DIY Battery Woes

meanjeans

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Nov 9, 2020
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Hello All,

I am new to DIY batteries and new to the forums as well. I'm having some trouble with (what I believe to be) a very simple battery pack and overall project and I think I might be missing something.

I'm building a small bluetooth speaker system (links to all components below) and the amp runs on 18 to 24 volts. I decided to add a 6S 18650 battery pack into the system and everything seemed to work fine through one charge of the battery. I bought the cells (Samsung 25R) from Liion Wholesale and soldered them to the BMS. I've read you have to be careful soldering to the batteries and I think I did a decent job of being quick with the iron and got everything put together. Each cell was reading 3.56v after assembly. I connected the charger I bought and it indicated that it was charging with a red light, and I watched the voltage creep up on individual cells.

The speaker was used several times over the next few days until it ran out of battery. I plugged the charger in, left if for a few hours, red light changed to green and unplugged the charger. The speaker would not turn on.

I took everything apart and the battery was reading ~13 volts but when I tested each cell I had three with nearly 0 volts and three at 4.6 volts. From what I gathered reading elsewhere, I have 6 cells that I should throw away. How did I get to this point? Any guidance would be appreciated!

Charger
BMS
Amp

Here is the schematic:


image_vrgoqs.jpg


Thanks!

- Josh
 

daromer

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Did you charge through the BMS?

There are 2 scenarios here

1. You didnt charge through the BMS and you didnt use a smart charger with balance function. This will cause this behaviour
2. You did charge through the BMS and the BMS is not working properly or not properly attached.


Since couple of cells are 0 you might have killed those cells. You can try to revive them but if they went all to 0.... They most likely are shorted and are better in the bin.
 

floydR

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None of the cheaper bms's I have seen have a positive connection on the bms, they control charging /discharging with the negative wire.
Pos-------------------- battery
Neg--------BMS------ battery
P- B-

later floyd
 

ajw22

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The link is to a 7S charger. It would have overcharged the cells of a 6S to a super dangerous level of 4.9V if you let it charge for too long.
The BMS should have cut off though... not sure what exactly happened without more data/tests.
 

jonyjoe505

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can you put a picture of your battery so we can see how the bms is wired. Even if your power supply is a 7s, the bms should have stop the charge/discharge before your cells got to 4.6 volts or 0 volts. With new batteries, they should all remain the same voltage during charge/discharge, you shouldn't have any balancing problems.
If you havent disconnected the bms, you might do that and check voltage again. Recently I had a 4s pack where some cells were reading high, I found a balance wire had come loose, once I remove bms the cells read normal.
 

floydR

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I admit I had not looked at this bms. It has a P- and a P+. As ajw22 and jonyjoe505 stated need more info and a pic or two would really help.
later floyd
 

meanjeans

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Nov 9, 2020
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Thanks for the feedback. I'm pretty sure it's wired as the diagram that is included with the BMS. One off the connections came off as I was getting out of my speaker.


image_cecivz.jpg


image_ziegjr.jpg


Regarding the 7S charger - I saw that in the title but I read somewhere that you want more than 24v to charge a 24v battery and I figured the BMS would cut off at the correct voltage. Is a simple charger like the one I linked to appropriate to charge a battery pack like this via the BMS?
 

daromer

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The BMS shut cut if off.

What wire did come off?

If you read my first post there are only 2 things that could have happened.
 

meanjeans

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The wire only came off (B5) after I took the battery out of the speaker. Everything was connected when it was charging. It was charging through the BMS - I have nothing connected to B+ or B-.

I'm just trying to establish what I did wrong and what to do differently if I build another battery. Different BMS? Different charger? Consider getting a spot welder (seems expensive)?

The thing runs off the 24v switching power supply, maybe I just choose to use the speaker near an outlet...
 

not2bme

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It's hard to tell but I suspect it is the BMS that is faulty. Next time I'd check to make sure the correct protection is in place.

1. Test for overvoltage (i.e. over 4.2V per cell or whatever is the overvoltage on that BMS)
2. Test for undervoltage (i.e under 2.8V per cell or whatever is the undervoltage on that BMS)
3. Test for one cell over voltage (i.e one cell over 4.2V, while the rest are under)
4. Test for one cell under voltage (i.e. one cell under 2.8V, while the rest are over)

Also I would increase to 7S instead of 6S, since your charger is for 7S (29.4V / 7 = 4.2V). Your charger is supposed to limit the charging current and voltage. At 6S it would be 29.4/6 = 4.9V (dangerous!). You shouldn't depend on the BMS to cut off the voltage. It's there for protection and not for stopping a regular charge. So it should ideally charge up to 4.2V while the BMS will disconnect if somehow it goes above 4.2V. Or if the 7S is too high, get a 6S charger instead at 25.2V.

And your wire should not just fall off, which may mean it didn't have a good connection to begin with. The wire may break, but it should not just lift off the battery with a simple tug. That's a cold solder which means it looks fine on the outside but it wasn't secure.
 

meanjeans

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Thank you for the input. I have one last question:

If I have a functional battery that is charging and discharging properly, If I choose to use the speaker connected to the 24v switching power supply, can the battery be wired up to that as well? Will it be damaged in any way, should I put a switch on the battery to take it out of the circuit for when I use the power supply?
 

daromer

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You cant parallell the PSU and the battery. You can attach the Charger and use it though. Unless you PSU is intended as a Charger?

Perhaps om missing something
 

Redpacket

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meanjeans said:
Thank you for the input. I have one last question:

If I have a functional battery that is charging and discharging properly, If I choose to use the speaker connected to the 24v switching power supply, can the battery be wired up to that as well? Will it be damaged in any way, should I put a switch on the battery to take it out of the circuit for when I use the power supply?

A typical switching power supply is not a charger, ie it won't self limit it's output current well & may shut off or overheat if the current the batteries pull is too high. You also risk putting too much current into the batteries (dangerous, causes heating). That said, 24V into a 6s pack would take the cells to a safe 4.0V each.

Best to use a proper charger for the pack, separate the battery when using the 24V supply (maybe use a schottky diode, low fwd drop).
 

ajw22

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meanjeans said:
Regarding the 7S charger - I saw that in the title but I read somewhere that you want more than 24v to charge a 24v battery and I figured the BMS would cut off at the correct voltage. Is a simple charger like the one I linked to appropriate to charge a battery pack like this via the BMS?

The terms "12V battery" and "24 battery" are very vague and can mean a lot of different things depending on context. Best to more or less ignore that and look at the exact voltages instead.

To fully charge aLiIon cell, itusually takes a4.2V source. With a 6S arrangement, that's 25.2V total. So you want to use a charger that outputs no more than25.2V, like this one:
https://www.amazon.com/25-2V-Charger-charge-Lithium-Battery/dp/B085RYV377
If you had a 7S arrangement, the charger you now have with 29.4V outputis just right.

Using a 7S charger to charge a 6S battery is... not ideal to say the least. Should only be done in emergencies to give the battery a limited charge boost.
Still not sure what went wrong in your case, but even in thebest case the cells will paradoxically not get fully charged (maybe to 80%).

Note that these chargers may look like a standard wall plug AC/DC adapter, but the key difference is that the charger limits the output current to a set level (0.95A with yours). If you tried charging the cells with a standard AC/DC adapter, a empty cell would likely suck several Amps of current, and overheat/overloadsomething.
 

meanjeans

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I have an update. I bought 6 new 18650s and I decided to use trays instead of soldering on the batteries. I purchased a 25.2v charger instead of the incorrect one that I bought originally. I used the same BMS as before, assuming that 29.4v didn't hurt it. Either 29.4v did hurt it, or it was broken for some other reason, because now I have 2 18650's reading 0v, 1 at 1v (and leaking fluid), 1 at 4v and 2 at 4.3v.

This is becoming an expensive battery pack!!

Should I keep the 3 batteries in the 4v range, buy 3 more and a new BMS and try one more time?
 

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pjones

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Oct 17, 2020
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Maybe throw away that BMS and get a new one, then experiment with a few decent shape used cells until you are happy with the results after a few cycles, then make it into a usable pack. If you want a cheap pack, you should buy one. :) Education isn't free, and that is kind of what you are really paying for.
 

italianusercs

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Feb 25, 2020
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What I do when I have a new device is to keep it under control at first usage, that is for the first 5 or 10 hours. And, yes, I'm sort of paranoid the next day, too, checking everything I can!

I do the first setup of BMS and batteries with the multimeter connected (even two multimeters) to see that everything goes right.

I do check everything before switching on and during first round because if it goes ok first go you'll probably be on the safe side. So, for sure, first things I check are, for e.g. PSU voltage output (I have some which are rated 12V on the label but output 14V), battery temperature and voltage during charge and do check for over-voltage conditions! That can be very dangerous because lithium cells burn at very very high temperatures. I remember the video of that cargo airplane which set on fire because of lithium batteries and nobody knew at that time what temperatures that fire would reach (if I remember correct if was over 1100°C).
 
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