the most ideal possibly, and maybe it would've been better splitting up the batteries into 2 or 3 separate batteries, but that has already been done.

Maybe next time

Sounds like the ISDT charger should be fine I guess, do you have anything else you'd like to suggest before I do so?

Thanks!
 
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Old 07-20-20, 02:12 PM
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I wonder how your charger and BMS interact when charging. If the charger is trying to charge each cell to 4.25 volts but the BMS is set to a max cell voltage of 4.2. The charger will never reach it's cutoff point and will continue charging while the BMS is either blocking the current or is bleeding it to resistors to burn off that excess energy.

---
I use a 6S1P pack with 5'000 mAh LiPo cells when camping. I bought a circuit that reduces the output to 5v and feeds USB sockets for charging my phone and other devices. Charging is safe and easy since the charger can see every cell and balance them. The big negative is LiPo have to be handled more carefully. They are not forgiving like 18650.

---
What voltages does your equipment need?
 
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Old 07-20-20, 02:38 PM
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Interesting...Shouldn't LiPo be much more safe than 18650?
I'm not really sure about the charger and BMS, I'll obviously monitor that and in the worst case just remove the BMS or something, even though it seems like most BMS's are rated at over-charge rate of 4.28V, and I think the balance charger is rated at 4.2V, so it(once again, hate this word) should be ok.

As for my equipment, it's mostly running on 12V, but I'm settings my voltage to a steady 12.4V using a meanwell transformer. I believe that on average I have a power consumption of around 4-5 amps/hour.

I initially started with a 75AH deep cycle battery, but since you can't discharge it much it was running too low.
It's also so much heavier than a lithium battery.
 
 

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