Mixing C ratings, in series or parellel?

[aaronski]

Hi guys, I have 4 6s5ah turnigy 30c rated packs, and a friend gave me 4 more 6s5ah packs, with about a hundred cycles, but only the 20c rated packs. I want to make a 24s10ah pack out of all 8, would it be better to build a 12s10ah pack rated 20c, and a 12s10ah pack rated 30c, then put those together in series, or parrallel each 20c pack with a 30c pack all the way up to 24s?

also, for charging, I'm currently breaking it down to 6s 20ah, and parralell charging with a balance harness, I'd imagine I'd need to do each type of cell separately.

 

[amberwolf]

Are you using the batteries at anything like their rated capacity? Probably not, so it won't really matter if they're in series or parallel. But since the internal resistance is different, there will be differnet voltage drop in the cells with different C-rates, when you do pull higher currents.

Shouldn't matter for charging, either, unless you are pushing charging currents higher than what the lower-rated packs are ok with.

 

[wojtek]

i dont think it will make any difference. 20c used cells will discharge faster so after your ride they will show lower voltage. And you will only get as much range as the 20c hit the low voltage cut off.

If you wanna charge them separately, best to build a 12s10ah pack rated 20c, and a 12s10ah pack rated 30c and then charge each of the subpack separately. This way you will charge them much quicker as all cells in each pack will have the same v and you won't waste time to balance them. [in case you are using eg hyperion eos 1420i net3 which balances all cells after charging them]

another question - are your cells and your friends' same brand? What you need to ensure is that both of those have same /similar discharge curve.

 

[aaronski]

Thanks, that's more or less what I thought.

the difference's should be: the 30c pack will have a lower impedance, letting more juice out to the wheel, and taking more juice into the pack during charging.
for discharge, under high loads, the 20c pack will sag more, and lose more amps due to sag/impedance in the cells, hence the 20c pack will discharge first.

if I were to parallel the 20 and 30c packs, the difference would be that the 20c would discharge more, sag more, and then be recharged slightly at idle by the 30c pack, which would add extra wear to both packs.

as for charging, I have a crappy turnigy eco8, that's worked fine for over a year now.

 

[dogman dan]

It would make sense to paralell the 20c with 30c, and let em equalize through the paralell connection.

I just had to do it different. I don't know why, but I just had to put my 20c lipo in pairs, and then one pair is 30c in one of my packs. Dicharging with a 40 amp controller, I see mostly about 1500w -2000w on the ca except for starts. So call it about 20 amps average. At 2c average, I can't for the life of me see any difference in how the packs discharge. Voltages identical, charge times within a minuite or two of each other, wh in nearly identical. Any difference is random, not the one pack different.

Basicly, unless you are shooting flames out the dick holes in your motor cover, it's not going to matter any. Even with a relatively strong 40 amp controller. It just doesn't pull 40 amps enough time to faze the lipo. In a race situation, that could change, but I doubt it would matter enough to care about even then. A really strong motor on a 100 amp controller might get you to a c rate that shows a difference, but then the flames come out so soon it may not matter even then.

 

[aaronski]

good point. I don't have a flaming dick mobile, just a 3k peak commuter. when it's in "big pack" mode it's like to be pulling 900 watts from a 10ah pack, so, um.. math... 2c? yea, that's not going to stress anything.

Last time I raced Luke, I was running 100v2.5ah, and even then, the phase wires went first.

-Aaron

 

[geetarboy]

I've paralleld several different configurations of 15c, 20c, 25c regular lipo and 25c nano-tech and found that the higher the c-rate the faster they discharge. When you parallel a 6ah 25c nano-tech with a 5ah 20c zippy, they almost even out by the time you hit LVC.

 

[dogman dan]

That makes sense, we were debating that on another thread about paralelling some really different c rates. In the real world test, the high c rate cells had to discharge some before thier resistance began to get closer to that of the lower rate cells.

Under intermittent use, they would show a voltage difference but then the low c rate cell would soon charge the high c rate cell. That's paralelled, not series.

At the end of the discharge, more of the power would be flowing from the low c rate. But again, at 2-5c I doubt you'd see a ton of difference, except the lower c rate lipo might be a tiny bit warmer than the higher rate lipo.

I guess the main reason I paralelled my packs by pairing like c rate, was so the paired packs would balance better when I do balance charge once in a while. In my case, I don't have the packs paralelled at the balance taps, untill actually doing a balance charge.

 

[John in CR]

I'm with Dogman, but I'd run some discharge tests to be sure, and if the capacity isn't close to identical then I'd parallel the different types. Your pack is only as good as your lowest capacity parallel group, and having different capacities in series increases the chance of one getting too low.

I've got some 6s5ah Zippy's and some 4s5ah Turnigy hardpacks, and due to the Zippy's being significantly lower in actual capacity, I'm going to have to take one of the Zippy's apart to get to 20s. I want to go full parallel cell structure anyway, so all the packs are getting open. I wish I didn't have to break up that Zippy pack, but I already tried two 6s + two 4s in series to get to 20s, but they got out of whack with every discharge. They're 25C and 20C respectively, but it's the capacity difference that matters.