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NiMh vs LiFe. What are the advantages and disadvantages?


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9 minutes ago, Martin McIntosh said:

When you already know that voltage alone will not indicate the state of charge then why fly a model so many times that you even get close to a flat battery? Have three or so flights then see how much a recharge takes. This is just common sense and applies to any type of battery. By the time that you get a low volt alarm then it is probably too late  anyway with a Rx pack.

Exactly what I do. Its a no brainer. Just keep an eye on the state of charge of the Life pack, as you do the Lipo. We have all got battery checkers these days, so easy to put an extender on the Life balance lead and job done. It really is that straightforward and certainly not a "black art"!.

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1 hour ago, SIMON CRAGG said:

Exactly what I do. Its a no brainer. Just keep an eye on the state of charge of the Life pack, as you do the Lipo. We have all got battery checkers these days, so easy to put an extender on the Life balance lead and job done. It really is that straightforward and certainly not a "black art"!.

Agreed. I think it's a no-brainer to check the voltage before every flight, with a load meter; it takes all of five seconds. Then one can envisage where the battery is on the discharge curve. (These curves seem to be rather similar for all batteries, although LiPOs fall off a cliff). A safe bet is not to fly once the voltage, under load, gets down to about 105 to 110% of the nominal battery voltage. I have never had any problems with NiMH batteries with this approach. The only problems I have had is when I have left a plane turned on by mistake at the end of a day - discharging NiMH batteries completely seems to destroy them.

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For what it's worth (although my telemetry voltage has compared well with measured voltages on test) I conducted a little exercise today after having set an arbitrary telemetry warning on my little Flair Legionaire and landing early yesterday due to it alarming.

 

On test, I took it down to the full discharge on my charger (set to cut at 2.9V per cell) at 300mA.  I then put it back in the model and after some extended control inputs - more than sufficient to perform a circuit and landing, put 3 servos on continuous test - they continued to perform perfectly for at least a minute.  Yes, there was a rapid drop off but there would have been more than enough time to land after an initial significant voltage drop was detected - especially as the load would be an order of magnitude less with only intermittent use of the controls instead of the continuous end to end movement on test.

 

My readings are distorted by the use of a silicon diode as a voltage dropper but I'm now very happy that an alarm set at the receiver voltage of 5.1V (let's assume a pessimistic 5.8V at the battery) will give a more than adequate warning threshold on this model - and there will be indications long before this is reached in a steady state if a few servos are used together.

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Further to my comments above on TGY Nanotech LiFe packs I was recently forced to use a brand new 1600 one on a model because the old Eneloop NiMh had virtually died due to negligence during the layoff. I had only 15 min. flying time with it but some Zippy 1100`s arrived in the post yesterday so I swapped it with one. On a discharge test the Nanotech only had 940 mA/hr left in! I was expecting around 1400 which would be reasonable since the model takes very little from a pack. Now a bit worried about my new 1:5 Chippy which has two of them via a Failover switch. Must do some more testing here but so far from the above I would say `don`t touch them with a barge pole`; Zippies are just fine. I flew my 2 servo Mercury Magna for about 10min. on a Zippy 700 and could only get 25 mA/hr back in.

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8 hours ago, Martin McIntosh said:

Further to my comments above on TGY Nanotech LiFe packs I was recently forced to use a brand new 1600 one on a model because the old Eneloop NiMh had virtually died due to negligence during the layoff. I had only 15 min. flying time with it but some Zippy 1100`s arrived in the post yesterday so I swapped it with one. On a discharge test the Nanotech only had 940 mA/hr left in! I was expecting around 1400 which would be reasonable since the model takes very little from a pack. Now a bit worried about my new 1:5 Chippy which has two of them via a Failover switch. Must do some more testing here but so far from the above I would say `don`t touch them with a barge pole`; Zippies are just fine. I flew my 2 servo Mercury Magna for about 10min. on a Zippy 700 and could only get 25 mA/hr back in.

These are the packs I use @ £4.05 each.  I have used Nano Tech, but have to say I am not a great lover of them.

 

Zippy Compact, the latest addition to our Zippy battery series offers excellent performance and high quality in a "Compact" size. These battery packs provide the same capacity and discharge rate as our Flightmax line but give you alternative sizes for applications that require a specific shape.

 

A perfect receiver size battery to replace those worn out old fashioned NiCD and NiMh receiver packs. Although only 1100mAh LiFePo4 batteries have a superior useable voltage range when compared with NiXX batteries. This battery comes with two output JR style plugs.

 

This pack will work with most receivers on the market without the need for a UBEC*

 

Specs:
Capacity: 1100mAh
Voltage: 2S1P / 2 Cell / 6.6V
Discharge: 10C Constant / 20C Burst
Weight: 65g (including wire, plug & case)
Dimensions: 72mm x 34mm x 17mm
Balance Plug: JST-XH
Discharge Plug: Standard JR Servo plug x 2

 

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  • 2 months later...

I have a couple of LiFe 2S2200 RX batteries that came with a model circa 6 years old, not much use and when I picked them up were around 50% indicated charge. The RX has a dual supply feature and air UC so the load is just 9 good size analogue servos hence nothing major for a 72 inch wingspan twin.

 

I have been setting the model up and using he existing RX batteries and recharged them a couple of times which got me thinking about testing them so see what state they are actually in.

On the charger in discharge mode and at 0.8 A (max rate!) it took a bit of time to get the 2200mAh down to just under 6V (2S pack) and then recharged at 1C. I was a bit surprised that the charge nearly equaled the capacity (for Lipo’s this is normally a good sign) and then ready for a discharge test.

Darn, my lipo discharger does not like the slightly lower LiFe voltage so it was old school resistor bank. For convenience I paralleled the 2S packs (2S 4400 mAh) and put them on a 2.5A load.

The surprise (or not!) is that while the current stood up and the voltage rolled quickly off to just over 6V for a couple of minutes the indicated capacity plummeted into single figures. Once the load was removed the capacity recovered, but only to sub 60% for a couple minutes at 2.5 A.

Batter info is:

2200mAh 2S 6.6volt Li-Fe Receiver Battery Specifications:

·         Application: : Aircraft Receiver, Aircraft Ignition

·         Battery Voltage: : 6.6V

·         Capacity: : 2200mAh

·         Cell Configuration: : 2S1P

·         Height: : 21mm

·         Length: : 70mm

·         Maximum Charge Current: : 2.20A

·         Maximum Charge Rate: : 1C

·         Maximum Continuous Discharge Current: : 44A

·         Maximum Continuous Discharge Rate: : 20C

·         Minimum Discharge Voltage: : 5.0 V (2.5 V per cell)

·         Normal/Standard Charge Current: : 440mA

·         Normal/Standard Charge Rate: : 0.2C

·         Watt Hours: : 14.52Wh

The question to the experts regarding LiFe batteries is: Does charge current (in mAh) normally have no bearing on battery discharge performance?

If his is the case then my concern (unlike lipo’s) is there is no indicator that there is a massive loss of capacity performance without doing a high discharge capacity test? Hence charging them up and putting  close to 2200mAh into them means I might only get 200 mAh out of them. Must admit I am surprised that the cells accept the charge without getting warm or false peaking and just stopping early.

 

Or have I just go a really knackered pair of batteries?

 

PS new ones on order and trying to work out how I test them on a annual bases to confirm they are ok to use.

 

 

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My inclination would be to do a representative test discharge.  An enthusiastic flight on an average model should use no more than 100 mA for 10 minutes - so say 600 mAh.  Your 2.5 A discharge rate isn't a fair test unless you can show that a new battery won't give similar results - there may be voltage drops in the thinner wiring and connectors typical of a receiver battery at higher currents. 

 

So perhaps repeat the experiment at 0.5 or 0.6A and see what capacity you get.  I'd be more than happy to do a discharge test for you as my charger is LiFe capable and I'd be interested in the results as I'm a long term proponent of LiFe cells.

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At the weekend I flew my 65" Chipmunk for a total of 14 minutes.

 

Started with a Life 1100 pack fully charged.

 

Charged after flying, and put back in 144mah.

 

There are several good discharge graphs available, so it is possible to compare your voltage (after each flight), with the graph to see where you are on the discharge curve.

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One thing I have noticed with some small lower capacity Life batteries is that they do have a significantly higher internal resistance than equivalent sized Lipos, so if you put any appreciable load on them you will see a noticeable voltage drop across the battery, not a problem on most sports models as the power draw isn't that high.

 

I do a 4s pack of A123 cells which I've used to power a model and now my IC starter, tried to find 2s packs using these cells but they don't seem to be available anywhere, if anybody knows a source that would be great.

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CW, you must always have your charger set to LiFe; it would appear that you were trying to use a LiPo charger. On my Overlander ones I scroll down to user program where the cell voltage can be changed from 3.7 to 3.3. It will then come up LiFe instead of LiPo. Most pack instructions say to charge at 1C.

They are not intended to be used under high discharge situations such as a starter or for electric power.

I regularly fly models with many digi servos on a single 1100, 2x 1100 or 1600. The recharge after one flight in each case is surprisingly low, and they can be left fully charged for long periods. Even after a year the self discharge is negligible.

You cannot test them with a battery tester because off load the voltage will always be 3.3V/cell.

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MM, I selected LiFe on my charger, that bit works ok, but what my charger won't do is more that 0.8A discharge in LiFE mode. The separate lipo discharger might be configurable via its USB port, but not by the cell or current selector buttons.

As stated above the pack says the charge current should be:

Normal/Standard Charge Current: : 440mA

·         Normal/Standard Charge Rate: : 0.2C

 

Don't know about other LiFe batteries but the spec on these are:

Maximum Continuous Discharge Current: : 44A

·         Maximum Continuous Discharge Rate: : 20C

·         Minimum Discharge Voltage: : 5.0 V (2.5 V per cell)

So we will have to differ of there discharge performance figures as I was only drawing 1.3A per pack for 2 minutes and the packs dropped over 40% in capacity.

 

I think I have determined the issue, they are knackered. I tried to recharge them from 60% change and they accepted about 50mAh and  then said hey were full.

 

Perhaps I misread the initial charge value, but they seem quite difficult to determine their actual capacity form just the voltage checkers.

 

I appreciate that people can fly around for long periods and draw little power, but what I am interested in is the headroom between RX battery pack and RX brownout voltage, its all well and good but if a servo fails, control surface binds or you need to move both sticks into various corners of the TX its no good it the RX pack can't supply the required power without the voltage dropping excessively. 

 

Thanks for the offer MH, I'll come up to the field Thursday and see if your view is the same as mine, but one thing for sure they are not going in a model  if they can't do more than 2 minutes at 1.3 A out of a 2200mAh pack.

 

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I managed to knacker a pack by setting the charger to the wrong type. They don`t like it. My chargers will only discharge at about the same rate as yours, if I am lucky, on any type of pack. Also, the cycle function does not work with them, same as LiPos.

They are still relatively cheap compared with other types and as long as they are used as intended they far outweigh anything else.

On a couple of large, fast WW2 models I have I use 2x 1800 via failover switches which really is belt and braces.

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MM wrote:

 

I managed to knacker a pack by setting the charger to the wrong type. They don`t like it. My chargers will only discharge at about the same rate as yours, if I am lucky, on any type of pack. Also, the cycle function does not work with them, same as LiPos.

They are still relatively cheap compared with other types and as long as they are used as intended they far outweigh anything else.

On a couple of large, fast WW2 models I have I use 2x 1800 via failover switches which really is belt and braces.

 

I double checked that the charger was set correctly, however the spec says 

 Maximum Charge Current: : 2.20A

·         Maximum Charge Rate: : 1C

but they seem to false stop at that level, however 0.4A seems more reliable

Normal/Standard Charge Current: : 440mA

·         Normal/Standard Charge Rate: : 0.2C

Cheap, you must be joking £80 for 2 x 2S2200 LiFe batteries is nothing like the price of a a pair of lipo, 

 

This model uses 2 x 2S2200 LiFe with failover, not belt and braces when both batteries don't perform, but more importantly its not easy to check if they are knackered or not.

 

The more this discussion goes on the more convinced I am to use a Lipo and a UBEC, fly models that need a 3S2200 as the flight battery. It soon becomes apparent with charging data and/or flight performance that the lipo is nearing its end of life. Batteries used as flight batteries which are then used as an RX battery (on a less onerous duty) seem like the safer bet IMO.    

 

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Where on earth do you buy your batteries? When they were introduced Zippy 700`s were £2.50 and 1100`s a little more. Even a 3S 1500 Tx pack was only about £7.00. So cheap that you could afford to replace them annually as a matter of course.

Rapid r/c stock the HK range but they more or less double the price. Still a fraction of a 5S Eneloop or a 2200 Lipo.

I have had so many Lipos puff up on electric models, particularly the NanoTechs after just one light use that I would not even consider one in an i/c model. Most of my scale models take around a year to build so I am very fussy regarding power sources.

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Chris brought his batteries to the field today and I've conducted what I feel is an appropriate test on them.

 

As they were handed to me fully charged, I simply put them on a fairly high discharge rate of double my estimated typical average rate of 600mA at 1.2A to ensure that I wasn't going to be misled by being too gentle with them.

 

On several occasions during the discharge, I doubled the rate to 2.4A to check for any possible excessive voltage drops which could occur with a binding servo or high demand situation. This resulted in average voltage drops of less than 0.1V per cell and even at virtually end of discharge after extracting the full rated capacity of 2200 mAh with the voltage curve dropping noticeably, the readings were 6.12V at 1.2A and 5.87V at 2.5A with never more than a 0.01V difference between the cells. I doubt this would trouble even an early Spektrum receiver.

 

Total delivered to the charger's cut off point of 2.9V per cell was 2250mAh for the first pack and 2280mAh for the second. 

 

I would happily use these packs in my own models - and certainly in preference to NiMH or LiPo packs via voltage regulators.

 

P.S. Chris was quite correct in his pricing - these are Spektrum branded batteries and Nexus list them at £39.99 a pop!  I suspect that you pay a massive premium for the OEM branding and cheaper alternatives are probably just as good.

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I just killed my 4.8v nimh by leaving the receiver switched on and whilst it does now charge, any load drops the voltage real quick and a fast charge pushes the voltage past 7v instantly which stops it charging fully. So I am in the market for a new rx pack but these life cost so much and I have never had trouble with nimh until I was forgetful. The arguments in favour are compelling but I still can't pay the same as I did for the model, for the battery. I found an old 4xaa holder in my bits box and for under £6 I can get 4xaa nimh 2100mah which I plan on using and then being able to pop out and trickle charge. I am thinking of getting these little battery holders in all my models so I only need 1 set of batteries and can swap.

 

Has anyone done similar? Why isn't it done more?

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24 minutes ago, Graeme White said:

I just killed my 4.8v nimh by leaving the receiver switched on and whilst it does now charge, any load drops the voltage real quick and a fast charge pushes the voltage past 7v instantly which stops it charging fully. So I am in the market for a new rx pack but these life cost so much and I have never had trouble with nimh until I was forgetful. The arguments in favour are compelling but I still can't pay the same as I did for the model, for the battery. I found an old 4xaa holder in my bits box and for under £6 I can get 4xaa nimh 2100mah which I plan on using and then being able to pop out and trickle charge. I am thinking of getting these little battery holders in all my models so I only need 1 set of batteries and can swap.

 

Has anyone done similar? Why isn't it done more?

If you have a charger with a forming charge function,  try a couple of cycles. Won't make it brand new, but might recover it to make it useful again.

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46 minutes ago, Nigel R said:

Is an absolute no on the basis that the cells can come loose or suffer poor contract with the holder. Ie most likely a quick way to kill the model. 

 

 

Agreed - dodgy enough in a cheap transmitter, but definitely not recommended in an airborne pack.

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