Life RX capacity

[Russ P]

I'm changing my rx batteries over to life 1100amh 6.6v. Having charged them to 3.6 v per cell, which is the recommended max charge, they are only charged to 62% of their capacity. If they are charged beyond 3.6v I get an alarm saying cells are over charged. Why are they fully charged but not to their full capacity?

[Martin Harris - Moderator]

How are you judging full capacity? With a LiPo checker perhaps? If so, it will tell you it's well below capacity. 3.6 V per cell is fully charged...

[Russ P]

I use a lipo checker, when balancing it shows the individual cell voltage, if using it as in a battery checker it's show sthe capacity as 62%.

[iqon]

you need another checker for life batteries

[Martin Harris - Moderator]

You can't use a LiPo checker to gauge the capacity of LiFe cells - it simply measures the voltage which is 3.6 V per cell and fully charged - your checker doesn't know it's a Life cell so thinks, hello - only 3.6 Volts eh? - so it's a part charged LiPo...I'll tell him that!

In fact, you may be overcharging the cells by the sound of it - you must use a dedicated LiFe program or charger.

[Bob Cotsford]

some checkers have a 'type' button which will cycle through LiPo, LiFe and NiMh. Mine defaults to LiPo which, as you've discovered, is a bit disconcerting when checking a LiFe!

[john melia 1]

The only way to check a life battery is to find out how much you put back into it after a session . The normal battery checkers dont work , efen the ones with the option to choose liFe

[john stones 1 - Moderator]

Iggy Pop ones work

John

[Peter Beeney]

Yes indeed, Russ, your checker seems almost to be saying that it’s actually not entirely truthful. It should in fact be telling you the pack is around 50% full. 3.6 is the correct voltage for a fully charged LiFe cell, but only half way there when charging a LiPo. The mid point between 3 and 4.2. Thus 4.2 - 3 = 1.2 ÷ 2 = 0.6 + 3 = 3.6 volts.

We consider the best way of monitoring a LiFe receiver pack is with an Hextronik on board monitor. These are very accurate. A LiFe soon comes off it’s top voltage of 3.6, down to 3.3, so the monitor will fall fairly quickly to it’s second green led, but then stays there and falls only slowly as the pack discharges. At the bottom there is a really bright red led which you cannot fail to miss. An essential safety addition, in my book.

PB

Edited By Peter Beeney on 08/08/2014 22:28:24

[Phil Green]

In developing a lipo 'fuel guage' display for Wayne Giles' LiPo ESR meter we found after extensive testing that a lipo is almost completely exhausted at 3.6v, more like 4% capacity, far from 50%. Half-capacity doesnt occur at the mid-point between 3 and 4.2, 50% charge is actually about 3.81 to 3.82v

Cheers
Phil

[Russ P]

Thanks to everyone for their comments.

will follow them up,

[Peter Beeney]

I think, Phil, that partly the problem stems from the fact that the manufacturer’s useful voltage range of a LiPo cell is considered to be from 3 volts to 4.2 volts. In the early days the centre point, or nominal voltage, was given as 3.6 volts, presumably so that the watt hours could be sensibly calculated, but then someone decided that he might get an edge by upping this to 3.7 volts. Now this is the universally accepted standard.

From your graph it would appear the useful capacity is 2,112 mAh, 2,200 less 4%, and the useful voltage range 3.6 to 4.2, with the ‘nominal’ centre point now at 3.9 volts. A 200mAh discharge rate is fairly gentle, I suspect that if this was a bit more aggressive, say at 10 amps, these figures would all be a bit sharper; and at 20C, 44 amps, quite pointed indeed!

However, it all does appear to highlight the difficulties in estimating a battery’s capacity from an open circuit voltage reading…

PB