3 wire 7.4 volt 2 cell lipo Battery

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
Hello

A battery pack I am trying to replace has circuitry I'm less familiar with or I understand less than I thought. It has three wires coming from two 3.7V cells wired in series making it a 7.4V. But instead of finding a thermistor or thermally variable resistor wired into the negative I find what I think is a fixed surface mount resistor on a small rectangular board on the positive side of one cell that seems unaffected by heat or cold. Multimeter set to 2K ohms reading .555 Ohms. Number printed on resistor is 195. I suspect this may be a discharge or overcharge sensing circuit.

Voltage between Positve and Negative are the same as reading from resistance circuit.

Incidentally the failed cell doesn't appear to have any monitoring circuit and is puffed up and is failing to function properly as a result of breakdown.

I cannot find a direct replacement battery assembly for the Bluetooth speaker I'm fixing. I found a similar but larger one with 3 wires but I can't be certain the circuitry is right.

Looking for advice from anyone willing to give it, on sourcing proper sized battery cells to build my own from what I have or if I should buy the battery that seems close and transfer the board from the damaged pack to the new pack. Or anything other than these options I'm thinking of.

Thank you in advance
Troy
 

Alec_t

Joined Sep 17, 2013
13,232
But instead of finding a thermistor or thermally variable resistor wired into the negative I find what I think is a fixed surface mount resistor on a small rectangular board on the positive side of one cell that seems unaffected by heat or cold.
That sounds like a protection circuit, perhaps for cell balancing when charging?
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
Thank you, I think you're right. It doesn't make sense to me why both cells wouldn't be monitored. Or it might be if there is additional circuitry in the device doing the math so to speak.

In the end though I need to make my own replacement I think. To save on funds I think buying appropriate sized cells would be good but I don't know where to source them other than to scavenge from a different new battery pack. Finding the right size to fit the compartment seems daunting so far as well.

One listing for the exact batt on a China site sells min 10.
 

LowQCab

Joined Nov 6, 2012
2,650
It would probably be cheaper in the long run to just buy a new Bluetooth-Speaker.

If, for some reason, your new Batteries don't play well with the existing Circuitry
You could find yourself with 2 new ruined Batteries, or, in the worst case, a fire.

Will your Bluetooth-Speaker work with a USB-Battery-Pack ?,
if so, I would completely remove the internal Batteries,
and just plug in a generic USB-Battery-Pack.
.
.
.
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
My mother bought one each for my kids and she's no longer living so I would like to do my best to repair it.

You raise an excellent point about just choosing any 3 wire battery and plugging it in. I wouldn't trust it without disassembly to see if it's made the same and if it wasn't wired the same I'm considering scavenging the tiny resistor circuit board and wire it in the same as it was.

It doesn't have usb charging, it has it's own power supply. It does play while plugged in but it's dying after a few songs unplugged. I will look at it's voltage to see if a separate power bank will work but I suspect it may need a battery inside and I can't leave a bad battery in it. Worth looking into at least.
 

Ya’akov

Joined Jan 27, 2019
6,857
Could you take some photos of the pack? Finding replacement cells should be hard at all, but maybe we will see something you are missing.
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
I just took a series of shots that might be helpful. I searched both the sticker numbers and the numbers printed on the individual cells. I was only able to find one source from China with the Gsp063093 for 10 pcs.

No luck when searching the number printed on individual cell.

Cell dimensions not including circuitry is about 87mm long by 28.5mm wide by 5.25mm thick.

One images show voltage between neg and pos. Another shows same voltage between negative and positive after resistor where blue wire is connected. No difference in voltage since there is no current flowing? Another shot set to 2k ohms between blue and red giving a value of .555 ohms.

One showing the small circuit board with resistor where you can see the traces go from positive side of cell not negative side.

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Ya’akov

Joined Jan 27, 2019
6,857
The normal marking on a LiPo indicates the nominal dimensions. If you measure the battery, you can find a replacement with that code. The numbers are sometimes slightly rounded, and construction can make something a little bigger than the nominal dimensions.

1670522879825.png
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
The normal marking on a LiPo indicates the nominal dimensions. If you measure the battery, you can find a replacement with that code. The numbers are sometimes slightly rounded, and construction can make something a little bigger than the nominal dimensions.

Very helpful to know what the numbers indicate. I will try again with the digits from the cell.
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
Thank you everyone for your thoughts and input. I've ordered 2 new cells that are slightly larger in size since I cannot find exact fit. It will require modifying battery housing but the project will be on hold until they come from aliexpress. Hopefully they aren't going to be paper weights. ;)
 

k1ng 1337

Joined Sep 11, 2020
701
BigCliveLive on YouTube does tear downs of random cheap power banks with little circuit boards hidden almost inside the battery. You may find some of the schematics he reverse engineers useful. They all seem to share the same components as far as the charge and discharge protection is concerned or at least a common approach.
 

sagor

Joined Mar 10, 2019
786
That is a 561 resistor (560 ohms), not 195. That is why your meter reads 555 ohms, close enough.
It is probably a sensor to detect uneven charging or if one cell is low or high compared to the other. That would be feedback to the charger, which would be designed to detect whatever the designer initially intended.
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
BigCliveLive on YouTube does tear downs of random cheap power banks with little circuit boards hidden almost inside the battery. You may find some of the schematics he reverse engineers useful. They all seem to share the same components as far as the charge and discharge protection is concerned or at least a common approach.
I've seen some of his stuff, I will go look through his vids again. Thanks.
 

Thread Starter

NegativeConfidence

Joined Dec 5, 2022
8
That is a 561 resistor (560 ohms), not 195. That is why your meter reads 555 ohms, close enough.
It is probably a sensor to detect uneven charging or if one cell is low or high compared to the other. That would be feedback to the charger, which would be designed to detect whatever the designer initially intended.
Thanks, I can see it now that you've pointed out my backwardness. I plan to transfer the circuitry to new cells I've ordered.
 
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