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Lipo and LiFePO4 - resting voltage following charge

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Gordon Whitehead 119/01/2018 13:59:52
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After over 15 years of using lipos and leaving them charged up between uses, I've become used to them charging up to 4.2V per cell, and then reading about 4.15V after a couple of weeks or so.

Recently I decided to try LiFEPO4 batteries for my latest plane. I bought Turnigy 2S 3000mAh for the Rx, and Turnigy 2S 2100mAh for the onboard glow, which is the HK 20-second glow job so is only giving current during engine starting. Both batteries work well, but the rapid voltage drop from 3.6V per cell to below 3.5 over one day following a charge is giving me cause for concern.

Clearly the normal charged resting voltage for a lipo is about 4.15V. And I know that when delivering current the terminal voltage drops to about 3.6V per cell depending on the quality of the battery.

The nominal voltage of a LiFEPO4 is 3.3V. What I need to know is what an acceptable value for charged resting voltage is for a LiFE cell. This large initial drop fools my battery capacity meter and after a day following charge it displays a charge capacity of about 67%, compared to, say, 97% for a Lipo that's been standing around charged for well over a week. So the capacity meter must be useless for LiFE batteries, even though it gives reasonable results for Lipos.

Gordon

Dickw19/01/2018 14:41:04
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All a capacity meter does is measure the voltage and display it as a pre-programmed capacity for that voltage, so a capacity meter designed for Lipo will be useless for LiFe because they are different voltage cells.

Just checked a LiFe pack charged a few days ago and the resting voltage is 3.45 volts per cell, so your 3.5 volts seems OK to me.

Dick

Edited By Dickw on 19/01/2018 14:42:36

Frank Skilbeck19/01/2018 15:32:12
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Part of the issue with LiFe packs as well is their very flat discharge curve, they drop off a bit at the beginning and then very little until almost the end, hence the voltage difference between 90% and 10% capacity is very little, so the capacity meters struggle to give an accurate % remaining. I was worried about this when i first started usinng LiFe cells, so would recharge them just before flying when the capacity meter said 65% left, to find they would only take a few mahs and then be full again.

Steve J19/01/2018 15:45:13
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Posted by Gordon Whitehead 1 on 19/01/2018 13:59:52:

The nominal voltage of a LiFEPO4 is 3.3V. What I need to know is what an acceptable value for charged resting voltage is for a LiFE cell.

I would expect to see c. 3.45V/cell fully charged and 3.3V/cell after a small amount of charge has been taken out.

Steve

Steve J19/01/2018 15:53:51
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Discharge curves for an A123 ANR26650 2500mAh cell.

anr26650.jpg

Steve

Edited By Steve J on 19/01/2018 15:54:45

Gordon Whitehead 119/01/2018 16:01:55
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Thanks for your help guys. I feel much better about using LiFEs now.

My capacity meter is one of those that can be switched to check the three main lithium battery types, Lipo, LiFE and Lion. Naturally I was using it on the LiFE setting. I also double-checked the voltages using a proper digital voltmeter.

What bothered me was that after setting up servo throws on a new model, say 10 mins adjusting etc, I checked the capacity and voltage just for a look, and the indicated capacity had dropped to 65% or so. Recharging put only minimal mAh back in. A discharge proved that the battery had been almost fully charged.

Your experiences and figures tell me that I need not worry any more, so thanks very much for that.

Gordon

Edited By Gordon Whitehead 1 on 19/01/2018 16:02:14

Gordon Whitehead 119/01/2018 16:16:48
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Oh, one more question. The last lot of A123s I bought, sometime around 2010 or even earlier, were the discounted 18V Dewalt cordless drill packs from Ebay, for use with EDFs. These were "manufacturers seconds" with one or two duff cells but overall they were good value. In case guys don't know, it is possible to solder to the aluminium cases using normal lead solder and resin flux so you can readily make up packs to whatever configuration you want.

Now my A123s are quite old I use them for powering my starter, fuel pump and air retract pump.

Looking on Ebay, the only Dewalt packs available seem to be full price ones, and there are a number of cheap drill packs claiming to fit the Dewalt drill, but I'm sure that they won't contain genuine A123s.

So my question is, where do folks buy genuine A123s from these days?

Gordon

Steve J19/01/2018 20:56:07
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I'm afraid that I can't help with a source for genuine A123's. Ecolux do metal cased LiFe and LiIo in various sizes -

**LINK**

Steve

Geoff Sleath19/01/2018 21:39:09
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2274 forum posts
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The only way I've ever been able to solder successfully to aluminium was when I've connected to aluminium electrolytic capacitors cases years ago. Even then I was never really confident the joint was good. The technique was to move the iron to scratch the aluminium oxide that form on the surface. Not sure I'd want to trust a connection like that to a flight pack but Gordon is an modelling guru so I hesitate to criticise

Having said a one time mate could weld aluminium just using aluminium off cuts and nothing special as a flux. He was well known for ally motorcycle fuel tanks and fairings etc so he was a genius, too

All the batteries we use for aeromodelling have very flat discharge curves so using the off-load terminal voltage as an indicator of stored energy levels isn't ideal to say the least. Really the only reliable way to measure how much energy is restored on charge and have an idea of the rate of energy use so you don't flirt with potentially dead batteries in use. Having said that, I'm as guilty as anyone for checking my LiPo voltages after use to see how much there is still left in the 'tank'.

Geoff

Geoff

chris larkins20/01/2018 00:21:05
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147 forum posts
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One thing that surprised me (it has also being discussed in another thread) is just how little it seems to take to recharge a LiFe pack as opposed to the usual Nicad or Nimh pack, it may be due to the higher voltage. For instance I have a LiFe pack in a model which has 8 servos, I was worried at the beginning about the difficulty in judging the state of charge by voltage alone, however after a 10 min flight it is rare to put back more than 80 mah into the battery, at this rate I could have about 30 flights on my 2500 mah pack. I have since measured this on other models and in all cases the current used for each flight is surprisingly low. In addition even after sitting for several months they remain full.

Allan Bennett20/01/2018 07:51:36
1263 forum posts
31 photos
Posted by Gordon Whitehead 1 on 19/01/2018 16:16:48:

. . . . In case guys don't know, it is possible to solder to the aluminium cases using normal lead solder and resin flux so you can readily make up packs to whatever configuration you want. . . .

My experience with A123 cells harvested from DeWalt battery packs was that they were quite easy to solder with a suitably large iron. They didn't need constant scraping like when I tried to solder to regular Nixx packs, which led me to believe their end caps are not aluminium. Checking with a magnet showed some attraction, which would seem to confirm this.

When harvesting from a batter pack you'll probably find that the cells all have spot-welded connecting bars anyway, which look like tin-plate, and are certainly easy to solder to.

Cuban820/01/2018 08:14:07
1738 forum posts
2 photos

After experimenting with a LiFe RX pack, I found that all my standard Futaba servos were quite happy but a couple of hi-torque Hitecs went crazy at the slightly higher voltage. Anything over 6.2V and they didn't want to know (not centring well , jittering & generally very jerky).

Gordon Whitehead 120/01/2018 11:47:41
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178 forum posts
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@ Steve J. Thanks for the LiFE link which I've bookmarked. The variety available and prices are excellent.

Re Soldering A123s, my technique (which I found on one of the electric flight forums way back) for soldering copper or brass strip to A123s to interconnect them is as follows:

1. Abrade the end of the case to clean it up using no 240 wet and dry paper.

2. Coat the wet and dry with fluxite, and then re-abrade the case, ensuring that there's an unbroken layer of fluxite over the aluminium surface to prevent re-oxidation.

3. Tin the case using a 40W iron and standard 60/40 lead solder.

4. Solder pre-tinned brass strips and connecting wires to the battery.

Works well and is as easy if not easier than soldering to sub-C nicads.

Gordon

Edited By Gordon Whitehead 1 on 20/01/2018 11:48:11

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