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Old Jan 11, 2004, 05:09 AM
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Some data on LiPo cell imbalance

I'm trying to collect more information on cell imbalance. So I ran a lab test on a Kokam1500H 3s1p pack. This pack has 15 cycles in a Speed 400 MicroJet, and it is equipped with LiPo-Balancers. I used a modified Ginzel Spectra II charger and other equipment to achieve accurate results.

I charged the pack to 4.205 V / cell. Then I discharged it at a very moderate (1.8A) current level to 3.0 Volts/cell.

When I recharged it to 4.205 V / cell, the difference between the three cells was approximately 3mAh. (35 flashes of one balancer before the others started to flash).

Assuming that the imbalance accumulates and that most chargers leave a safety margin of 100mV / cell which corresponds to roughly 100mAh for the 1500H, the imbalance (if the pack had no balancers attached) would become critical after approx. 35 cycles.

We need more tests to see if results for other packs and higher discharge currents are similar.

Unfortunately, measurements at this level of precision are difficult and time consuming.

I know that this raises more questions than it answers, but it's a start. I will try to think of better tests and post my findings. Any suggestions are welcome.
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Last edited by Suzanne; Jan 11, 2004 at 05:17 AM.
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Old Jan 11, 2004, 07:37 AM
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Re: Some data on LiPo cell imbalance

Quote:
Originally posted by Suzanne
... Any suggestions are welcome.
Charge all cells in a pack individually or in parallel. Avoid the problem.

- RD
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Old Jan 11, 2004, 08:32 AM
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Does that really work? What if you had a pack that was 75% capacity and one that was 50% capacity. If you charge both of those in parallel do they still charge correctly?

Thanks,
Kenny
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Old Jan 11, 2004, 08:47 AM
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Re: Re: Some data on LiPo cell imbalance

Quote:
Originally posted by RD Blakeslee
Charge all cells in a pack individually or in parallel. Avoid the problem.

- RD
Yes, that's fine for small packs, if you can live with the additional wires & connectors, but not practical for my 10s4p Joker pack.

Even a 4s4p pack for a Logo10 would take forever to charge in parallel, and most chargers would time out several times before it's full.

Parallel charging is a clumsy workaround, not a solution.
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Old Jan 11, 2004, 08:50 AM
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Quote:
Originally posted by kenny_dilger
Does that really work? What if you had a pack that was 75% capacity and one that was 50% capacity. If you charge both of those in parallel do they still charge correctly?

Thanks,
Kenny
No. There would be a very high current initially flowing from the higher pack to the lower pack until they are balanced. This current would by far exceed the 1C max charge rate of the lower pack.

Therefore, the manufacturers say that packs that you want to charge in parallel must have the same voltage.
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Old Jan 11, 2004, 09:04 AM
luc
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The best thing is first to charge in serie and end in //; You save time, and at the end the amps slow down, so the equilibrium between packs is perfect.
Kenny, if you want to charge at the same time packs with a lot of voltage difference, no problem: just put them in // for max 1 hour before charging them. They will be much closer by that time and charging in // will close the remaining gap.
Sorry, suzanne to disagree with you, but if people can live with some multiplugs, this system has no flaws (and is free).
It allows also to use the same packs xP in different size applications: 3SxP (3 packs), 5s xP (5 packs)....
Lots of advantages .
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Old Jan 11, 2004, 10:43 AM
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Re: Re: Re: Some data on LiPo cell imbalance

Quote:
Originally posted by Suzanne
... not practical for my 10s4p Joker pack.

Even a 4s4p pack for a Logo10 would take forever to charge in parallel, and most chargers would time out several times before it's full.
Just don't use "most chargers".

For less than the price of your Joker pack, you can buy a charger which will do up to 30V and 20A:

http://www.radios4you.com/mastech-hy3020D.html

- RD
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Old Jan 11, 2004, 11:14 AM
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The advert says that Radios4you only deliver in the continental USA.

Can anyone recommend where to get the Mastech HY3020D, or a psu with the same spec, in the Eurozone, or preferably in UK? A google seach only turns up Radios4you

Thanks

Gordon
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Old Jan 11, 2004, 11:57 AM
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Re: Re: Re: Re: Some data on LiPo cell imbalance

Quote:
Originally posted by RD Blakeslee
Just don't use "most chargers".

For less than the price of your Joker pack, you can buy a charger which will do up to 30V and 20A:

http://www.radios4you.com/mastech-hy3020D.html

- RD
Even with two high current and voltage outputs (Mastech power supply), those of us flying larger electrics, and taking 5 or 6 10S4P packs to the flying field, don't want to spend half the week watching LiPos charging! Parallel charging for smaller packs is an excellent safety practice; however, I don't see it as a practical alternative to those flying with large packs. I agree with Suzanne on that issue. I believe it's the manufacturer's responsibility to come up with a safe power system across the full range of packs from small to large.

Michael
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Old Jan 11, 2004, 01:14 PM
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Critical after only 35 cycles? That's pretty crappy. Anyone more info on this?
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Old Jan 11, 2004, 01:22 PM
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There must be a better way than "parallel cell" charging ( a needless mega-amp demanding-meaning expensive, potentially more heat producing dangerous if just 2 wires get shorted or crossed and time consuming confusing with all those wires-connectors) to simply keep the cells balanced during charging and Suzanne's balancer boards seem like the safest, best "keep the charger affordable, simple and away from such "high" total recharging amp rates while keeping recharge times pratical........There's no doubt that parallel cell configuration charging probably self equalizes but i just don't like such "long" term high amp things going on with the charger requirements.......Just my intuitave confort-zone "feeling" about LiPo charging scene.......... kw
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Old Jan 11, 2004, 02:12 PM
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I'm afraid all you have shown is that your cell balancer creates extra imbalance, during charging, which it then re-balances.

I know from my own experience and real life use, that properly made packs do not have anywhere near that level of imbalance build up.
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Old Jan 11, 2004, 03:30 PM
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that's not true......Suzanne's balancer (automatic, precision, u-power voltage comparator controlled current shunt circuit) is one of a few proven, known methods (in addition to indivual cell charging) that uniquely and simply works with your existing, affordable CV-CC standard LiPo charger and tapped packs.......If you're into approved, pratical, series connected, 2 hr C1, standard pack charging, you're much better-off using Suzanne's or similar type balancer........ kw
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Old Jan 11, 2004, 04:15 PM
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Quote:
I'm afraid all you have shown is that your cell balancer creates extra imbalance, during charging, which it then re-balances.
No.

It was one of my design parameters that the balancers must not create a noticeable imbalance themselves.

The balancers draw a MAXIMUM idle current of 4uA @ 25C, rising to 8uA worst case @ 85C. Idle current variance between different units is approx. 1uA.

So the balancers alone would need more than 3 months to create an imbalance of 3mAh! And the time for the measurement was less than 3 hours.

But I agree completely with you on the other point:

Different packs will show different amounts of imbalance. Most likely, many packs will be better than mine, while some others may be even worse. As I stated above, we have very little hard data available, and I would help to have more.
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Old Jan 11, 2004, 04:46 PM
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Quote:
Originally posted by gotak
Critical after only 35 cycles? That's pretty crappy. Anyone more info on this?
As I said, that's the conclusion for this particular pack. It's true only if imbalance accumulates, which I suppose, but I don't know for sure, since all my packs have balancers.

I try to measure every pack that I can get hold off, and I have seen packs with major cell imbalance after only 12 cycles, and also packs with minimal imbalance after more than 50 cycles.

I'm still trying to figure why some packs hold up well and others don't. One key factor seems to be self discharge varying between cells and with temperature. That might be the reason why most often the inner (=warmer) cells show lower voltage.

My current theorie is that this "natural" imbalance may cause an unintentional deep discharge of the weakest cell, even when you discharge to an average of 3V/cell. This will further hurt the weakest cell, leading to a quickly degrading pack.

But again, at this point, there is much speculation and little hard data, and I will be the first to admit it.

All my friends are now using balancers too, so it has become difficult for me to obtain any more real world data of cell imbalance.
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