Jump to content

LiFe battery life


Tim Kearsley
 Share

Recommended Posts

No pun intended in the title!

I've not a lot of experience with LiFe batteries, so posting here for comments from any battery experts in the forum community. 

I have a Hangar 9 Spitfire - the 82-inch 30cc model, which I've electrified and runs on 12S LiPo power.  For the Rx I have two Vapex 2S, 2000mAh LiFe Rx batteries which feed into a 10-channel FrSky Access Rx for redundant power.  Now, the model hasn't been flown for probably three years and the LiFe batteries not touched in that time.  I thought I would cycle the batteries by discharging and then charging.  So I set my Graupner charger to discharge at 1A (0.5C).  To my surprise, the Voltage dropped to the cutoff Voltage I'd set on the charger (2.7V/cell) almost instantly.  Likewise, charging at 1A reaches the LiFe cutoff of 3.6V/cell very quickly.  Strangely, charging and discharging at 0.5A gives a much more sensible result.  The IR of the battery is measured by the charger at around 80 mOhm.  Both batteries exhibit the same behaviour.

So, are both batteries U/S?  It's disappointing if so because they've had hardly any use. 

Link to comment
Share on other sites

The batteries on our electrified pedal cycles are LiFe - 10s 36v 11AH.   They're the only ones I have experience of.  They have a fairly easy life (gets confusing doesn't it?) because the maximum power is 250 watts (which we use very rarely) and that's just shy of 7 amps. We've had them for 5 and 3 years and they still seem to be OK.  I get at least 100km/60 miles per charge with still about half left (measured by voltage which is a dubious measure).  They seem to be more robust than LiPo and charging seems not to be so critical. 

 

However, 80mohm ESR/IR does seem quite high for a 2S set up.  I usually consider a LiPo with an ESR higher than 10mohm/cell on its way out and really only suitable for modest performance.  I think 3.6v for a LiFe cell is its nominal voltage - just as 3.7v for a LiPo  is.  Fully charged it's nearer 4.2 volts/cell.  The charger for our bike batteries (10s) is 42 volts and there's a built-in charge control in the battery itself.

 

As far as nomenclature goes I get confused with LiFe (Lithium iron) and Lithium-ion.  Being neither a chemist nor a physicist (at least as far as batteries goes) I'm generally in the dark.

Link to comment
Share on other sites

Yep, I have found that quite a few of the soft LiFe cells have quite high internal resistance and give a big voltage drop if asked to deliver any load. The A123 round cells in a metal case though are a completely different kettle of fish, they are quite happy chucking out the amps, I have a 4s A123 pack that must be 10 years old (I bought it from Puffin Models), balance lead has broken off but I use this on my Glow/Petrol engine starter no problems turning over a 38cc engine (biggest I've got). Recently got some A123 round cells from Robotbirds with the idea of using one to drive dual glow plugs in series, but it looks like they will be too powerful.

 

But I also have had some pouch type Life cells that have been very good and hold a pretty constant 6.4-6.6v all day in my 1/4 scale gliders.

Edited by Frank Skilbeck
Link to comment
Share on other sites

I have had good service from LIFE batteries for quite a few years, and use them in both transmitter and RX . 
 

I have found their storage discharge to be very slow. However, if they are left in very cold environments or over discharged  I understand  they can be damaged though.

 

I wouldn’t risk flying your plane if the batteries go to minimum voltage on a 1amp drain. It’s better just to replace them .  LIFe batteries do need careful balance charging and monitoring what is going in and out of them on each flying session. I do believe their convenience makes it worth it . 

Link to comment
Share on other sites

Thanks very much chaps for your rapid and helpful replies.  What puzzles me is that if I drop the discharge rate to 0.5A they go on quite happily for a long time.  I realise that the discharge curve for LiFe is very flat and they sit at 3.3V/cell for most of the curve and then drop very rapidly right at the end.  I would expect them to supply 1A without any problem.  To my thinking, if the IR really is around 80 mOhm then 1A should only cause the Voltage to drop by 80mV.

On balance, I think the only safe thing to do is replace them.

 

Link to comment
Share on other sites

Measuring the internal resistance can be quite misleading as it can change with just about everything you do to a battery. It's not just something that can be done under one set of contritions. The things that can affect the IR are...

  • State of charge
  • Rate of charge or discharge
  • Temperature
  • Age
  • Number of cycles completed

Also, and dare I say it how cheap they were to buy.

 

 

 

Link to comment
Share on other sites

18 hours ago, Geoff S said:

The batteries on our electrified pedal cycles are LiFe - 10s 36v 11AH.   They're the only ones I have experience of.  They have a fairly easy life (gets confusing doesn't it?) because the maximum power is 250 watts (which we use very rarely) and that's just shy of 7 amps. We've had them for 5 and 3 years and they still seem to be OK.  I get at least 100km/60 miles per charge with still about half left (measured by voltage which is a dubious measure).  They seem to be more robust than LiPo and charging seems not to be so critical. 

 

Small off-topic observation... The maximum power your e-bike is putting out is likely to be way more than 250W, probably more like 500-700W. Street legal ebike motors from the likes of Bosch, Brose and Shimano will all be labelled with that 250W number but that is the maximum constant power the system is allowed to average, not the maximum peak power. However there is a lack of clarity over how that average is measured; basically it seems like a self certification by the manufacturers at this point.

 

The fact the motors go over 250W is universal though - here's data from a Specialised Turbo Levo eMTB showing a 700W plus output on a CE marked and 15.5mph limited bike! The pic below is also from Specialised's "independent" ? testing of some of the leading motors a few years ago...

 

Specialized-Turbo-Levo-2-point-1-motor-p

 

18 hours ago, Geoff S said:

...As far as nomenclature goes I get confused with LiFe (Lithium iron) and Lithium-ion.  Being neither a chemist nor a physicist (at least as far as batteries goes) I'm generally in the dark.

 

Lithium-ion is the overall family of lithium chemistry batteries that include LiPos andLiFe packs. For more detail see the Battery University site...

 

https://batteryuniversity.com/article/bu-205-types-of-lithium-ion

 

 

Edited by MattyB
Link to comment
Share on other sites

I assume since the horizontal scale indicates cadence that these are bottom bracket fitted motors.  Ours are brushless motors fitted in the front wheel hub of normal bikes (I've converted several for ageing cycle clubmates as well as for ourselves).  There are 5 levels of pedal assist and I usually have it on zero but level 1 on a drag (which I claim merely compensates for the extra weight!) and very rarely go over level 3.  I get just as out of breath as I did when I rode unaided up hill.  As our motors are physically disconnected from the pedals but require them to be turning before helping they may not behave as the curves indicate. 

Link to comment
Share on other sites

31 minutes ago, Geoff S said:

I assume since the horizontal scale indicates cadence that these are bottom bracket fitted motors.  Ours are brushless motors fitted in the front wheel hub of normal bikes (I've converted several for ageing cycle clubmates as well as for ourselves).  There are 5 levels of pedal assist and I usually have it on zero but level 1 on a drag (which I claim merely compensates for the extra weight!) and very rarely go over level 3.  I get just as out of breath as I did when I rode unaided up hill.  As our motors are physically disconnected from the pedals but require them to be turning before helping they may not behave as the curves indicate. 

 

OK, so you are right the hub motors won't behave quite like that as there is generally no dependence on cadence, they just want you to be pedallng to some degree. Even so I can guarantee that just like those in the graph your motor will give out more than 250W peak power in the higher assistance levels - if it didn't it would probably not be sufficient for most buyers interested in an ebike, especially in hilly areas. Also most of those conversion kits are sold globally and regulations do differ; there are plenty of 1kw kits available on ebay etc.

Link to comment
Share on other sites

I live in a very hilly area (Peak District) and you'd be surprised how much help even 250 watts offers when combined with a bit of leg muscle.  The only time I used bottom gear after a cycle camping tour of the Pyrenees was the hill into our village after riding from Manchester airport  - mind you I was mere slip of a lad in my 40s ?  Now over 80 I need a bit of a shove.

Link to comment
Share on other sites

On 11/11/2021 at 16:40, Geoff S said:

I live in a very hilly area (Peak District) and you'd be surprised how much help even 250 watts offers when combined with a bit of leg muscle.  The only time I used bottom gear after a cycle camping tour of the Pyrenees was the hill into our village after riding from Manchester airport  - mind you I was mere slip of a lad in my 40s ?  Now over 80 I need a bit of a shove.

 

I have ridden lots of ebikes (mostly bottom bracket mounted, but a few rear hubs too) so am well aware of what they are capable and the wattages involved. I am not commenting on your specific riding, just stating that whilst 250W may be what is written on it the vast majority of motors will peak out at way more than that - if they didn't they would be insufficient for omost riders when they got to a hill. The easy way to tell is to look at the controller unit - most 36V units tend to come with a 15-20A, showing they are capable of far more at the peak. 

 

Here is a thread from Pedelecs regarding the difference between the continuous rating (which is the 250W the manufacturer marks up the motor with) and peak. There is lots of other good info on there too, though personally having ridden a lot of the more recent BB mounted motors I don't think I could go back to the hub conversion kit motors which jsut don't have that same natural ride feel.

Edited by MattyB
Link to comment
Share on other sites

There is another thread on this topic somewhere here, something like LiFe v NiMh, with lots of input (quite a bit from me).

My Txs are mostly on HK 1500 LiFe on which they state to charge at 1C so I do. They last very much longer than 1900 Eneloops but do not believe the Tx voltage meter since they will always register 9.9V until they suddenly die. I soon got used to how much flying to do before a recharge was necessary.

Most of my i/c models are now on these ranging from 700, 1100 to 1800, often dualled up via an electronic switch. They are all Zippy Flight Max brand with exception of four 1600 Nanotechs which were all I could get during a lockdown. One of these failed a few seconds after a full charge so they are all destined for the bin.

The Zippies have never given a problem and even after a year of zero use will usually take no more than 20mA/h to come back to full charge. Fight times are considerably greater than with NiMhs in the same model but the only way that you can tell how much has been used is to recharge them since the voltage is constant.

A 2s is 6.6V which is lower than a freshly charged NiMh and gives me no problems on any of the vast number or servo types I have tried.

The only time I managed to kill some was by forgetting to disconnect from an electronic switch for two or three months!

Some of my club mates also swear by them and at the price you could replace the lot for very little cash.

Link to comment
Share on other sites

I thought I'd better post an update here as I realised this morning that I'd made a very silly schoolboy error!  I'm almost too embarrassed to own up, but it might just save someone making a similar error. 

Because I wanted to charge the LiFe Rx batteries in situ (not ideal, I know,  but getting at the batteries is a pain) I had made up a charging lead about two feet or so long.  Because I'm a twit, I used servo cable.  It turns out that the combined resistance of this, plus the internal wiring in the model,  plus the existing charge lead on the charger was dropping enough voltage to fool the charger.  So when I tried to discharge the battery at a couple of Amps the charger was "seeing" the battery voltage minus the Volts drop in the leads.  The result was that it was cutting out prematurely.  Conversely, when charging, the battery was never reaching full charge due to the Volts drop.

The moral of this story is keep your charge leads as short as possible and use decent wire! 

  • Like 1
  • Thanks 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...