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Old Jun 29, 2015, 10:17 AM
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LiFePo4 packs puffy?

I received two of the pictured packs today and they're both a bit puffy with cells not very well balanced. I'm tempted just to send them back, as I assume the puffiness isn't normal? I would have ordered Lipo's, but they need to be charged in the model, so that's something I won't do.
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Old Jun 29, 2015, 01:11 PM
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How well balanced?

Quote:
Originally Posted by GordonTarling View Post
I received two of the pictured packs today and they're both a bit puffy with cells not very well balanced. I'm tempted just to send them back, as I assume the puffiness isn't normal? I would have ordered Lipo's, but they need to be charged in the model, so that's something I won't do.
What are the cell voltages measured on your LiFe pack? LiFe and A123's do have a bit of variation between cell voltages, especially a day or so after coming off of the LiFe charger. If any cells are lower than around 3.3 VDC, you could have a problem.

What is important is the individual cell voltages under under a few Amps load. I'd charge it up with your charger. When each cell hits 3.60 Volts DC it's charged up. Don't worry about the battery flaming up like a LiPo can. A couple of years ago, I saw a 3200 Mah LiFe pack bent at 45 degrees after a straight in crash of a 50 cc gasser. (The models tail came off in flight) The LiFe pack was smoking a little bit from internal shorts. But after around 15 minutes the smoking stopped, and the pack still measured 6.6 VDC.

Then connect an automotive tail light or similar load across your battery. One lamp that will work is the #2057. The bright filament pulls around 2 Amps, the running filament pulls around a half Amp. Keep that load on the battery for around 15 minutes, then measure each cell individually while under load. They should be very close in voltage, within a few percent. If not, send it back.

DO NOT forget about the battery pack and run it dead. If the individual cells drop below around 2.0 VDC, That will damage them.

Out of curiosity, have you considered the A123 batteries? That would take 8 cells as 4S2P to match this LiFe, for $80 plus shipping. And you have to build up your own pack.

You can run a 4S2P pack at up to 80 Amps without issues. I've got a whole pile of 6S2P, one 6S1P, a couple of 4S2P's and a few others. They last a long long time, and with an aluminum jacket, no worries about puffing or similar. At 80 Amps for a 2 parallel A123, you get around 2.8 Volts per cell.

They can be charged at up to 10 Amps per cell, so with a high powered charger, you can charge a 12S2P pack configured as 6S4P for charging in 15 minutes. That's what I've got on my two giant scale models pulling near 3000 Watts.
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Last edited by vollrathd; Jun 29, 2015 at 01:37 PM.
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Old Jun 30, 2015, 07:14 AM
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I measured one pack at 3.193,3.198,3.286,3.232,3.208. I didn't bother measuring each cell on the second pack as #1 was 4.03 volts and the rest were about 3.2.

I gave the worst one a couple of cycles on my charger and that high cell always remained at over 4V when the pack was charged. They shouldn't be puffy and they were both dated as being checked in February, so I've sent them both back.

I've used A123's in a couple of my larger models up to now and love them, as you do. I'd dearly like to use them in this new model, but there just isn't space under the cockpit floor to accommodate another 20 cells. I've already got 4 installed (2S2P) for a receiver pack, have modeled my power setup in a couple of Calc programs and found that 10S2P A123 is about as close as I could get to the performance that the old Nicad pack gave.

I'd much prefer to use LiFe packs of some description, as half the cockpit has to be dismantled to get the batteries in and out. However, I can't find anywhere selling packs that will fit, work and be reliable. For now, I'm going to fly it on a couple of Lipo packs that I already own and will suffer the pain of adding lead to get the CG and of getting them in and out. I'll reconsider my options over next winter and maybe I'll have found something better by then.

Thanks for the help.
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