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Nickel Cadmium - Nickel Metal Hydride - Lithium-Ion & Lithium-Polymer

Nickel Cadmium (Ni-cads)

Cycling of cells seems to have become part of modelling mythology, with no-one really knowing where the idea originated. If you consult a ni-cad applications manual from a battery manufacturer it will tell you that deep charge/discharge cycles are harmful. For 'deep charge/discharge cycles', you could read 'electric flight'.

All rechargeable batteries are subject to a loss of capacity in normal use. The internal resistance will also rise. This means shorter flights and less power. The general theory is that 'cycling' will bring back the lost capacity. Quite how it is supposed to do this, we are not told. What is clear is that cycling will give an apparent increase in capacity. The truth is that this 'increase' will disappear the first time you return the batteries to their normal duty cycle.

If the batteries are used for electric flight, you are cycling the batteries every time you charge and fly them. Why waste several flights on more cycling? You are throwing away useful life. It used to be accepted in the electric car world that the best batteries around at the time, the 'SCR' (alas, now gone) cell, should only be charged about once every four days. The 'SC' cell could be charged once per day but gave inferior results.

Another accepted maxim was that you harder you charged a cell, the more power it gave ('the faster you put it in, the faster you can take it out'). One professional car racing team were said to charge their 6 cell packs by connecting them directly across a healthy 12 volt car battery. Less publicised was the fact that a good percentage of cells only lasted one cycle!

If you are using the cells to power an R/C system, the situation is a little different. In most cases you will be carrying far more battery capacity than you really need so you need not worry about lost capacity. The biggest problem here is the possibility of a cell going bad in flight. In fact, the real problem here is if a cell goes bad and you carry on flying. Most systems will work with bad cell - giving reduced range - but not for long.

A 'fail-safe' system that cuts the motor when the battery voltage drops below a preset figure is a far better investment than a battery cycler. Give the battery a full 10-14 hour charge before you go flying and forget about it.

No I haven't mentioned 'memory' here.
a) Because it's a different subject, and
b) Because I've never experienced it - not with ni-cads, anyway.

At this point I find myself wishing that I had kept better records of all the ni-cads that I have used over the last 40 years or so. Much of what I think I know is not backed up by real evidence. I suspect that this is true of most people.

For use as an R/C receiver battery pack, I think that ni-cads are still by far the best option. Their flat voltage curve and ability to survive ill-treatment makes them second to none.

Nickel Metal Hydride (NiMH)

Modellers being what they are, the cycling mythology has been carried over into the use of NiMH batteries. This is strange because one justification for cycling is to eliminate 'memory' effects and we are told that NiMH batteries don't suffer from these.

It is common advice to give Nickel Metal Hydride batteries at least one charge at the C/10 rate before you start fast charging them. This works for me, but I have learned that NiMH batteries don't like changes in their duty cycle. It used to be common advice on a well-used mini helicopter forum to give your batteries a slow charge after every 10 fast charges. The result would be a poor flight. This might be expected, but it took up to 5 normal fast charges before you got back to anything near 'normal' performance.

These cells also operate in a fairly narrow temperature window. Change the temperature and they will give poor performance. Doesn't all this sound like memory? Sorry, we know they don't suffer from that.

Cycling them - like flying them - does nothing except wear them out.

When I stsrted using these cells, I decided that I would keep some record of their use. My main usage was to power mini R/C helicopters (MS 'Hornet'). I have logged their use for over 1200 flights to date. Now a helicopter gives a pretty good indication of battery performance, particularly if flown indoors, because you start with a fully charged battery and fly until the model can't lift itself off the ground. It soon became clear that the best flight from any battery pack was on about cycle number 4 or 5, From there on, it was downhill all the way. I've already recorded that a change in the cycle gave a poor flight. Flying the cells when warm immediately after charge gave better performance. That isn't surprising, but leave the cells for an hour and the model may not fly at all.

Every flight in this situation is a cycle and cycling the cells would be completely pointless. With this treatment, the average cell life is around 100 cycles. I bought a large quantity (48) of Vanson 700 mAh 'AAA' cells from Maplin Electronics for future use at an atractive price. I made the mistake of taking all of them out of their packages and giving them one slow charge before storage. When I came to use some of them some 18 months later, I got 2 or 3 good flights and then a rapid tail off in performance. Most of these ended up being scrapped after 20 cycles or less, yet some of the first packs that I made up are still in use. It's clear I should have left them in the containers until needed.

In desperation, I tried cycling some of the useless packs - well, I'd nothing to lose and proving yourself wrong can be character building. They would then accept what appeared to be a normal charge with the normal mAh input. The model would lift off with what appeared to be normal power - and literally fall out of the sky after 30 seconds or so. At this point (sometimes earlier), my Graupner charger would refuse to charge them.

What is interesting about those 700 mAh cells is that the first charge may well accept 700 mAh, or more, but number two will be about 680 mAh and they will be down to 600, or less, after 10 flights/cycles. Leave them for 3 months and you may get close to 700 for one charge. The chances are that it will give a poor flight.

The far more expensive Sanyo 'Twicell' batteries, rated at 720 mAh give virtually identical results.

I would have severe doubts about using NiMH batteries for an R/C receiver battery pack. They are excellent for transmitters where there is a steady current demand. I think it would be all to easy to get an in-air failure of these cells, only to have them check out as healthy.

Lithium-Ion (Li-Ion) & Lithium-Polymer (Li-Poly)

So far, nobody seems to have suggested that these batteries should be cycled. GOOD, let's keep it that way.

Old ones (say, 50 cycles) do suffer from the temperature window effect. They don't like cold anyway. Sounds remarkably like human beings to me.

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