Hi, I have a bluetooth speaker that has a very short battery life due to the fact that it has only one 18650 lithum cell. I have 3 others that I want to add to expand the capacity. The speaker was built to have only one cell and I think the picture attatched is a BMS. Can i only use one battery or more if I hook them up in parallel? Thanks!

 

If they are all identical batteries and fully charged, you should be able to connect them in parallel.
But they will all have to be charged individually.

 

If they are all identical batteries and fully charged, you should be able to connect them in parallel.
But they will all have to be charged individually.

Thanks for your help! I though that it would act as one large capacity cell, but it makes sense why not, do you know why they discharge together but can't be charged together?

 

why they discharge together but can't be charged together?

When the discharge, if one has a slightly lower impedance, it will just discharge a little faster until its impedance rises and the others catch up, which is not a problem.

But when charging, if one has a lower impedance, it will hog the current and possibly overcharge with disastrous consequences (lithium-ion batteries have a tendency to overheat, blow-up, and catch fire if overcharged ).

 

If they are all identical batteries and fully charged, you should be able to connect them in parallel.
But they will all have to be charged individually.

That is totally incorrect ... Providing they are all single cell (3.6 nominal V) you can connect in parallel and the resulting pack can be treated as one cell and charged as if it were one cell

To keep things neat you may want to replace the single cell you have with a cell with good capacity . Manufacturers of such products invariably use low capacity , or even used cells (salvaged on an industrial scale from scrap laptop batteries) ... often the capacity is below 1000mAhs .
You can get cells with a capacity of up to 3500mAhrs ... Buy a recognized brand , Panasonic or Sony you cannot trust the capacity listings of unknown brands , no cell has been made with over 3500mAhrs capacity.

 

...But when charging, if one has a lower impedance, it will hog the current and possibly overcharge with disastrous consequences (lithium-ion batteries have a tendency to overheat, blow-up, and catch fire if overcharged ).

Totally ridiculous scare mongering .... If two or more cells are being charged then the very low heat generated by internal resistance is shared among the cells and dissipated more easily ... Multiple cells will Always be cooler

 

I think the picture attatched is a BMS.

One of its ICs has come unsoldered.

 

One of its ICs has come unsoldered.

It does look that way, doesn’t it? But it also looks almost as if it was intentionally connected like that. Notice the pad on the right which looks tinned and untouched. It’s hard to tell in that photo, though.

 

How is the charger connected to the speaker? By any chance is it through a USB connector? possibly mini or micro USB? IF that's the case, I'd suggest adding an external battery pack or power bank. That would make it a simple off-the-shelf fix if it's even possible.