LiPo's in parallel?

Roger,

Please don't do this. You won't be able to be sure that all the packs are at the same charge level before you connect them in parallel especially after use.

The only time that it is relatively safe is to connect the packs immediately after charging when they are all fully charged and even then only do it after checking the pack voltage.

A better approach would be a multiple output charger: each output charges and monitors one LiPo cell but they can stay in series and all the charging circuitry can be fed from the same power source. That way all you are doing is creating copies of lower wattage charging circuits rather than one big charging circuit. Ok it means more work to calibrate and more expensive & complicated construction but it would make it easier to keep the cells within tolerances and safe.

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I agree with Chris.

You are literally playing with fire when trying to charge packs in parallel. If it were that simple, you wouldn't see so many different sizes of packs available.

Ed Cregger

Yeah, but you can buy packs already wired in parallel. The key should be to have them at the same voltage level before connecting the leads. Another RCer told me that they connected a 4.2V cell and a 3.8V cell and only had 5A flow between them and it diminished quickly. Alghough I don't recommend any connection that would cause cause more than 1C current to flow into a cell, I don't think paralleling equal voltage cells would be any more dangerous than charging LiPo's is. However, if one pack would go bad, it may be relying on the fuses to protect the other packs.

I think I'd be better off with a multiple (4 or 5) port charger that has

250W power like the Hyperion 0610i does. I wouldn't have to match the cell voltages before connecting. Just trying to come up with an easier way to handle multiple packs without lugging around multiple chargers.

RogerN

Yup. The thing here is that as long as the packs are of the same chemistry/construction style and from the same manufacturer, *even if the cells are of differing capacity* you can connect two packs in parallel *once you are sure they are at an equal state of charge*.

If you are not sure, don't risk it.

Now once they ARE in parallel, you can charge and discharge as a single pack.

But the moment you use them independently you MUST go through the above procedure all over again to make sure the state of charge is the same.

The reason for this is that *as long as the cells are identical chemistry and construction style*, there is a completely unique relationship between voltage and charge state. So they will distribute charge and current correctly, and stay in balance. This is of course how large capacity packs are made up: From identical cells in parallel..

What is distinctly dangerous, in order of increasing insanity, is mixing cells of different quality - say 15C packs versus 25C cells - from the same manufacturer, mixing cells from different manufacturers, or connecting cells in parallel when at a different state of charge.

You MAY be lucky. Do you feel lucky today?

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If it was not you or Red Scholefield saying this, I would not have believed it would ever be recommended. I learned something today.

Ed Cregger

Point well taken and I would prefer to error on the side of safety but... My charger doesn't have a separate setting for 15C LiPo's vs 25C LiPo's. Since they all charge to the same 4.20V/cell, and all are recommended to not discharge below 3V/cell, and they all charge from the same algorithm, shouldn't they work in parallel if you're not pushing the limit? I mean if the maximum recommended charge current is 1C, you don't want to exceed that but as long as you stay within the recommended bounderies I would think it would be OK. It's not like paralleling LiPo's with LiIon or some other chemistry that has a different charge algorithm.

To match them, what I was thinking on doing is after use is balance charge/discharge them all to the same voltage, then parallel them with a small resistance and a fuse between each connection. Then they could all be held at storage voltage or all be charged together. If one cell went bad, it would blow fuses at perhaps 2C and not be able to electrically damage the other packs. The fire might damage the other packs though! :-)

But I do think the better solution would be to connect all packs to a multi-port charger and charge them independantly. That way you wouldn't have to worry about getting them to the same voltage before connecting them.

RogerN

The last time I got into parallel cells-- needed added resistors would work if they had the right positive temp coefficent. series diodes plus a small value resistor can be a viable alternate solution if the few tenths of a volt drop across the diodes is acceptable.