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I have a 2s lipo baterry pack I would like to charge with a multi-purpose battery charger I use quite often for bigger balanced pack. That one (see picture) does not have a balance wiring harness and has only 3 wires and from other post of this site i understand the white one is a temperature control.

Should I connect the white to ground before charging? With a resistor?

enter image description here

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Unless the person telling you the "white for temperature" story is a manufacturer's representative, I'd be very suspicious. Its common for a LiPO battery pack to have a tap in between every cell, so 3 wires makes sense for a 2S pack. Assuming the color codes are typical, where black is 0V and the Red is the full output, check with a voltmeter I'll bet you'll see about 3.5V between the black and white, and about 7V between the black and red.

The unfortunate downside of LiPo packs is that you can not simply charge them with a "general purpose charger" as we used to with Ni-cad packs. With Ni-Cads, you could simply pass a current of about 1/10th the amp-hour rating through the whole pack, and this simple "trickle charger" would often work well for years with the pack for years.

But that's not the case with Li-Pos! LiPo cells in pack tend to get progressively out of balance over time, so they have to be charged with individual monitoring of the voltage across each cell. This, as you hinted at, is balanced charging, and requires the charger to shunt current away from cells that reach their full charge (about 4.2V), so that other cells can continue charging without damaging the first. That's important because overcharging a LiPo beyond 4.2V is a sure way to damage it. (another is letting it's charge drop much below 3V!).

So trying to charge LiPO cells without a charger specifically made for Li-ION/Li-PO cell technology will degrade the pack over time. And that is the best you can hope for. The worst is that the pack could become sufficiently out of balance to make one of the cell's chemistry very unstable, which could become a fire hazard.

I'm not trying to discourage you... I'm a big fan of LiPo rechargeable cells and packs. But they do require some extra investment, either in designing a true balanced charger, or purchasing one ready made. Fortunately the latter is often the easiest and cheapest option, because the hobby world uses so many LiPO packs, that there are plenty of good chargers around at reasonable cost.

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  • \$\begingroup\$ I do have a li-ion/Li-Po charger (IMAX B6) , what I want to know is how to plug that battery in the charger \$\endgroup\$ – Gael Cazes Nov 1 '18 at 0:17
  • \$\begingroup\$ I'm telling you that a charger that does not have connections for each tap position in a Li-PO battery pack simply is NOT a charger correctly designed for such packs. It is not possible to do balance charging on multi cell packs with just a single +/- input. It may be fine for NiMH packs, and it may claim to be a balance charger for Li-POs. It is not. \$\endgroup\$ – Randy Nov 1 '18 at 4:55
  • \$\begingroup\$ Consider getting one of these. Its good for 2S and 3S packs. They are cheap, re-labeled by several companies on ebay, and will do the job correctly. Your cell pack should plug right in with its existing plug. This is a China supplier. If you look around you might find a local supplier, for a few more bucks ebay.com/itm/… \$\endgroup\$ – Randy Nov 1 '18 at 4:58

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