Overview of electric flight
Symposium by Keith Shaw
Public Newsletter Articles
How to rebuild an Astro motor
Weight reductions for electric conversions
A low cost thermal peak detection charger
Making Printed Circuit Boards
Aerobatics for Electric Airplanes
Nicad Care and Handling
The Shuttle ZXX
Getting the Most Out of Ferrite Motors
What Difference Does A Bit of Wire Make Anyway?
Using 1100AAU Cells for the Speed 400
"Squirrel" - Construction - Speed 400
A Weight Comparison of some Lightweight Coverings
Motor Comparison
Keith Shaw on Props
Building Foam Models
Electric Flight Box
Nicad Care and Handling
by Rod Wooley

Keep your batteries in shape!

There's no doubt that the one subject guaranteed to raise a heated discussion amongst modelers is nicad care and handling! However, we all need to be expert at battery handling for our radio gear and glow ignitors etc, not to mention it's useful knowledge with the modern day prevalence of camcorders and portable power tools etc. For the electric-flight enthusiast good battery management is a vital to continued success and happiness, and so this month I will dare to say a few words on the subject.

To cut the verbiage I'll use point form:

1. Charge new nicads and ones that have not been used for many weeks at a low current of C/10 for 15 or 16 hours. e.g. Charge a 600 mAhr cell at 60 mA. Continuing to charge fully charged cells at C/10 for another day or two will not harm them (except it does slowly reduce their usable lifetime due to cadmium migration into the separator.)

2. Cycle cells or batteries four or five times from new, or after three months or more of storage. This can significantly increase the capacity of a battery up to the rated value. (NB ratings are derived at relatively low discharge currents. Don't expect the same capacity on heavy loads!) Once cells drop below 80% of rated capacity they should be replaced.

3. Overcharging cells at currents much above C/10 causes overheating and venting and rapidly leads to cell failure.

4. Nicad cells can be stored charged or discharged. It is safer to store them discharged as any nicad can deliver a very high current if shorted. Discharged cells can develop internal shorts but this is unlikely.(I had a pack that was discharged for most of ten years and it still worked fine.) It is not good to leave a load on a nicad battery as it will eventually cause damage once one or more cells are discharged. It is wise not to make a practice of discharging receiver or transmitter batteries by leaving them turned on.

5. Rx and Tx batteries can be managed as follows. Blank off a 15 hour period on a plug in wall timer. (E.g. duct tape) Remove all "on" and "off" plugs except one "off" one at the end of the 15 hour period, and one "on" one 15-20 minutes before. Plug in the C/10 transformer that came with your outfit. A full charge from "new" or "flat" is set up by manually turning on the timer and setting it at the beginning of the 15 hour section. Once charged, the short boost each day will make up for the small self-discharge, and the batteries will be full and ready for flying at any time. (Check voltage "on load" before flying if you do this!) Leaving Rx and Tx permanently connected to the charger is not recommended. (see 1 above)

6. After flying some people estimate the percentage usage of their Rx and Tx packs, add a bit, and charge accordingly. (E.g. 500 mAhr is good for say 100 minutes, 40 minutes of use represents 40% used: charge for 7 to 8 hours) It is also acceptable to provide a full 15 hour charge at C/10, but this is time consuming! Personally I find this too "hit and miss" and slow, and prefer to peak-charge at 500 mA and then put the packs back on the 15 minutes a day "maintenance charge". ( Use of peak chargers also means that you need to "top up" the charge at C/10 every two or three charges. It is easy to wind in 2 to 3 hours on the timer to compensate for cell imbalance and bring all cells up to a full charge.)

7. Try to avoid discharging a battery below about 1V per cell. (Actually 1.2(N-1) V! This is to avoid cell failure: deep discharges cause a few cells to reverse charge.) A single cell can be taken to a lower voltage and even be shorted (once its voltage is below 1.0 V!), but should not be left in a discharged state for long periods with even a light load.

8. For electric flight applications many modelers have found Sanyo cells -provide unrivaled performance. SCR cells are a fast-charge type of cell and have a low internal resistance and so are ideal for high-current applications. E.g. a 1000 SCR can be charged at 5C or 5 A in 15 minutes, and can be discharged at 50 A (or more!)

9. Small motors, e.g. speed 400, only draw 10 A or less, and to keep weight down cells like the KR600AE are used. These should only be peak charged at just 2.5 C (1.5 A) and used once, followed by a 15 hour C/10 charge (60 mA in this case), or can instead be peak-charged and topped up at C/10 for two or three hours to bring all cells up to full charge.

10. Nicads are damaged by heat and overcharging at higher currents. Use of a good peak charger is recommended, but beware of the "mis-matched cells problem": usually Sanyo SCR's are well matched, but for example with camcorder nicads, repeated peak charges gradually leads to a low capacity. This is because some cells are empty before the others, and the effect is accumulative. Eventually the lower efficiency cells will fail. A good long charge at C/10 will often bring such a battery back to full capacity if things haven't gone too far.

11. Nicads typically maintain a very level voltage over the 10 to 90% portion of the charge discharge cycle. A fully charged cell on charge can be as high as 1.6V. In use, this voltage quickly drops to 1.2 V and then slowly sinks to 1.0 V. After that it drops off quickly during the last 10 % or less of discharge. (On a very heavy discharge the cell voltage can drop immediately as low as 1V as a result of the cell's internal- resistance, e.g. 4mohm x 50A = 0.2 V).

12. Nicads can be soldered without damage, but clean the terminals well e.g. with a slow Dremel tool using a flexible disc-sander. Avoid shorts due to solder "blobs" around the positive terminal. (Homemade masking tape seals are good). Tin the cell and connection. Use a light smear of flux and multicore solder, and a good powerful iron with a clean flat bit, so the joint is made very quickly before the cell heats up internally. ( Take care! Its easy to damage the nylon seal on the positive terminal!)

13. An accurate digital voltmeter, battery discharger and capacity tester and are very useful equipment for checking batteries. Keep a record of the voltage at the end of a charge and battery capacity. Capacity tests and voltage checks will quickly detect a battery that is starting to fail by detecting shorted cells and cells that are no longer holding a full charge. A good test is to measure capacity once immediately after charging, and then measure capacity again 3 or 4 days after a charge. Any pack that shows a marked drop should be replaced. A drop of 10% in capacity is the most one should consider safe after this time. (Obviously if a pack has not been used much in recent weeks it's worth cycling the pack 3 or 4 times before checking capacity) A good test of an individual cell is to take it to zero volts and then short it out for 24 hours. Remove the short and see if the cell will re-establish some voltage greater than zero. If it does this it means it is free of any high-impedance internal shorts.

14. On electric models it seems to be best to store the propulsion batteries discharged until just before use, and then to peak-charge them. Land as soon as the pack starts to drop off so you still have control, and then run the battery down until there is not much "umff" left. Thus full charge, full discharge seems to work well and provide maximum capacity, although I am not sure why it is superior. It may be because it helps to avoid the mismatch problem mentioned above, perhaps because empty cells can't self-discharge at different rates and get out of step!

Rod Woolley (Ottawa RC club)