Parallel-ing batteries

SeeFernando · August 28, 2019, 11:05:21 AM

Hey Y'all,

Quick question about using batteries in parallel: Do C rating matter? Can I use a 4S 2200 40C in parrellel with a 4s 2200 75C?

I would fly it like a 40C pack, but I'm not sure if there are any concerns with doing this.

Thanks!

davidk · August 28, 2019, 11:26:17 AM

The only reason I'm taking a stab at this is because it's all new to me. And... this is a theory for me. No practical experience.

The C rating tells us how much power can be safely drawn from a battery under load.
If your flying is under 40C, then the 40C battery will happily deliver the power requested.
The 75C battery will also happily deliver power requested.

In a sense, the batteries don't really know their C rating... until you go beyond it. They don't care what power you're drawing, as long as it's below their rating.

Have I understood anything you guys have been telling me?

SeeFernando · August 28, 2019, 11:35:11 AM

I think I understand! If I were to try and fly at 60C, the 40C would not like it.

Or does the C rating get split/averaged?

davidk · August 28, 2019, 09:01:56 PM

There's no circuitry to average the draw... so the 40C could present with a lovely plume of smoke. While highly attractive, it's not conducive to a healthy plane.

pmackenzie · August 29, 2019, 11:33:41 PM

My 2 cents:

No additional circuitry is needed, the pack resistances will produce the current sharing.
Depends on the cell resistances exactly how things are shared.
C rating will be somewhere between 40C and 75C.

Pat MacKenzie

davidk · August 30, 2019, 12:15:35 AM

Right... the C rating would between 40C and 75C because 1 battery is 40C and the other is 75C.

As long as you discharge the mix at less than 40C then it should be fine.

VadimKirillov · August 30, 2019, 02:36:08 PM

It is actually even better , if batteries are in parallel you can draw twice as much Amps, so it would be equivalent of 80C from two battery pack (still should not draw more then 40C from the lower rated Battery) same goes for parallel charging.

pmackenzie · August 30, 2019, 07:04:42 PM

Quote from: VadimKirillov on August 30, 2019, 02:36:08 PM
It is actually even better , if batteries are in parallel you can draw twice as much Amps, so it would be equivalent of 80C from two battery pack (still should not draw more then 40C from the lower rated Battery) same goes for parallel charging.

Two 40C in parallel would not be an 80C. It is still 40C, but both the maximum current and the total capacity are doubled.

I thought about mixing different C rated packs some more, it is not so simple.
I think burst C ration would be average of the two, but continuous rating perhaps slightly higher than the lowest one.

My reasoning is this:
Under steady load the higher C rated pack would contribute more current initially. Simple circuit theory.
But this can't go on for long because its open circuit voltage would be dropping faster than the other pack, due to its higher state of discharge.
(terminal voltage is the same for both because they are in parallel).
As the run progresses the higher C pack will always be a bit more discharged than the lower C one, so towards the end of the run the lower C pack will have to
contribute more of the current.
Averaged over the whole run both packs have to contribute about the same total capacity.

Overall I think this needs to be tested , but I would suggest only paralleling packs of similar C rating.
A simulation would be interesting, using the known pack resistances and standard Lipo discharge voltage curve.

Pat MacKenzie

davidk · August 30, 2019, 07:34:10 PM

Quote from: SeeFernando on August 28, 2019, 11:05:21 AM
Hey Y'all,

Quick question about using batteries in parallel: Do C rating matter? Can I use a 4S 2200 40C in parrellel with a 4s 2200 75C?

I would fly it like a 40C pack, but I'm not sure if there are any concerns with doing this.

Thanks!

Chris,

What's the practical application of the question? What are you thinking of doing... with what plane, etc?

sihinch · August 30, 2019, 07:49:28 PM

He wants to power his house with Lipos......he's just working out how many in parallel!

SeeFernando · August 30, 2019, 08:00:58 PM

Quote from: sihinch on August 30, 2019, 07:49:28 PM
He wants to power his house with Lipos......he's just working out how many in parallel!

Diy Tesla wall? Hmmm

@David well my cub flies best with a 4000 but I've got plenty of 2200s of various C ratings. So figured I'd make a parallel cable for it.

I'm no engineer but I'm agreeing with Pat's thoughts.

So how do I fly it like a 40c? Is it just less full POWAH and more glide?

bweaver · August 30, 2019, 10:40:20 PM

In order to fly it like a 40C, when you see it smoken, back off a bit.

Chris, aren't you glad you asked this question?

pmackenzie · August 30, 2019, 11:23:38 PM

Are you sure you need 40C capability?

True 40C discharge would mean about 1.5 minutes of full power.
If you are flying for 6 minutes, pretty short by modern electric standards, that is about 10C.

40C on a 4400 mAhr pack is 176 amps.
On 4S that would be about 2600 watts , over 3 horsepower.

Hard to imagine a Cub needing 40C discharge

Pat MacKenzie