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DIY Battery Woes
#1
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:

 

Thanks!

- Josh
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#2
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.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#3
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
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#4
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.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
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#5
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.
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#6
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
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#7
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.

 
 

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?
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#8
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.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#9
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...
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#10
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.
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