LiFePo4 Batteries

Nicad, NiMH, Lithium, A123? If it's transmitter or receiver batteries you want to talk about, do it in this forum.

LiFePo4 Batteries

Postby beau0090_99 » Wed May 26, 2010 2:15 pm

Alright, I got my first set of LiFe batteries yesterday. I am pretty sure they are simply A123 knockoffs, they are the 26650 size (2.3 ah). My question is what is their relaxed voltage supposed to be? I am used to charging Lipos where once they are charged, they hold a voltage of about 4.2 v/cell, and with the LiFe, the charge voltage was set to cutoff at 3.6V/cell and it immediately put about 1300 mah into the pack (I made it a 2S for my Rx), but I never checked the voltage afterward until this morning and it reads about 6.72V, or about 3.31v/cell. I know the nominal is about 3.3, but I thought they wouldn't relax this much without being discharged first. What is your experience with these? Bruce, I think you have been using them as your Rx pack for a while, any thoughts?
Thanks,
Curtis
User avatar
beau0090_99
 
Posts: 28
Joined: Tue May 04, 2010 7:09 pm
Location: Eden Prairie, MN, USA

Re: LiFePo4 Batteries

Postby RCModelReviews » Wed May 26, 2010 9:07 pm

That voltage sounds about right.

LiFePO4s have a much flatter discharge curve than LiPos so they start out much closer to nominal and stay pretty much at 3.3v per cell until they're virtually flat -- then the voltage drops like a stone.
RCModelReviews.com, just the facts.
User avatar
RCModelReviews
 
Posts: 2120
Joined: Tue May 04, 2010 3:40 am

Re: LiFePo4 Batteries

Postby GeicoGecko » Sat May 29, 2010 1:51 pm

Would this voltage curve almost suggest they are not suitable for our application? What's the method of telling when they're about to drop or how much juice the packs have left in them? I'd be extremely concerned about a battery pack which I couldn't measure to work out its charge state.
GeicoGecko
 
Posts: 18
Joined: Thu May 13, 2010 12:32 pm

Re: LiFePo4 Batteries

Postby RCModelReviews » Sat May 29, 2010 9:31 pm

The interesting thing about LiFePO4 cells is that you can get a pretty accurate indication of their charge-state by throwing them on the charger.

They use a constant-voltage/constant-current charging system which means that during the first part of the charge, the voltage across them is slowly increased so as to maintain the programmed charge rate. However, once the voltage reaches 3.6V per cell under charge, the current starts to fall off.

By measuring the charge current you can get a pretty close idea of how much charge is in the cells.

For example, when I charge my 1100mAH A123 receiver packs, I know that once the charge current has fallen to 600mA the batteries are 85-90% full. At 300mA they're around 95% full.

It only takes about 30 seconds on the charger for the charge-rate to settle to a rate that gives this pretty clear indication of the charge-state of the cells.
RCModelReviews.com, just the facts.
User avatar
RCModelReviews
 
Posts: 2120
Joined: Tue May 04, 2010 3:40 am

Re: LiFePo4 Batteries

Postby GeicoGecko » Sat Jul 10, 2010 3:43 pm

What charger do you use to charge A123 batteries?
GeicoGecko
 
Posts: 18
Joined: Thu May 13, 2010 12:32 pm

Re: LiFePo4 Batteries

Postby RCModelReviews » Sat Jul 10, 2010 8:19 pm

I just use the cheap iMax 4-button charger.
RCModelReviews.com, just the facts.
User avatar
RCModelReviews
 
Posts: 2120
Joined: Tue May 04, 2010 3:40 am

Re: LiFePo4 Batteries

Postby labuscj1 » Sat Jan 01, 2011 12:46 pm

can I use 1350mah lifpo4 from Dealextreme to make RX pack for 40 size planes, and is it posable to put 2 sels in paralele to get a 2700mah battery packs.
Last edited by labuscj1 on Fri Jan 07, 2011 5:26 pm, edited 1 time in total.
labuscj1
 
Posts: 10
Joined: Sat Dec 11, 2010 1:45 pm

Re: LiFePo4 Batteries

Postby RCModelReviews » Sun Jan 02, 2011 5:47 pm

You can parallel LiFePO4 cells (just as you can with lipo) to get more capacity.

It would pay to cycle any cells you get from DealExtreme to check that they do deliver the rated capacity as a friend bought some (which turned out to be blue rather than green like mine) and his weren't anywhere near as good as the ones I've been (and still am) using.
RCModelReviews.com, just the facts.
User avatar
RCModelReviews
 
Posts: 2120
Joined: Tue May 04, 2010 3:40 am

Re: LiFePo4 Batteries

Postby labuscj1 » Thu Jan 06, 2011 12:36 pm

ok thanx for the tipp. but when I make the battery do I have to make it 2 seperate packs or can I make It one pack and ballance the 2 cells together
labuscj1
 
Posts: 10
Joined: Sat Dec 11, 2010 1:45 pm

Re: LiFePo4 Batteries

Postby Kamu » Sat Jan 08, 2011 8:40 am

Hi there,

Where do you get your cells from RCModelReviews? As well as your balance/charge leads with their associated JST connectors.
I would otherwise buy a Tx battery from HK but I usually order only when placing a big enough order to justify the shipping costs. If I can get these parts elsewhere sooner and make up my own pack, that's preferable than waiting till my next big order at HK (a month or two down the track).

Thanks,

Kamu
Kamu
 
Posts: 34
Joined: Sat Jan 08, 2011 8:25 am

Next

Return to Receiver/transmitter Batteries

Who is online

Users browsing this forum: No registered users and 0 guests