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NiMh vs LiFe. What are the advantages and disadvantages?


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No, I got my numbers direct from Vapex website, direct links were copied in for reference, you are welcome to check them. Same brand, and as close in energy capacity as I could find. The nimh pack is 12Wh (2.5Ah x 4.8V nominal), the LiFe 13.2Wh (2Ah x 6.6V nominal).

 

Low discharge nimhs can be fast charged, in an hour.

 

If you're using a smaller, lower energy LiFe, ok, but, then we are not comparing like to like. Comparing Eneloops, a premium brand nimh, to a cheap brand (hobbyking?) LiFe isn't exactly equivalent either. I have no particular irons in either fire here so to speak, but do like comparisons to be as fair as possible.

 

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For myRX LiFe battery packs I use https://www.componentshop.co.uk/

 

They do some excellent LIFE batteries. I’m hoping they will be at Wings and Wheels at the end of June so I can but some for my new build . I typically use their 1600mah 2s packs . On most of my models I seem to get a power consumption of less than 100mah per flight . 

Edited by Tim Flyer
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31 minutes ago, GrumpyGnome said:

@SIMON CRAGG is that just 6-8 flights even in the vintage type? If so, I need to add extra capacity to some of my models..... 

Anyone have any idea about cells powering a cdi for 15 to 20cc petrol engines ? How long would an 1100 pack be good for ?

 

GG  

I did several tests with my Playboy Senior (3s powered).  I must have completed at least twenty 10 minute flights, and the model used between 80-100mah each flight. Tim Flyer reports getting similar. There are several good graphs available, plotting voltage reading against percentage, so I used one of these and my battery checkers to plot the usage. The voltage drops a tiny amount each flight, but if you are worried about using Life packs its a worthwhile exercise. I am not an expert on electrics, so I tried to complete simple real world practical tests on a "safe" model before installing in my larger models.

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I use a pack of 700 mA/hr, regulated to 5V, on a CDI and have no problems.

I am not familiar with Vapex so cannot comment, but if a model takes 80-100 mA/hr on Eneloops then you will find that on LiFe`s you will get far less back in for some reason. Everyone I know who has changed to these reports a similar situation. So far the HK ones have proved to be ultra reliable which is more than I can say regarding various NiMh`s, especially those 2600 ones which can fail at the drop of a hat and are unable to deliver sufficient current to drive even low power analogue servos.

A  2s 1800 Life is way lighter than a 5s 1900 Eneloop and would give far more flight time if you so wish.

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I think Peter C has it right, horses for courses. For typical sports models a four or five cell NiMh battery is fine (check your servo specs before using on five cells)

After 4 flights with my 90 Four stroke powered Aeromaster biplane (Usually 12 - 15mins per flight) My charger puts back 400 - 500 MaH into the five cell 2000 mAH Enerloop battery.

I use five cells for the increased speed and power I get with the Hitec digital servos in the model. When I have checked the current draw with all servos moving it is 0.5 to 0.6 Amps rising quickly to over an Amp if I restrict the movement of say the rudder.

 

Once every month or so or if a model has not been used for a while I will discharge a NiMH battery to 1 volt per cell to check capacity just to check all is well.

 

As the servos and control surfaces move quicker using five cells it stands to reason that they are using more energy but for slightly less time than with a four cell battery so I would expect that power consumption per flight would be little different.

 

I recently purchased some Hitec High Voltage servos and have tested them with a two cell Lithium Ion battery which like Lipos are 8.4V fully charged. Boy those servos move quick and will put a sizeable dent in your thumb if you try to resist their movement.

 

A bit different to the Skyleader SLC1 servos I used with 4 cell Deacs in 60 powered aerobatic models in the '70s!

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A club member's model crashed due to total loss of control due to Rx battery. He was using a small 2s LiFe and had not charged it since previous flying session. The Rx voltage showed 6.4V on Tx.

 

So, my question is - What is the lowest voltage one should go down to with a LiFe to be safe?

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6 hours ago, ASH. said:

A club member's model crashed due to total loss of control due to Rx battery. He was using a small 2s LiFe and had not charged it since previous flying session. The Rx voltage showed 6.4V on Tx.

 

So, my question is - What is the lowest voltage one should go down to with a LiFe to be safe?

 

volts reading is only half the story Ash

Simon's chart shows voltage and sudden voltage drop off

Flyin Brian's Regime of fly then check, then On field charge, should be followed.

the full story Is voltage and capacity, and using a load tester after 3 or 4 flights.

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7 hours ago, ASH. said:

A club member's model crashed due to total loss of control due to Rx battery. He was using a small 2s LiFe and had not charged it since previous flying session. The Rx voltage showed 6.4V on Tx.

 

So, my question is - What is the lowest voltage one should go down to with a LiFe to be safe?

I've been using some 700 mah 2s Turnigy LiFe cells in a couple of small models and was getting voltage drops down to 5.5v when the servos were moved, my charger measures cell resistance so I checked them and it was around 250 milliohms per cell, so thats 0.5 ohms total, so any appreciable load and the voltage drops off. For comparison an 1100 mah 2s LiFe had around 100 milliohm per cell, and my lipos are typically below 10 milliohms per cell when new. Both LiFe cells had been used, but even my worn out Lipos never get an internal resistance that high. This would suggest the LiFe cells do have a higher internal resistance and will drop more voltage under load, so maybe on your club members model the combination of partially discharged and high load led to a short brown out.

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I'm trying to work out whether my transmitter (which needs 7.4-15V input voltage) is better off on a LiFePo4 pack or an 8-cell stick-type NiMH.

 

I'm thinking a sealed NiMH pack from Overlander with a more obvious discharge curve negates the possibly inaccurate/uncalibrated internal voltage measurement that would lead to a LiFe pack hitting that steep voltage drop-off point without realising it. There's better early warning from the NiMH pack I think.

 

What I'm thinking is, if I'm able to get a 4 cell LiFe pack, I could run 14.6V peak voltage, which keeps the voltage up in the transmitter and even on a LiFe pack that does hit that 20% value, I'll still be above 12.8V - the Tx will carry on working and I can get the model down safely. On a NiMH pack, 12V will be my peak, fading to 9.6V for the majority of operation, but by the time the pack is dead, say 0.9V/cell, it'll be beneath the Tx's minimum 7.4V.

 

Which is the lesser of two evils? - lower in-operation voltage levels (NiMH) or the voltage off-a-cliff behaviour of LiFe?

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For a transmitter it's not so much an issue as the load is very constant and quite low, even a back lit colour screen should still draw less than 300ma. Also the 7.4v is probably not the txs minimum operating voltage but that it will work on a 2s lipo which has a nominal voltage of 7.4v, the low voltage alarm will be set lower again.

 

What transmitter is it, does it have a LiFe setting, if not and you select Lipo then the low voltage alarm will be set for a Lipo not a LiFe.

 

BTW my main Tx runs a 1s LiFe, but does report mah used and remaining operating time (as well as voltage and % capacity)

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I have been running a couple of Life packs in my 14SG for years. I get virtually double the stick time compared to NIMH. On the odd occasion that the battery does get a bit low, it triggers the on board alarm system and gives plenty of time to land. IIRC correctly the Futaba RX I use will still work down to something ridiculous like 3.8v!. This means I would really have to push my 1100mah RX Life pack before it all went pear shaped.

I always check the on board Life after a few flights anyway, whilst swopping Lipos etc.

Have to say, I would never go back to NIMH.  

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3 minutes ago, Frank Skilbeck said:

For a transmitter it's not so much an issue as the load is very constant and quite low, even a back lit colour screen should still draw less than 300ma. Also the 7.4v is probably not the txs minimum operating voltage but that it will work on a 2s lipo which has a nominal voltage of 7.4v, the low voltage alarm will be set lower again.

 

What transmitter is it, does it have a LiFe setting, if not and you select Lipo then the low voltage alarm will be set for a Lipo not a LiFe.

 

BTW my main Tx runs a 1s LiFe, but does report mah used and remaining operating time (as well as voltage and % capacity)

 

Hi Frank it's a Radiolink AT10ii. Draw is rated at 100mA but I guess that would be at 7.4V as it's the only number reported. It has a customisable voltage setting rather than a chemistry selection setting, so you can set where it cuts off if you want telling earlier or later than nominal. I know the Tx gives me the opportunity to calibrate the pack voltage relative to what the Tx reads by default, but I need to check how linear that is.

 

Looking at it again, a 3S LiFe would also do the trick. I'd have to satisfy myself the Tx keeps pace with the reading of a separate multimeter reading to the tune of 0.6V seeing how one cell can drop 0.2V over 80% of its useful charged life. 

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10 hours ago, ASH. said:

A club member's model crashed due to total loss of control due to Rx battery. He was using a small 2s LiFe and had not charged it since previous flying session. The Rx voltage showed 6.4V on Tx.

 

So, my question is - What is the lowest voltage one should go down to with a LiFe to be safe?

The computer side of a computer radio works of 3.3 volts BUT the RF output stage works off 5.0 Volts, you lose RF output below 5 Volts so whilst a LiFe 2 cell will go down to 4.0 Volts you lose RF signal before the battery goes flat, having said that on a LiFe pack the discharge curve from about just below 6.0 Volts is so steep you really don't want to be pushing it that far. 

 

The only real way to know the state of charge of a LiFe pack is by what you put into it or what you have taken out of it, voltage is NOT reliable enough. 

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55 minutes ago, Philip Lewis 3 said:

The computer side of a computer radio works of 3.3 volts BUT the RF output stage works off 5.0 Volts, you lose RF output below 5 Volts .....................

Surely that depends on the radio brand/model/system?

just checked my Tx battery and it reads 3.86 volts  (1s2p LiIon) and plenty of flying time left in it yet. I suppose there could be a voltage booster in the RF stage.

 

Dick

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As I believe Ash was talking about the Rx voltage I've tested receivers (FrSky) down to 3v, at which point the servos have no power at all.  I set LiFe alarms at 3.3v per cell for 2 seconds as the initial low battery warning and 3.1v for the Battery Critical warning.  The 2 seconds allows for the momentary drops when running digital servos.

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Yes, it's the Rx battery voltage using LiFe. The audio warning came on - "low battery" and straight after it crashed. Rx battery was showing 6.4V so I presume it 'fell off the cliff'.  Should the alarm be set at 6.5V ? Or, is anything below 6.6 in danger territory.

 

Edited by ASH.
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In my opinion straightforward battery voltage is your best bet if you have nothing more sophisticated.  6.6v will give a safe warning level though it can also give false warnings if servo movement causes a momentary dip.  A better bet is to charge the LiFe pack before flying!

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39 minutes ago, ASH. said:

Yes, it's the Rx battery voltage using LiFe. The audio warning came on - "low battery" and straight after it crashed. Rx battery was showing 6.4V so I presume it 'fell off the cliff'.  Should the alarm be set at 6.5V ? Or, is anything below 6.6 in danger territory.

 

Well I have had Rx voltage telemetry warnings on my 2s Life setup with the 700 mah battery saying 5.5V (alarm set at 6v) with no adverse effects. Also the default alarm setting on Multiplex telemetry receivers is 4.5v (based on a 4 cell Nimh) and I have run them in small models with a 1s Lipo for power, as Dick says the Rx will work down to voltages below 4v before cutting out and rebooting. 

 

But if for some reason there is a high current draw on the Rx battery, e.g. faulty/binding servo, then with the small LiFe cells you could get quite a big voltage drop due to the internal resistance. BTW a few years back I was setting up a glider and kept getting low voltage warnings every time I moved the servos, it was the new switch I was using that had very high resistance, and it was meant to be a heavy duty one.

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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.

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