Desford Flyers
Lithium Ion Cells
For many years modellers have been using
NiCd or NiMH cells for supplying power to the radio
control equipment in model planes and helicopters fitted
with glow engines.
Following successful experiments I offer an alternative with greatly improved performance.
Firstly why do we still use NiMH cells? They are easy to fit and can be fitted near to the front of a plane, usually just behind the firewall and underneath the fuel tank, where they assist in providing some balance weight to assist with the production of correct centre of gravity. Unfortunately they don't hold much useful charge which necessitates recharging them at the flying field every two or three flights. This in turn requires flyers to lug about heavy lead-acid batteries for this purpose.
Greatly improved performance can be had by using LiPo cells connected to a UBEC (Universal Battery Eliminator Circuit) which converts the higher voltage of these cells to 5 or 6 volts required by radio equipment. Unfortunately these batteries are not ideal to bury out of site in a model plane because they are occasionally prone to burst into flames when being charged. Thus they are only suitable when the design of the plane allows for their easy removal for charging.
I have a very high power torch which uses a rechargeable Li-Ion cell which far exceeds the performance of any other torch I own, and it uses a Samsung 18650 cell which is widely available for Vape users. These cells are 2200 mAhr at a nominal voltage of 3.7 volts. After searching availability I rashly ordered a number of similar cells via Amazon which are rated at 5850 mAh. I should have smelled a rat because this capacity is unreasonably high, however when they duly arrived I was initially pleased because they looked very pretty!
I made up a pack of three cells using a newly acquired spot welding machine, charged them with my charger set to LiIon mode, and performed a discharge test. The results were so bad that I re-performed the test with a second set of cells which resulted in similar disappointment.
I relayed my findings to my brother and he asked "Have you weighed them Bruv?" You have to realise that I am a retired qualified electrical engineer and my brother's skills are more in the computing area so I initially dismissed his thoughts. However, I decided to take up his idea and popped a cell onto the kitchen scales. It weighed 15 grams, whereas a good quality Samsung 2200 mAhr cell is specified at 48 grams. I took the Samsung cell out of my torch and confirmed that this was true. So it seems that all you need to do to find out if you've been sold a duffer is to weigh it.
I then purchased some Samsung cells from Fogstar which turned out to be a very good purchase.
I show below the results of a discharge test comparing three Li-Ion cells connected via a UBEC to a 5 volt 1 amp load against a similar test with the same load connected to a 4 cell NiMH battery. The results speak for themselves. I don't think any of us would start a flight if our battery was only reading 4.7 volts, therefore when looking at the results the battery is only good for 30 minutes. Compare this with the 4 hours available from the Li-Ion battery pack and we can see an improvement of 8 times. If we assume that we can get 3 flights from a NiMH battery before charging then the Li-Ion battery is good for 24 flights; good enough for most flyers I think.
4 cell NiMh 0.5 amp discharge
3 cell Li-Ion via UBEC set to 5 volts 0.5 amp
The Samsung Li-Ion cells data sheet gives the assurance that they are resistant to catching fire during over charging and discharging and that provided they are charged with quality chargers they will hold their charge to 80% over 3 months. Their weight and size is such that a 3 cell Li-Ion battery pack is roughly equivalent to a 5 cell NiMH battery pack.
As a final safety precaution I have fitted a temperature sensor into the battery packs that I have assembled. Most good quality chargers are equipped with a Futaba type socket for connection of a sensor, and I have successfully tested many chargers, ensuring that when the sensor is heated, the charger terminates its charge. The sensor I have tried is type LM35DZ. This device looks like a three legged transistor but is actually a self contained integrated circuit which requires 5 volts across its outside pins and it produces a voltage on its centre pin which is proportional to the temperature.
Unfortunately there appears to be no standard pin-out with charger manufacturers, some having opposite polarity across the outer connections.
Finally it should be noted that some cells have
built-in protection circuitry.
NiCd or NiMH cells for supplying power to the radio
control equipment in model planes and helicopters fitted
with glow engines.
Following successful experiments I offer an alternative with greatly improved performance.
Firstly why do we still use NiMH cells? They are easy to fit and can be fitted near to the front of a plane, usually just behind the firewall and underneath the fuel tank, where they assist in providing some balance weight to assist with the production of correct centre of gravity. Unfortunately they don't hold much useful charge which necessitates recharging them at the flying field every two or three flights. This in turn requires flyers to lug about heavy lead-acid batteries for this purpose.
Greatly improved performance can be had by using LiPo cells connected to a UBEC (Universal Battery Eliminator Circuit) which converts the higher voltage of these cells to 5 or 6 volts required by radio equipment. Unfortunately these batteries are not ideal to bury out of site in a model plane because they are occasionally prone to burst into flames when being charged. Thus they are only suitable when the design of the plane allows for their easy removal for charging.
I have a very high power torch which uses a rechargeable Li-Ion cell which far exceeds the performance of any other torch I own, and it uses a Samsung 18650 cell which is widely available for Vape users. These cells are 2200 mAhr at a nominal voltage of 3.7 volts. After searching availability I rashly ordered a number of similar cells via Amazon which are rated at 5850 mAh. I should have smelled a rat because this capacity is unreasonably high, however when they duly arrived I was initially pleased because they looked very pretty!
I made up a pack of three cells using a newly acquired spot welding machine, charged them with my charger set to LiIon mode, and performed a discharge test. The results were so bad that I re-performed the test with a second set of cells which resulted in similar disappointment.
I relayed my findings to my brother and he asked "Have you weighed them Bruv?" You have to realise that I am a retired qualified electrical engineer and my brother's skills are more in the computing area so I initially dismissed his thoughts. However, I decided to take up his idea and popped a cell onto the kitchen scales. It weighed 15 grams, whereas a good quality Samsung 2200 mAhr cell is specified at 48 grams. I took the Samsung cell out of my torch and confirmed that this was true. So it seems that all you need to do to find out if you've been sold a duffer is to weigh it.
I then purchased some Samsung cells from Fogstar which turned out to be a very good purchase.
I show below the results of a discharge test comparing three Li-Ion cells connected via a UBEC to a 5 volt 1 amp load against a similar test with the same load connected to a 4 cell NiMH battery. The results speak for themselves. I don't think any of us would start a flight if our battery was only reading 4.7 volts, therefore when looking at the results the battery is only good for 30 minutes. Compare this with the 4 hours available from the Li-Ion battery pack and we can see an improvement of 8 times. If we assume that we can get 3 flights from a NiMH battery before charging then the Li-Ion battery is good for 24 flights; good enough for most flyers I think.
4 cell NiMh 0.5 amp discharge
3 cell Li-Ion via UBEC set to 5 volts 0.5 amp
The Samsung Li-Ion cells data sheet gives the assurance that they are resistant to catching fire during over charging and discharging and that provided they are charged with quality chargers they will hold their charge to 80% over 3 months. Their weight and size is such that a 3 cell Li-Ion battery pack is roughly equivalent to a 5 cell NiMH battery pack.
As a final safety precaution I have fitted a temperature sensor into the battery packs that I have assembled. Most good quality chargers are equipped with a Futaba type socket for connection of a sensor, and I have successfully tested many chargers, ensuring that when the sensor is heated, the charger terminates its charge. The sensor I have tried is type LM35DZ. This device looks like a three legged transistor but is actually a self contained integrated circuit which requires 5 volts across its outside pins and it produces a voltage on its centre pin which is proportional to the temperature.
Unfortunately there appears to be no standard pin-out with charger manufacturers, some having opposite polarity across the outer connections.
Finally it should be noted that some cells have
built-in protection circuitry.
TIME
0
15
30
VOLTS
5.2
5.0
4.9
45
4.7
60
4.6
75
4.5
90
4.3
TIME
0
15
30
Li-Ion
12.1
12.0
11.9
45
11.7
60
11.6
75
11.4
90
11.3
105
11.1
120
11.0
135
10.9
150
10.8
165
10.7
180
10.6
195
10.5
210
10.3
225
10.1
240
9.8
UBEC OUT
5.1
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
5.2
