LiPos in Parallel

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LiPos in Parallel

Postby OpticalFlow » Sun Jul 25, 2010 11:35 am

Hi, I believe this is my first post here, so hi !

I've been enjoying the technical articles on this site for the last few months. Excellent, great work. They bring together a lot of technical knowledge in one place. In contrast to searching information on some of the big forums where really useful snippets of information are buried in threads that are many 100 pages long your well-written articles are fun to read with all the useful information in one place.

So, now a question: I have a 450 size electric helis and about 6 flightmax 3s 2200mAh (some 20C some 25C) batteries for it. Now I just finished building my first quadcopter with 4 DT750 motors. The first prototype frame I knocked together is made from solid wood and fairly heavy so the 4 motors draw a lot of current. In order not to have to buy new, large capacity lipo batteries I fitted the Quad with 2 XT60 connectors in parallel so that I could attach 2 of my 2200 mAH at the same time. I'm starting to wonder whether this is a good idea after all. Here's why: if I pick any two batteries out of my 6, they never will exactly have exactly the same voltage - even when straight out of the charger. So I have a slight voltage difference between the batteries (say up to 0.5 V) - I guess this will in effect cause one battery to charge the other one - until they're at equal Voltage, with a current between the batteries that will depend on the internal resistance of the packs. I wonder what Voltage difference between the packs is acceptable in order not to damage the LiPos. I know that the internal resistance of the packs will limit the current between the two packs, but I assume that this internal resistance is fairly low - otherwise the LiPos wouldn't be able to provide such large bursts of current.
Now, I have seen references to people using LiPos in parallel, but I haven't found a good technical discussion on how to handle this. Do you have to balance them during charging in a similar fashion as serial cells are balanced ? Any tips ?

BTW. I notice that there a three "Electric Power" sections in the forum, two for airplanes, one for helis. I would like to suggest that they could be merged into a single subforum as most things electric apply to both rotorcraft and fixed wings.
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Re: LiPos in Parallel

Postby RCModelReviews » Sun Jul 25, 2010 10:48 pm

You can safely parallel lipo packs but it's a good idea to parallel all the cells individually... ie: a 3S pack would have each cell paralleled to the matching cell in the other pack -- this is to reduce the effect of cell imbalance.

Taken to its logical extreme... imagine if one cell in one of the packs was so bad that its voltage dropped to (say) 2.7V rather than 3.7 and the packs were only paralleled by their two output leads.

This would mean that (in the case of a 3S pack) you'd have 11.1 V applied across the pack with the bad cell but the two good cells would end up with 4.2V across them rather than 3.7. In effect, they could end up becoming over-charged by the flow of current from the good pack and that could cause puffing/fire.

In practice, you'd be fine, so long as you balance charge the cells individually and avoid using any pack that seems to have one or more failing cells.

Or you could parallel each cell through the balance leads -- but they aren't designed to carry much current so that could create other issues if you end up with a dud cell unexpectedly.
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Re: LiPos in Parallel

Postby Mr Ed » Thu Jul 29, 2010 2:32 am

I guess I have been doing it wrong then, but the only thing I look at is the C rating and the MAh to make sure they are both the same size. I've hooked two 3S 2200MAh packs together one several occasions and never had anything happen. Both batteries were 20C rated and about the same age. I do balance charge all my batteries and try to keep the sells around the same age together for setting up a 6S or 8S pack and I even made a 7 cell pack and a 5 cell pack for testing a motor out. I used a 3S along with a 2S for the 5S and a 4S and a 3S for the 7 cell battery. I have never noticed any one cell going down faster then any other sell and since I always balance charge and all cells are pretty close at the end of the charge. I use the Cellpro 4S charger from FMA and I've never had a problem out of a battery.

I do have one question though. I was told by one guy on another forum that I can use a 2200MAh in parallel with a 3300MAh battery if I want to. Now I was always told that all cells are going to contribute equally in a parallel system and if you have a 2200MAh along with a 3300MAh battery, the 2200MAh is going to give out before the 3300MAh battery does. Well, from everything I have learned, that is a no no and not to do that. It seems to be wrong to me, but then again, I could be wrong. I have been wrong before. Any thoughts on this??

Ed
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Re: LiPos in Parallel

Postby RCModelReviews » Thu Jul 29, 2010 3:00 am

There's nothing wrong with just paralleling the main leads -- it's just that you then have the small risk of toasting a cell (with the resulting potential for fire). It'll work just fine (until it doesn't) but so long as you keep an eye on your batteries and don't use any that show significant imbalance you'll be okay.

You can also parallel batteries of varying capacity without too much trouble because they'll automatically deliver according to their capacity.

ie: when the 2200mAH pack starts to get down, its internal resistance will rise so the bigger pack will deliver more of the current being drawn.
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Re: LiPos in Parallel

Postby Mr Ed » Thu Jul 29, 2010 7:47 pm

Thanks. I guess that answers that question. Now I have another one. Don't you pretty much have the same risk of cooking a cell in a 3S pack that you would in a 6S parallel pack? I know they are in series, but you can still get a week cell in one and cook that cell too. I know it doesn't happen all that often, but if a battery is always changed through the discharge leads and never balanced, then sooner or later one of the cells will go low or high and make for a bad situation. You can also get a weak cell in a new battery. I had this happen on a new pack and only used once. I gave it the initial change and ran it through a balance charger and it looked good at the end of charging. I ran the battery and when I went to charge it I had 3.1, 2.8 and 3.1 on the cells. I did manage to get it changed back up and ran it again on the ground and got the same results. One cell low. I returned it and got a new battery and things have been fine since, but that one cell never did charge right. At the end of each charge, all three sells read 4.2, 4.19 and 4.2. It would seem that they were OK but in fact, that one cell discharged more then the other two. The dealer I got the battery from said there was a weak cell in there and changed it out for me, which is great, but that held me down a little as I was just getting started and that was the only big battery I had.

Thanks for taking the time to answer all of our dumb questions. It really helps. I have been in modeling for quite some time, but just started in electrics about 3 years ago and am still learning.

Ed
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Re: LiPos in Parallel

Postby OpticalFlow » Sun Aug 01, 2010 11:05 am

Hi, thanks for your reply to my question. I probably won't connect the balance leads (at least not permanently) because then I can't use the batteries individually on my 450 helicopter anymore (which sits broken on the shelf at the moment anyway). I will make sure though that I only parallel packs that have well-balanced cells and are at the same voltage.
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Re: LiPos in Parallel

Postby ransideous » Thu Aug 05, 2010 4:26 am

paralleling 2s packs for weight distribution on crawlers for 2 months now, no problems/issues so far in discharge or charge. test cell volts pre and post charge and discharge. mated 2 old 300mah 2s mini cub packs that are of different age and cycle counts, and then mated a bunch of new rhino 360mah 2s packs. so far nothing but convenience and convenience. quicker charging and better configuration options. i feel confident now in using this method on my planes. i think i will balance charge individually packs once every month or 2, just to catch out a bad a cell if 1 develops. just remember when balance charging in parallel through stock balance charge leads(on balance chargers that charge through balance lead), that most are rated for only 4amps! so make a new cable.
optical flow"Do you have to balance them during charging in a similar fashion as serial cells are balanced ? Any tips ?"
do you have to balance charge a single pack, no, but its better if u do.
in series balance charging you can monitor each individual cell on every separate pack. with parallel balance charging each cell count is paired, eg 2 times 2s lipo = pack one c1 paired with pk 2 c1, so charger sees one cell voltage. until i find a lipo balance charger that separates voltage and cell count this is the only way i have found to balance charge lipos in parallel. like the other bloke said, you can get a bad cell being hidden, so balance charging in parallel is a must. and individual pack charging once in a while helps to monitor bad cells.
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Re: LiPos in Parallel

Postby ransideous » Thu Aug 05, 2010 4:33 am

here's 50 odd pages of info, testimonials..... http://www.rcgroups.com/forums/showthread.php?t=932319
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