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Old Feb 07, 2011, 03:14 PM
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I tried searching, I promise... max voltage for LiFePO4 pack?

I swear I tried searching the forum first and I couldn't really come up with a definitive answer. So, here goes the question that's probably been answered 1,000 times:

Situation: I am an all-electric flier with a large range of airplanes. Everything I own flies off a BEC of some sort, with the exception of two planes... my Morris Hobbies "Big Boy" (80" WS, 7 standard analog servos) and my GP Ultimate Biplane (65" wing, 7 high-torque/analog servos). Both of these have NiMH receiver packs; one is a 4-cell pack and the other is a 5-cell pack. My Futaba 9C transmitter has an 8-cell 1900 NiMH pack as well.

Problem: My beloved and trusty Ace Super DigiPulse charger is dying... I had two of them, and one died last year. Procured another one in a trade and this one is experiencing problems as well (the display is non-operational; only have LED lights that indicate charging.... if it loses power, there's no way to reprogram the charge rates without the display). It's just a matter of time before it dies completely, leaving me no convenient way to charge these two planes as well as my transmitter. I thought this would be a good time to switch my remaining items over to LiFePO4 batterties, rather than try to buy yet another DigiPulse. LiFePO4 packs do not require a voltage regulator, and I have the infrastructure to charge them properly.

Question: What is the max output of LiFePO4 cells? I know that LiPo cells are 3.7V/cell nominal and 4.2V/cell full charged, and LiFePO4 cells are 3.3V/cell nominal... I do not know the voltage of a fully-charged LiFePO4 cell so I want to know this to make sure I don't fry my transmitter, 2.4 FASST receivers, or servos.

Thanks!
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Old Feb 07, 2011, 03:18 PM
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Old Feb 07, 2011, 06:02 PM
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Thank you, Rampman. So a fully-charged 2-cell LifePO4 pack would be 7.2 volts. How does that compare to a fully-charged 5-cell NiMH pack? They are close, I assume, which is why it can be used with regular servos and receivers?
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Old Feb 07, 2011, 06:11 PM
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Depending on the specific brand of LiFePO4, 7.2 volts would be the maximum that you should see when you 'plug it in'. My A123 cells typically 'settle' to ~3.45 volts a few hours after charging. A 2S pack would be about 6.9-7.0 volts. NiMH cells are typically ~1.45 volts after charging. 1.45 x 5 = 7.25 volts.

Bottom line - If a 5 cell NiMH pack works for you, you'll have no issue with a 2S LiFePO4 pack.

Mark
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Old Feb 07, 2011, 06:14 PM
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http://www.rcgroups.com/forums/showt...light=receiver
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Old Feb 07, 2011, 07:34 PM
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Thanks for asking the question Mike, I have been looking for simliar info lately as well. I have similiar set ups and have been considering going the LiFe route but in my planes I have digital servos. It seems I read somehwere that the LiFe and digital may not work well together. Does anyone have any comments to that?

These just seem so much nicer than the BEC route. I just don't understand taking a 6S and converting it to a 6V for the rx....
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Old Feb 07, 2011, 08:07 PM
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Perhaps use the link I posted and read post #1,26,27 and 29 at mininum.

Charles
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Old Feb 07, 2011, 08:18 PM
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Thanks for the replies, everyone. I'll go ahead and order a Hyperion LiFePO4 pack or two.
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Old Feb 07, 2011, 09:18 PM
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Quote:
Originally Posted by Mike_Then View Post
Thanks for the replies, everyone. I'll go ahead and order a Hyperion LiFePO4 pack or two.
What is the benefit over a123?
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Old Feb 07, 2011, 09:54 PM
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It's funny you guys ask this question. I read a ton about LiFeP04 and then decided to go with them. I"m running them on two helicopters and love the setup. I've occasionally forgot some of the data regarding these cells and have had horrible times finding it again.

Charles' thread is very helpful but occasionally I feel stupid reading them. He's providing a ton of information but like any good teacher he leads you to your own conclusion rather than telling you his opinion. Does make it more of a challenge though sometimes when I read it and then reread it and still am not sure what it all means.

My 2 cell LiFe's come off the charger at 7.2 volts but they then quickly settle (5-10 minutes I guess...haven't timed it) down to 6.7 or so volts where they will basically stay for a long long time (even under load).

The thing you need to be careful of is that the power curve is super dooper flat. They hardly drop at all so it's very difficult to estimate how many mAh is left in them. I just peak them every night or right before I fly. I bring 3-4 batteries to the field every day and so far my flights are pulling 100-120 mAh out per flight. I'm running a 1400 mAh pack so I figure I can fly 4-5 before I'm even at half a pack. Been working like a charm so far.

Fingers crossed it stays that way.

I do have a quick question that I swear I've read but have had trouble finding again.

It's No problem storing fully charged right? It's not going to shorten their lifespan like Lipo's.....right?
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Old Feb 07, 2011, 10:16 PM
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That shouldn't be a problem for me as these are flight packs, and I always charge them the night before flying. Now I don't have to kick myself if I forget to charge them as they can quickly be topped off at the field.
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Old Feb 07, 2011, 10:16 PM
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Unlike a lipo, storing at full charge is OK.
Rick
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Old Feb 07, 2011, 10:45 PM
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Quote:
Originally Posted by rampman View Post
Unlike a lipo, storing at full charge is OK.
Rick
Thats a bonus!
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Old Jan 13, 2015, 10:59 AM
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I just bought a ZIPPY Flightmax 8400mAh 4S2P 30C LiFePo4 Pack

http://www.hobbyking.com/hobbyking/s...dProduct=20645

I am going to use it on my experimental aircraft (hy-tek hurricane) to power the avionics and starter. After a full night charging, I was dismayed to see only 3.45, 3.46, 3.49, and 3.51v per cell, for about 13.9v total.

Since I am used to seeing 4.2v per cell with Lipos on my R/C planes, I was getting ready to send this LiFePoS back to Hobby King!

Then I read this thread, and learned the voltage will settle down after 10 minutes of charging to right around what I am reading.

Thanks for the info! Now to finish soldering this baby in!
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Old Jan 13, 2015, 03:06 PM
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Looks like everyone keeps forgetting about Google. Amazing what they know.

Les
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