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More on batteries and charging

Previous notes

It must be stressed that the use that we are making of all batteries used in electric flight applications goes far beyond anything that they were designed for. Some of them stand up to this extremely well, but life will always be short. As a rule of thumb, 50 cycles is about all you can expect before there is an obvious drop in performance - yes, I've changed that.

The story so far...

When I first started playing with electric R/C models nearly 30 years ago, the first 'Sub-C' ni-cads were appearing. These were of 1200 mAh capacity. They were steadily up-dated (mainly due to the R/C car fraternity) until the advent of the 1700 SCR cell. In 2001 I bought a 'Hornet' helicopter which led to the use of NiMH batteries and the acquisition of a high tech charger. My aircraft interests then tended towards smaller models and cells and a switch to Li-Poly batteries.

Nickel Cadmium - Nickel Metal Hydride - Lithium-Ion - Lithium-Polymer - Here's where I go out on a limb...

Nickel Cadmium (Ni-cads)

Most of my usage of ni-cads until fairly recently (for electric flight) has centred on the flying of a Kalt 'Whisper' electric helicopter. This was originally designed to use an 8 cell pack of some special 1100 mAh SCR cells which soon became unobtainable. These weighed around 10 ounces. I then used a pack of 1400 mAh 'red' SC cells which gave similar flight times and weighed 14 ounces.

For some years I then used a total of 3 packs of 1700 mAh SCR cells which, again, gave similar flight times. Each pack weighed 16 ounces. There's a distinct pattern here, increasing capacity meant increasing weight and no increase in flight time.

Then, disaster, no more 1700 SCR's. I tried a pack of 2400 mAh 'N' cells (18 ounces) with disappointing results. This could have had something to do with the fact that I could never get them to accept more than about 1700 mAh on charge. Flight time was down and, more important, there was a marked reduction in power. I soldiered on with this and the last of my 1700 SCR packs for some time, getting similar performance.

I looked around for a replacement and was offered a pack of Tornado cells labelled 'RC 1920+' in large letters. I was encouraged, initially, by the fact that it only weighed 14 ounces. After accepting 2250 mAh on the first charge, this has never since taken more than 1750. In use it has about the same power and flight time as my tired 1700 SCR pack. For the record, the 1700 pack has 152 cycles on it and the 2400 a mere 70. When you consider that the new pack has only 5 cycles on it and is already showing signs of reduced performance, the outlook is depressing.

Whisper packs

Comparison of early 1100 mAh pack with current 1920 (?) mAh pack.
The early pack had better performance.

The problem that this particular model presents is that spare parts are impossible to obtain and the next flight could be its last. In those circumstances I'm not inclined to spend around 60 - 70 on a suitable Lithium-Polymer battery.

Charging

In the early days, charging consisted of a resistor and a stop watch. Sophistication took the form of a clockwork timer. An 8 cell pack was simply connected to a car battery for 20 minutes! Delta peak chargers were a great improvement but I only used one for the 'Whisper' packs. In recent times I have used a Graupner 'Ultra Duo Plus II', but mainly for NiMH batteries.

Nickel Metal Hydride (NiMH)

I have recently scrapped many of my NiMH batteries. These had not been used for a while - not regularly anyway. When I came to use them, I found that the Graupner charger wouldn't charge them. At first, I thought I had a charger problem and tried other methods, a different charger, or a slow charge. These didn't help because I would get what appeared to be normal power but a very short flight. The ESC would cut the power very quickly after a rapid tail off in power. Other batteries charged normally and gave normal flights.

Many of these cells were quite new and only had a few cycles on them. The commen denominator appeared to be that all of them were bought some time ago as part of a large order of Vanson cells from Maplin. They had all been removed from the packages and given one slow charge to initialise them and then stored in that condition. It seems that this was a mistake and that I should have left them uncharged in the packs. Another lesson learned. NiMH seem to like being used regularly, if you charge them, use them.

All of the above were 750 mAh cells. I have packs of 1600 and 2000 mAh cells in transmitters which seem to work normally - a different duty, of course. I have one pack of 8 x 2000 mAh cells used in the 'Streaker' with a Speed 400 motor (now my largest model) which have lost a lot of power, but which still fly the model and give reasonable flight times.

Charging

As previously described, this is done with the Graupner charger set to 'Sensitive' and the charge current set manually to 1C. Sometimes a pack will cut out after a very short time. Unplugging the pack and then reconnecting normally sorts this but it probably means that the pack has problems approaching. If it repeatedly cuts out, the problem has arrived.

Lithium-Ion (Li-Ion)

I only have one Lithium-Ion pack and this is rarely used.

Lithium-Polymer (Li-Poly)

Maybe I simply haven't been doing enough flying recently, but several of my Li-Poly packs are showing signs of a reduced capacity. I try to spread the use as evenly as possible, with the result that they are all have about the same number of cycles on them. I first thought that I had a charging problem in that I would get a very short flight with a rapid run down of power. This was mainly noticable on my 'Hornet' helicopters, but it soon became clear that the same problem applied to all. I had just one 3 cell pack, this being of 145 mAh Kokam cells and it displayed the same characteristic.

Charging

Having charged the cells again and tried other methods, I then began to suspect my charging source. Actually, I have three of these, two 12 volt 7.5 Ah gelcells and a car battery and they all gave similar results. Charging while 'floating' the battery across a power supply made no difference either. So, what else?

My regular charger, the HE Cell type described previously has LED's to indicate the charge state. A red LED shows that it is charging at constant current. When it switches to constant voltage, the red LED goes out and a green comes on. This goes out when charging is complete. It is difficult to check exactly what is happening because the charger pulses and an ammeter gives confusing readings. However, I did establish that charging does not actually stop when the green LED goes out, but continues for some time at a reducing rate. It pays to leave it on for a while.

I had been using custom built chargers for my smaller cells, so built a similar device for the 1200 mAh Kokam cells that I used for the Hornets. Now I knew that I was getting a complete charge but still got poor flights. OK, buy two more 2 cell packs. These gave normal charging and normal flights, so it was the old cells. Next move was to buy two more packs which were on offer from Indoor Flyer at reduced price.

For the record, my first two 1200 mAh 2S1P packs were bought at different times and one was clearly marked as 'Kokam', while the other looked similar but had other markings. This second one was never as good as the 'Kokam' pack. The next two packs were bought together and, again, one pack was marked 'Kokam' and one wasn't. Right from the start there was a marked difference in flight time. I'm not sure what can be done about this as the packs are now being 'sold-off' at reduced price.

So what exactly is happening?

As the cells age, two things seem to happen. One is the cell recovery already mentioned, where the power will drop off and recover after a short rest. This effect seems to get worse as the cells age. Another effect is that the cells appear to become increasingly sensitive to temperature. They don't like cold - we are not talking about below zero here, just a few degrees lower. If they are old and cold, forget it.

The recovery effect seems to have received very little attention except in the instructions of a few toys. The temperature effect seems to have received no attention except in terms of very low temperature.

Here's where I go out on a limb...

All cells are subject to 'warm up'. It's fairly well established that ni-cads get better as they get warm. I had a powered glider (Nitto 'Kitty') that would float around in ground effect for a couple of minutes before climbing away.

NiMH cells show the same effect but will lose power - and need a rest - when they get too hot. For best results, they need to operate in a fairly narrow 'window' where temperature is concermed.

This could be the first time this has appeared anywhere, but Li-Poly cells show the same effect! My 'Hornet' helicopters are equipped with my warning light and both will lift off showing low power and with the light on. After a couple of minutes struggling around in ground effect, they will then climb away and the light will go out. This can reach a point where I am struggling to prevent the model climbing into the ceiling indoors. Needless to say, it gets worse with age and low temperatures (just like me).

Now, apart from the effects noted above, all of these different types of cell appear to have the same tendency to dislike change. If they are regularly used in a certain way (duty cycle) they will give poor results if something is changed. You can call this effect anything you like, but I call it MEMORY. Don't try to tell me that Li-Poly cells don't suffer from memory effects. More.

A chart of my recent battery history can be found here.

This page will be updated.

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