See:

www.aliexpress.com/.../32987013457.html

I don't think that product has the same voltage. Need to be careful with overcharge ... also do we need to worry that the chemistry of the two batteries might not be the same, so the charger profile may not be the same?

Just being caution, as lithium batteries are not nice when things go wrong.

Hmm usually Lithium polymer in a soft pouch. If there are 2 wires it is not too problematic, as these are bare cells. If there is a third, then inside the 'battery' is at the least a temperature sensor, and in some cases a management chip. It is often possible with care to do giblets transplants int a new pack but it is a delicate soldering job and not for the faint of heart.

In this case I think the protection circuit is on the main PCB under the gold-coloured 'kaptan' style tape.

There is no universal numbering but where there is a standard of sorts it is to use the dimensions in mm as a part no with the thickness in units of 1/10 of a mm.

Does any of the documentation mention the battery capacity in mA hours ? apart from  cell type,  that is the main thing to get at least roughly right, I would not recommend it for professional use, but I  have done repairs for home items where I  have simply hot-glued a cell of the wrong shape but the right capacity on the outside of the case and led wires inside.

Mike

There are three wires and one does go to the PCB at the rear of the unit as what seems to be thermal protection. Unfortunately I have no documentation regarding the batteries so do not know what the capacity is. I will be replacing all three of the batteries at the same time. From what I can see online most of these are 3.7v not 3.8v. As far as I can determine the numbers do not match the dimensions in any way. They obviously mean something to the mystery manufacturer. 

Random nos could just be  a factory line no and date code. The 3 wire thing may be your hardest part - you will need to know what is in the battery end to keep the bit on the PCB happy. The simplest are a temperature dependent resistance to the negative terminal, but some are more complex.

Can you get clearer photos of the wiring and where it goes.  It looks like 2 cells strapped in parallel,

If so you can probably replace with a similar capacity, in a single cell of that is easier to get hold of. From the size you give I'd expect 500 - 800 mA Hrs per cell. so 1 to 1.6AH for the pair.

Also they do look like unprotected cells Far  more common is like this example below - where under the tape on a thin PCB is the low volt lock-off and charge limiting circuits, that prevent the cell being deep discharged or over charged (much below 2.5 V per cell internal shorts develop, and attempts to recharge can occasionally be explosive, and much above 4.2 per cell there is a risk of out-gassing - bad in a sealed cell ! ) There is also normally a fuse or the equivaent is performed in electronics. I assume these protective functions are elsewhere in the unit in your case.

Assuming it takes some charge you can confirm the end of charge voltage with a meter by charging it open as you have  it and putting a meter on the terminals. You may wish to tape/protect other parts while you do this.

Knowing the full and empty charge voltages helps identify the chemistry the charger is expecting - quite often the rated voltage of the cell relates so some odd average state between charged and flat and is the one voltage it never is.

Mike.

40*40*50