RC Model Reviews

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

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.

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.

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.

What charger do you use to charge A123 batteries?

I just use the cheap iMax 4-button charger.

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.

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.

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

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