boris114

I was considering upgrading my NiMh receiver packs to LiFe primarily to save weight and wanted to see what the weight savings is since everyone says that they are lighter. For comparison I selected a 2000 mAh NiMh 6V pack (http://www3.towerhobbies.com/cgi-bin...&I=LXNHS4&P=ML) and a 2100 mAh LiFe pack (http://www3.towerhobbies.com/cgi-bin...&I=LXNHS4&P=ML). I figured that the difference of 100mAh is not significant and that the 6V NiMh was the proper comparably to the LiFe packs. According to tower, the weight for the NiMh pack is 152g and the LiFe pack is 143g. I heard a lot of people discuss that LiFe receiver packs are lighter than NiMh, but is this the difference that we are talking about. I am looking to put the pack on a 90 size plane, and frankly I don't think that 9 grams will make a noticeable difference. Am I missing something? I found manufacturer comparisons (http://www.lifesourcebatteries.com/reasons.html), but they are using a wrong weight for the LiFe battery, which is of course in their favor.

Where is the weight savings? Has anyone looked at this closely? Also, this assumes no regulator on the LiFe pack. With a regulator and comparing against a 4.8 volt NiMh pack, the LiFe would be a much heavier option.

Thanks.

flyinwalenda

My Feedback: (5)

I would be comparing to the Hydrimax NiMh packs as that's what I am replacing. The LiFe packs I'm using are  GForce.
A 2000ma Hydrimax 6v is 152g(5.4oz0 and a GForce 1800 Life is 105g (3.7oz)and a 2500 LiFe is 139g(4.9oz). They weigh a bit less but their life cycle is much longer than NiMh packs too.

acerc

Plus they charge much quicker and in my experience it seems that I don't use as many mah's with the LiFe as with the Hydrimax NiMh per flight's. And in my situation they are much lighter because I only use one instead of two batteries.

Truckracer

My Feedback: (19)

To fairly compare the batteries, you also have to consider the C rating. In general for a similar capacity rating, the LiFe batteries will be capable of delivering much higher current to the servo load than the NiMh batteries. Capacity alone just doesn't tell you how a battery will perform. The voltage from the Hydrimax and similar NiMh batteries will pretty much fall on its face with only moderate servo loading. In general, if the NiMh battery has a high capacity rating and is in a AA sized cell, when compared to most any LiFe battery, the NiMh will lose most every time. Good NiMh batteries are available but they are not light weight.

acerc

Thank's Truckracer. I hadn't thought of that being the reason for the less mah used with the LiFe until I just read your responce.

KW_Counter

acerc,

Another factor in your comparison is the self discharging of NiMhs.
They will loose up to 5% a day just sitting there, 30% from Sunday afternoon
to Saturday morning. You need to recharge weekly just to cover that loss.

Good Luck,
KW_Counter

acerc

KW you may be misunderstanding my meaning. When I first switched over to Lipo I noticed that I was not useing as much mah's as I had previously been useing with NiMh. That would be because of the Lipo's higher output capability. Could also mean I was slightly under powering with the NiMh slightly. That is what I realized readingTruckracers post.
I have been switching out the Lipo's for LiFe and also have wondered how in the world did we do what we did many moons ago with alcaline's and NiCd's.

Truckracer

My Feedback: (19)

Ace, not aimed at you, just a general observation! Don't want to upset anyone here.

It is hard to know exactly how much you take out of a NiMh or NiCd battery in use by measuring how much you put back in during a charge. This is because the methods our chargers use to detect full charge are not very efficient and our indication of full charge comes sometime after the battery is fully charged! The (2) most commonly used methods of detecting full charge and terminating the charge cycle are the voltage depression usually referred to as peak detection and temperature rise detection. The first method became quite common for most chargers and temperature is rarely used though is is quite accurate. Summary .... you almost always put quite a bit more energy back into a NiCd or NiMh battery during recharge than you actually take out in use. The best way to test a Ni battery is to capacity test it after use and see how much charge is left in it rather than measuring how much goes back in during charge.

Similarly, many NiCd and NiMh batteries have way more charge left in them than most people would want to believe. Highly misunderstood technology even though they have been in use by modelers for decades!

Most of the lithium chemistry batteries are quite efficient when they charge and give us (or should I say our chargers) a very nice current decrease as they approach full charge. Our chargers charge in two stages starting in constant current mode then they switch to constant voltage mode when the cells reach the desired level for the particular type of Li cell we are using. So summary here ..... Li batteries require very little more current (energy) for a full charge than what we take out of them in use and they give (our charger) a very nice indication of when they are full charged. So measuring the amount of energy that goes back into a Li battery is a good indication of how much was taken out in use.

Probably more than anybody here wanted to know or even cared about .....

Darn, I must sound like some kind of defender of Ni battery technology or something .... which I am not. I use LiFe and A123 just like everybody these days because it is a superior technology. But I do get tired of people bashing Ni battery technology because it is simply not as bad as many make it out to be.

Truckracer

My Feedback: (19)

Kinda like how did our elders live to such ripe old ages when they were surrounded by lead paint and asbestos everywhere around them. Re. our older batteries we got by using them and they got by because that is what was available to use at the time.

dirtybird

My Feedback: (5)

Wall Mart has some NIMH batteries that come precharged and are guaranteed to lose less than 5%/yr. of their capacity.
I bet Boeing wishes they had used NICD's

Truckracer

My Feedback: (19)

Sounds like an Eneloop clone or something similar. I use Eneloop batteries in several of my transmitters and like them very much in that application. Extremely low self discharge rate and after several years of use, their capacity is holding right in there. They are expensive though.

Scota4570

http://hangtimes.com/weightcompare.html

You will need five NIHh cells to equal two LIFE. You do not need or should you desire a regulator for LIFE. I believe LIPO will fade away as an RX pack due to the regulator issue. Nicd is probably going to go away due to the enviornmentalists. For simplicity, holding a charge, and amps available LIFE is best. NIMH and NICD are certainly valid options though. To get big amps with NIMH you will need to go to sub-C cells. They will work just fine. AA cells that hold 2000+ maH are more fragile and may not survive. In a large plane that needs a bunch of lead in the nose weight is not an issue. Just use less lead; it all comes out even. LIFE requires that one learn new things and buy new stuff. IF that does not appeal, then use whatever you want, it will all work. There is no decisive advantage to any of them.

KW_Counter

Scota,

Using a regulator with a LiFe isn't just because of the higher voltage
and protecting your equipment.
It is also because at higher voltage your equipment draws more
current and your batteries become drained sooner.
A well designed regulator lowers the voltage without wasting the capacity.
Some draw the same current but send less to the equipment and waste the difference.
I am referring to a switching regulator vs. simple zener diode circuit.

In addition, why use higher voltage to speed up the servos on a trainer or Sunday Sport Flyer.
On a fully aerobatic plane it makes sense.

Good Luck,
KW_Counter

Scota4570

"In addition, why use higher voltage to speed up the servos on a trainer or Sunday Sport Flyer. "

Because there is no downside to running the voltage the RX and servos were designed to run on. Using a regulator adds unnecessary weight and is another thing to fail. Each connection is another potental failure point. IF a connection in the power supply fails you crash. That is why some use dual batteries and switches.

dirtybird

My Feedback: (5)

There is no way a regulator can reduce the voltage without wasting capacity.

Truckracer

My Feedback: (19)

+1 for dirtybird! Exactly the reason they have a big heat sink on them. I have never used a regulator in any kind of RC application and I think it is safe to say, I never will.

4*60

My Feedback: (41)

Unless you have a modern switching regulator.

ZAGNUT

My Feedback: (1)

the article in your link is a year old and since then every one has started making high voltage servos compatible with unregulated 2S LiPo/Li-Ion. the receivers themselves have always been compatible.

the newer ignition modules are also 7.4V compatible and that's besides the point that many are simply eliminating the ignition pack and switch harness and using an IBEC instead.

dirtybird

My Feedback: (5)

Even a switching regulator wastes power. First you have to chop the incoming voltage up then you have to filter it at the level you set.
The level set is an adjustable circuit controlled by a potentiometer. You don't get all of this for free.
All of this circuitry is subject to failure. A failure could mean loss of your aircraft. Plus the square wave used to chop the voltage could cause you RF problems.
With the availability of LIFE or A123 for receiver batteries you are foolish to include a regulator. Its just another package of parts to fail along with the hazard of LIPO's
About the NIMH's, the package I saw at Wallmart guaranteed less than 5% loss of voltage per year. Buy a couple and keep them for a year. Then if they don't you can get your money back.

Truckracer

My Feedback: (19)

dirtybird, the forums have quite a few examples of switching regulators that have caused radio problems.... just as you suggested. Rarely happens with a linear device or better yet, no regulator at all.

4*60

My Feedback: (41)

Well I use life and no regulators so I guess I learned something.  about regulators.  

Four Stroker

Unfortunately a switching regulator is only 60~80% efficient. Better than a linear but not great. The Futaba PS-10 switching reg. caused battery failsafe repeatedly (3.7 volts) AND reduced my range by a factor of 4 - FASST. A couple of 10 A diodes is better than most switching regulators. Switching regs. work OK with KNOWN constant loads. Our servos dancing about are the opposite end of the spectrum.