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New battery cell balance question

Old 12-05-2014, 08:42 AM
  #1  
xmech2k
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Default New battery cell balance question

OK, this makes me feel like a newb again, but gotta ask.

I got a couple of Admiral 6s 5000mAh packs from MotionRC. Got them, and as usual hooked up my battery checker immediately to see what state they're in. They were at an OK storage charge, but one of them had the cell voltages far enough apart to set off the imbalance warning in the batt checker. I guess the warning is just some arbitrary figure the people who made that checker programmed in, but it was a bit disconcerting.

I figure I'll cycle the battery a couple of times before I risk trying it in my EF Phantom. So I balance charge it 1C on my Hitech X1-200. It finishes, I plug it into the batt checker and warning again. Here's the numbers I saw:

Cell 1 - 4.178 Cell 2 - 4.156
Cell 3 - 4.165 Cell 4 - 4.175
Cell 5 - 4.135 Cell 5 - 4.165

The other battery had voltages ranging from 4.175 to 4.205.

When I used the charger to return them to storage charge, that same batt still set off the imbalance warning. I failed to record mA in and out. Will try another cycle and remember to record them this time.

Am I being too anal about hundredths of a volt? Though some are closer to a tenth of a volt. These batteries were over $100 each, and being from MotionRC was expecting better. Will cycling them help any, or is it time to give MotionRC a call?
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Old 12-05-2014, 08:50 AM
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Personally ? 4.135 vs 4.165 ... that's enough for me to question batterys condition after 1 cycle ...

But in honesty - I rarely check batterys on arrival ... I plug in - charge and use ... I would get inbalance alarm though if they were to be as you found even with my crap B6 ..

I am not a believer that cycling improves this sort of matter though ... LiPo's are not like NiXX .. that's what I found anyway. What I've seen is if a LiPo cell is down compared to others after charge - then it will usually always be down .. regardless of cycling / charge regime. Even if it does recover - it soon displays reluctance to play ball.

Nigel
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Old 12-05-2014, 10:40 AM
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I suspect it may say more about your charger or about your voltage checker than about the battery.

Those little voltage checkers are not reliable if you are looking for accurate measurement down to hundredths of a volt, and to be fair they aren't intended to be precision instruments. Much better use a multimeter before jumping to any conclusions about the battery or the charger.

Having said that, the charger you are using only has 200mA balance drain current (same as most 50W chargers). That means it will tend to struggle to balance large batteries if they are in any way significantly unbalanced. If you are charging large batteries or several batteries in parallel you should check the charger spec and get one with as high as possible balance drain current, many overlook this when selecting a charger but it's quite important.
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Old 12-05-2014, 11:16 AM
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Fastest balancer I know of is the Turnigy 420 charger. Charges 2S to 4S with selection for 500 to 2000 ma in 500 ma increments. The reason it is a fast balancer is because it handles each cell separately, thus balancing by charging at up to 2 amps.

A side effect is that its OK to plug in 2 different size 2S packs to charge as if 4S. Recommended you plug the larger in the port for cells 1 & 2 and smaller in 3 & 4 (and use the rate for the smaller capacity or lower charge current rated pack)
You can't safely do that with other chargers/balancers.

When doing mixed 2S packs
You can unplug and even change in a different pack on the 3 & 4 port without stopping the charge on the 1 & 2 port. Pulling the 1 &2 will stop the charge. cell 1 must be connected for 2 to charge, 2 must be connected for 3 to charge 3 must be connected for 4 to charge.
But no current goes to a full cell. even if 4 is the low cell

If the end voltage is wrong on any cell (and always the same) there are pots inside to adjust the stop voltage for each cell. Should only need to get this adjusted once... if the factory goofed. My 420's came within 4.19 to 4.21 on every cell... not worth the effort to dial it closer.

Cheap strange little charger.
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Old 12-05-2014, 05:44 PM
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Ugh. More to learn about electrons... Stupid question: Even if the charger had a low balance discharge, shouldn't it eventually get them to 4.2v, give a warning that it can't keep up with the balance, or even just time out eventually? I've experienced it timing out before on some well used batts. Or what if I didn't balance charge (Which I pretty much do every time), but did a normal charge?

Thanks for the info, and I'll look into stepping it up (again) with a better charger. I thought this one was pretty good, but I didn't know about the balance discharge rate thing at the time.

And I think I'll shoot MotionRC an email and see what they think.
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Old 12-05-2014, 06:27 PM
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Eventually an Astro Flite Blinky will balance anything that can be balanced... but 0.1v differential on a 5000 mah pack can mean a very long balance at 200 ma

Some of those stupidly long balance charges (timing out instead of saying done at 2 hours when set at 2C charge rate) are just the slow balance rate.
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Old 12-06-2014, 05:19 AM
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I've got an Astroflight "Blinky" balancer set up for A123 cells. At least for A123 cells, and their flat voltage curve, that POS was worse than useless for balancing.

For a big LiPo, that Blinky might balance a way out of whack battery in a day or three.

As for me, I've got a pair of $$$$ Cellpro Powerlab 8 chargers that have a one amp maximum balancing capability. That's enough to balance just about any way out of balance battery pack.

I'm charging my 6S2P 4600 Mah A123 packs at 20 Amps with the '8 chargers. Charge time is generally around 15 minutes. Before buying the second '8 charger, I was charging my giant scale 12S2P packs as a 6S4P pack with one Cellpro Powerlab 8 charger set at a charging rate of 35 Amps.
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Old 12-06-2014, 06:36 AM
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The matter of time to balance is valid - but I throw out this comment ... if a pack takes so long to balance that the charger defaults out on time - then maybe the pack is not as it should be ?

I don't use high power chargers - I am old school and still cling to the belief that slow charge is good charge ... I therefore buy extra chargers to augment my capability instead of one larger.
I have a few packs that do default out on time when balance charging ... needing a second START to finish .. usually not a long deal on second start. Those packs are good for low to moderate power models - but put in a heli or high power job and they show weakness in one or more cells.

Correct me if wrong - but I tend the think that if a pack is good on the charger - then it will generally be good in the model. If it shows any 'quirks' on the charger - it can be expected to show strange effects when put to use.

Nigel
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Old 12-06-2014, 07:26 AM
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Extreme current can drive even very high quality packs out of balance. No big deal if you balance every charge. But some still use chargers that don't balance.
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Old 12-06-2014, 08:08 AM
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Originally Posted by fhhuber View Post
Extreme current can drive even very high quality packs out of balance. No big deal if you balance every charge. But some still use chargers that don't balance.
Totally agree ... and it is also that a pack is actually not a good thermal design. You have for example a 4 cell pack .... the outer cells have access to cooler exterior ... but the inner cells are surrounded by heat ...

Push packs hard and that thermal issue can prove disaster for cells.

Also no two cells are same .. one will tolerate high demand better than another even from same batch production.

I think the difference is whether the pack is habitually going off balance or whether it's a one-off job.... and if it takes excessive amount of time to balance up.

Nigel
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