Battery Life Expectancy

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Ni-Cd Life - or why is down so quick?

C. Scholefield

While volumes have been written on this subject I would like to relate it to the specific application of R/C , separating fact from fiction and enabling the R/C fraternity to focus on more serious issues of the day, like convincing your wife it's too foggy to clean the pool so you're going flying while the field is not so crowded.

The primary failure mode of Ni-Cd cells (outside of user abuse) is separator deterioration. This will occur in all Ni-Cd batteries as they age. The separator breaks down allowing the plates (electrodes) to touch and short out the battery. Millions of testing hours on thousands of cells has established the mean time to failure of a single cell to be 8 years for cells/batteries maintained at 25C (77F). Higher temperatures will significantly reduce these numbers. Mean time to failure means the time that it takes for half the cells in a given population to fail. As the cells are built into packs the mean time to failure decreases. For a 4 cell receiver pack the mean time to failure comes out to be 5.7 years while an 8 cell transmitter pack falls to 4.8 years. Now it is completely possible that the average R/C modeler doesn't want to tempt statistics to the point where half of his battery packs should have failed. A more reasonable number would be the expected time for 0.1% of his batteries to fail. The number comes out to 58 weeks for a receiver pack and 49 weeks for a transmitter pack. For the more adventurous willing to live with 1 failure in a hundred, he can stretch his receiver pack to 103 weeks and his transmitter to 87 weeks. Does this mean that he should rush out and buy new packs at these intervals? Not really. Proper battery monitoring, while it may not significantly increase life, will give you ample warning that your pack should be considered for replacement. Remember, normal failure is the deterioration of the separator system. As the separator deteriorates (oxidizes) self discharge rate of the battery increases significantly. A pack that looses 15% or more capacity over a week of open circuit stand is at risk. A pack that looses 10% overnight should be used for ballast only. Check your pack with a cycler or some technique that gives you the amount of capacity available immediately after charge and then (after fully charging again) after a rest period of 5 to 7 days.(NO, this isn't MEMORY!). Doing this at least quarterly (if you are fortunate enough to live where you have a flying season longer than 3 months) will greatly increase your odds of crashing by some other defect than battery failure.

The number of cycles you put on your battery is secondary in the life equation, again, assuming you don't abuse them by high rate over charge, vibration or exposure to high temperature. I know of very few people that totally exhaust their battery packs while flying (at least not as a matter of course) so the packs seldom see a full discharge and the risk of cell reversal is nil. Test have demonstrated that hundreds of cycles of reversal where 140% of the rated capacity is taken out in a driven discharge resulted in a capacity loss that was barely measurable. Many multi speed power tools use the technique of tapping the battery for speed control with no adverse effects on the battery. A single cell can be discharged through a load to zero volts without damage. In fact this is a good way to determine if a cell has suffered from separator deterioration. A cell discharged to zero volts will recover to over 1 volt open circuit if left to stand. Those that will not are approaching the steep part of the failure curve and could be a crash waiting to happen. Bottom line: the number of full charge/discharge cycles that can be accumulated by today's Ni-Cd technology is in the 400 to 500 cycle range. Of course partial discharges seen in the R/C application can extend the use cycles to significantly more than this. It doesn't take a battery expert to figure out the amount of flying time you can accumulate on 500 full discharges. We are talking in excess of 1000 hours. If you put in a full two hours a week in the air every week year round, you would be well into the next century before you reached 500 cycles. Separator failure or old age will probably do you in before you run up 500 cycles. Meticulously recording the number of discharge cycles to establish a replacement schedule can be a study in futility and should be left to the electric R/C indoor microfilm pylon set. Don't worry about reversal. If you have left your switch on overnight or for even a couple of days, just give the pack a good long slow charge using your regular charger supplied with the system for 48 hours and you will probably be OK. It would be prudent to run a capacity check cycle after such an incident just to make sure.

Long term overcharge, leaving your packs plugged in to the charger supplied with the system, while considered an acceptable practice for many consumer applications can contribute to a reduction in battery life. First, as a battery goes into overcharge, oxygen is generated on the positive electrode and then recombined on the negative electrode. This oxygen rich atmosphere only accelerates the oxidation of the separator. As the oxygen is recombined on the negative it generates heat.We all know how to make a chemical reaction speed up, turn up the heat.

One further phenomena recently brought to light after years of testing is that of cadmium migration. This is a transfer of cadmium metal through the porous separator structure to form a conductive bridge between the electrodes. In simple terms a high resistance short which causes the cell to self discharge, shunts charging current to where the cell takes longer to charge and ultimately, if left of continue, become a hard short which, if happens during a period when batteries are part of, or contributing to the direction of an airborne operation, result in a rapid depletion of model resources. The same testing reference also confirms that the same amount of charge put into the battery in a short period significantly reduces the cadmium migration. Therefore using a simple appliance timer to switch your charger on for about an hour a day minimizes the overcharge and yet maintains the packs at peak charge should an airborne operation be called for at any time. For the experimenter, a charger designed to charge the battery at C rate (1 hour rate) run at a 10 to one duty cycle (on 0.1 second, off 1 second) is more effective than charging continuously at the C/10 (10 hour rate common to most system chargers) and will enhance battery life. For a maintenance charge a 25 to 1 duty cycle is recommended. This pulse charge is better than even a very low trickle charge for maintaining the battery as cadmium migration is driven by passing current through the separator (charging) over a period of time. The rate of cadmium migration does not seem to increase proportionally to the current density, leaving us with the conclusion that getting the job done (replacing charge loss through inherent self discharge) quickly by a pulse of charge current is better than dragging it out with a long sustained overcharge. While this gives battery a break it will probably give rise to a new generation of exotic (expensive) chargers focusing on the dreaded cadmium migration phenomena (hereafter referred to as CMP, people only take three letter problems seriously) and leave the dreaded memory effect (DME) alone for awhile. Just remember that you can do the same thing with a $5.00 timer and spend the savings on a subscription to your favorite R/C magazine, RCM.

Storing the battery is no big deal. Living in Florida where there are no cool (damp, dark, moldy) basement work shops, I store my batteries in the refrigerator on off flying season (July 3rd 9:30 AM to July 4th 7:00 AM). Those living in Northern climates don't really have anything to worry about (there must be some advantage) but should remember about the trunks of cars and what happens to batteries you leave them in there when you are visiting us for a winter flying vacation.

Looking at the battery voltage after several months of storage is an excellent way to pick out a weak cell (use straight pins to probe each cell). If a cell voltage after several months drops noticeably below any of the others, beware. You have a potential problem and the pack should be relegated to some benign surface application. While we are on the subject of measuring battery voltage, consider getting one of the little digital voltmeters available through electronic hobby outlets. They give you a precise reading and are well worth the modest investment. Second piece of advice. don't listen to the R/C car guys when it comes to batteries, they have never experienced the thrill of real rip roaring, crank shaft bending, dirt in the transmitter, kind of crash and as a consequence take liberties with batteries that would make Leclanche and Volta turn over in their graves to say nothing about causing me just a little heart burn when they get me cornered in "technical" conversations.

Now that we have addressed the life of the battery pack to some extent we should take a quick look at the life of the plane, which is directly connected to " How long can I fly on a fully charged pack?

Know what your system consumes in the way of energy per minute of flight. This can be determined by first charging a pack and then discharging it on a cycler to determine how much capacity it has - fully charged. Then recharge and go fly. Record your system on time and immediately discharge the pack when you return home. This will tell you how much capacity you have left. Lets say you fly for 40 minutes and when you discharge the pack you get 390 mAh. From your initial discharge from a fully charge pack you got 585 mAh. This would mean that you discharged 195 mAh in the 40 minutes you flew or about 5  mAh/min. From this you would know that your pack is good for 116 minutes of flight time under the actual flight loads. Now we don't  want to take it this close so give yourself (and your plane) some margin of safety -25%. So this would set your SAFE flight time to 75% of 116minutes or 1 hour 27 minutes. This should be done for each plane.  Also it should be done for your transmitter at least once just to accurately characterize its "flight time". The system usage will vary, depending on your flying style, size of the plane and number of servos used. (Excerpt from article on  Keep your batteries in the ready state without damaging overcharge.  )
 

CLS 1/24/91 (published in RCM 6/91)
Rev 10/31/96
Rev  2/24/04

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