LiPo battery packs in parallel

[Ron]

Being of an inquisitive nature, and having a couple of LiPo battery packs that I'd like to parallel up to give improved flight times, I was wondering - idly to myself, the way you do, what conditions may apply to the charge state when you connect them.

If one is (say) half charged, and the other is fully charged, does the full one try to dump loads of current into the half charged one?

Various sources on the web say you can parallel up series packs, so you could have - 3S 20C 2P - but that is in one already made up 'pack'.
So presumably all cells would be charged and discharged together.

But can you parallel up separate packs?
Sounds a bit dodgy to me.

Ron

[Happy Days]

Hi Ron

The answer is Yes,……….what was the question? :shock:  (I’m sure some TV ad used to have that as it’s punch line. Didn‘t it? :lol: )

I have a powered plane that uses two sets of batteries in parallel. The system has worked fine for the past 3 years.

As to your question about one “half charged” pack being “charged” by another pack with a higher voltage on it, I’m not too sure. :?  
Logic would suggest that it would, all be it at a very slow rate as the different potential between the cells only amounts to a very small percentage of their overall voltage.
However, I have a little thought that’s pricking the back of my mind :twisted: and telling me that Ohms Law stops one cell of the same type from charging up another. But having just written it down on paper I can’t see why one battery shouldn’t charge up another.  

HHHmmmmmmm Worthy of further thought Ron. :?:

Keith

[Happy Days]

Hang on a minute,………..The higher charged battery can’t charge the lower battery because there is infinite resistance between the battery contacts. i.e. there’s no circuit!

If you connect the positive and negative sides of the “pack” into a circuit load all the cells will start discharging, all be it at different rates depending on their internal resistance > state of charge. (Ignoring changes in internal resistance that occure through ageing and other minor matters.)

Well, I think that's right............. :roll:

Having thought some more on this matter, I've decided that the above is not valid.
I think the cells don't charge eachother because the potental difference is too low between the cells to over come the resistance of the connecting wires.

So endeth the lesson!

[Ron]

The answer is Yes,……….what was the question? :shock:

Ah I like that answer.
I think the question was 42.
No that's the answer to another question...

As to your question about one “half charged” pack being “charged” by another pack with a higher voltage on it, I’m not too sure. :?  
Logic would suggest that it would, all be it at a very slow rate as the different potential between the cells only amounts to a very small percentage of their overall voltage.

I guess this is the real answer.
With LiPo especially the voltage varies so little.

However, I have a little thought that’s pricking the back of my mind :twisted: and telling me that Ohms Law stops one cell of the same type from charging up another. But having just written it down on paper I can’t see why one battery shouldn’t charge up another.  

That's the thing. It seems like there would be nothing to stop it happening, but somehow.....

HHHmmmmmmm Worthy of further thought Ron. :?:

Oh no, not more thinking :cry:
My brain hurts  

Ron

[DaveMc]

Yes, you can connect 2 packs in paralell.

Make sure both packs are equal and are both fully charges to within 100 - 200 mV of each other.
Also make sure both packs are good and will discharge their full capacity.

It is easier to think of Voltage and Current in term of Water.

Imagine 2 buckets of water. Bucket 1 is full and Bucket 2 is 50% full. Now connect both of them with a pipe. What do you think will happen to the water level in the Full bucket?

The water levels will try to balance. Water from the full bucket flows to the half full bucket and both buckets will end up with the same water level.... ie 75% full each.

2 paralell packs of say 11.1V 2200mAh when connected in paralell will try to balance out any potential difference between them. Of course the above water analogy will not result in a fire
A weaker pack will result in a fairly massive current drain placed on the stronger pack.
Best case.... the good pack may get damaged and you will have 2 bad lipo packs.
Worst case... your model plane/car/house goes up in smoke.

Charge your packs seperately and look after them.
The harness you use to make the paralell connection must be able to handle the increased capacity of your new battery pack and therfor any potential increase in current draw.

God i hope the 6  pack i have just drank this evening has not caused me to advise you to do something which will cause a big bang!!!

D.

[Ron]

Thanks Dave,

Yes, I've been in electronics all my life so no bother with the buckets of water  :lol:  :lol:
But don't know much about the chemistry of LiPos, and knowing their potential for dishing out the current I was worried about huge currents out of one and into the other, before they balanced out.

Not sure, overall, if it's a good idea.
Maybe we need an electronic switch that can detect the falling voltage level of one and switch to the other.
(Thinks.. :?:  :?:  - marketing idea   )

A six pack?
Would that make it 6P ?
Or does that come later :lol:

[DaveMc]

Ron,

Yes I had a 6S 1P evening last night.

I think so long as you make sure both packs are individually charging and discharging equally, and that your paralell harness is rated to handle the potential amps involved all should be safe.

What we need is an ESC with a Dual input and breed that with Balance charger.

I might hook up a couple of packs in paralell myself. My 50" Edge used Lipo's and I am getting pretty tired of landing after 5min... So I am trying to avoid buying more Lipo's and just use the ones I have.
Lipo's are a bit of a pain in the arse to store and manage and I can't see myself trying to maintain any more than the 6 packs i have now.

Go for it Ron, it will be fine.

D.

[Happy Days]

Dave,

The harness won’t be handling any more current than it would with a single pack. Just that each battery pack is giving up half as much as it would, so it'll run twice as long. Think about it………The motor isn’t pulling any more current, it’s just running for a longer period.
So the same cables that are used for a single pack would be fine.

K.

[Aidan]

Yep, What Dave said.

You can connect packs of the same series cell count in parallel but they should be close to the same charge state when you connect them as they will equalise and you don't want any high currents while that happens.

I've been charging my indoor lipo packs in parallel for a while now (Up to 20 at a time!) which works in the same way, just reversed. I make sure all packs are within about 0.2V before connecting them to the harness and leave the packs to equalise for a while before starting the charge. While checking the advisability of this method I did come across some figures for the sort of currents likely to occur when you connect packs of dissimilar voltage but I can't remember the figures and never measured it myself. I decided 0.2V was safe and went from there.

So, I'd suggest you check your packs are at similar voltage before connecting them to the harness and let them equalise for a short while after hook-up before use. Other than that you should have no problems. There's no reason you couldn't charge them in parallel too but you wouldn't be able to use the balancing function on your charger (if it has one) as it's the overall pack voltage and not the individual cells that will equalise voltage. If in doubt, charge separately.

Aidan

[Ron]

Thanks guys.  :clap:
Yes I guess it's a matter of keeping an eye on fully charged voltage per pack.

Was thinking I might try a Schottky Diode to separate the packs, like this one.

http://www.westfloridacomponents.com/mm5/graphics/ds2/40CPQ060.pdf

And it will handle both packs.
But even so, you're still talking 0.5v drop at 20 amps.

Ron

[Aidan]

Thanks guys.  :clap:
Yes I guess it's a matter of keeping an eye on fully charged voltage per pack.

Was thinking I might try a Schottky Diode to separate the packs, like this one.

http://www.westfloridacomponents.com/mm5/graphics/ds2/40CPQ060.pdf

And it will handle both packs.
But even so, you're still talking 0.5v drop at 20 amps.

Ron

I don't think there's any need for the diode. It may even cause additional problems and is certainly introducing an extra potential point of failure.
The fact that the packs will equalise is not a bad thing! - you just need to ensure they do so safely which is as simple as checking the voltages before connecting them. The qualisation ensures that both packs are sharing the load and will maintain a similar charge state in use (providing they are similar type so that voltage sag under load is similar).

Aidan