Mystery third wire?

[PaulJ]

I have nearly completed my latest conversion and realised that there is not much room for the battery . I wanted to install a 2s 800ishMah battery but couldn't find one that would fit. However I found an 800 single cell and ordered two of them, intending to tape them together and connect them in series. When they arrived I found that they each have a small circuit board in them and THREE wires.

What is the third wire, can I still connect the cells in series and if so, what do I do with the white wires?

Paul

[Phil_G]

Hi Paul, it could be a temperature sensor output or an extra positive intended for balance leads when used in multi-cell packs, or it could be a 'raw' positive if theres a protection board under the heatshrink. Most of these small cells are intended for consumer electronics and as such many have a tiny PCB which cuts off the charge at 4.1 or 4.2v and cuts off the output if the cell drops below 3v or if excessive current is drawn. I think I can see a mosfet under the Kapton tape which suggests yours might be a protected cell. If you see battery volts across red & black then you can probably ignore the white wire.

If you're not confident, for transmitters I'm a big fan as you know of the PP3 Lithiums, EBL etc, these are 2S and 600mAh upwards, eg
https://www.amazon.co.uk/Rechargeable-K ... B083J2G1NN
https://www.amazon.co.uk/EBL-600mAh-Lit ... B01CQAVS7U
https://www.amazon.co.uk/EBL-Rechargeab ... B08HK8H564



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[PaulJ]

Hi Phil,

Many thanks for your speedy response. 'Fraid I have no idea what your ringed component is. I haven't charged them yet but if it helps, there is 0V between Black and White, 4.0V between Black and Red and 3.95 V between White and Red......?

I have tried a PP3 battery and it is a very tight fit with absolutely nothing to spare and if it moves at all it will foul one of the pots. However I have just found this, which I think should go in quite easily......https://www.ebay.co.uk/itm/235491126227 ... 5656378690 but I haven't got it yet.

Paul

[Phil_G]

Those voltages suggest the white is a protected charge/discharge lead as we thought. Just use red & black as usual, ignore the white.
Is it the 17mm thickness of the PP3 thats the problem Paul?
Width of two 14500 cells is at least 28mm, 3mm bigger than a PP3.
Length is about the same 50mm if you include the PP3 snap connector.

[PaulJ]

Phil, The maximum dimension I can accommodate is about 46mm...... If I use a PP3, there is no room for the plug-on connector on the top and I will have to solder the wires to it...... That may yet have to happen but it would not be ideal.

2 x the 1S cells that I started this thread with will fit easily, and maybe the 2S that I have ordered. If they neither work out, I will use a PP3 but mounting and securing it will be tricky.......

Paul

[Tobe]

Commercial LiPo in consumer products are equipped with a short circuit and reverse polarity protection and this can come in different set up. The third wire is called a sense or temperature wire, most of the set ups will work fine by just using the regular red and black but there exceptions.
If it's a sense wire normally it needs a signal to allow charging....but you could just remove the board and connect directly to the 2 terminals on the battery itself.

[PaulJ]

Many thanks Phil and Tobe for your responses. I took your advice and removed the mystery board from each of the cells. I then put the cells together, soldered the tabs in series, added a lead and balance lead from an old knackered battery and BINGO! I now have a 2s 800mAh Lipo that fits! I will post a picture of it in place when the tranny is finished.....

For what it's worth, and I don't know if you can tell what it is from this picture, but this is the board that was in the top of each of the cells.....

Paul