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Disclaimer
This article deals primarily with care and maintenance of nickel cadmium (NiCd) batteries, primarily radio packs, because they are the most prevalent in R/C applications. However, most of the methods described herein will work well for nickel metal-hydride (NiMh) as well. Lithuim-ion cells are a completely different breed, and none of the information here is intended for these types of batteries.
Background
Nickel cadmium batteries have a few quirks that must be understood for proper battery care. First, NiCds have a unique voltage discharge curve, and packs quickly drop voltage at the end and beginning of a charge. Second, NiCd voltage varies greatly at a given charge level, depending on factors such as temperature, how the pack was last used, and the load on the battery. Third, NiCd batteries will self discharge if left on their own for extended periods of time. Lastly, the total capacity of a pack doesn't fluctuate, yet slowly drops as the pack ages.
NiCd voltage does not decrease steadily as the charge is depleted. There is a sharp drop within the first 10-20% of the charge, and another sharp drop in the last 5-10% of the charge. This leaves 70-85% of the charge where the voltage is nearly constant. As a consequence, there is little to no difference between the voltage of a 20% depleted battery and an 80% depleted one.
The voltage of a nickel cadmium battery varies with many factors. First, voltage will decrease as the charge is depleted, which is a well known fact to most modelers. Lesser known are the various other factors which affect voltage. Temperature, the charge rate of the previous charge, the discharge cycle since the pack's last charge, and current load can all affect the voltage of a pack. These complex interactions between the pack's usage and voltage are usually more significant than the change in voltage due to the depletion of the majority of pack's useful charge.
If left on their own, NiCd batteries will slowly lose their charge. Typically, this loss amounts to about 1% of the pack's capacity per day, though this will vary with the type of pack. Additionally, this self discharge tends to tire the pack, and the voltage will not be as high as a pack which has been discharged immediately after charge. Fortunately, a full discharge and charge on a pack will usually remedy this latter effect.
I contend that NiCd batteries have no "memory" on the total capacity of a pack. Total pack capacity slowly decreases with age and abuse, but does not fluctuate from charge to charge. I have found, however, that the useful capacity of a pack might vary, simply because the voltage may be so low at a certain charge point that the rest of the charge may be unusable by the system the pack is powering. The voltage can be depressed in this way by any of the factors mentioned earlier, though it is most commonly observed by self-discharge tiring a pack.
You will notice that throughout this background information, I have elaborated on the various effects on the voltage of a pack, and for good reason. What we want to know at all times is the remaining capacity of our pack. Capacity, however, can only be measured by fully discharging a pack, something we can't do at will when we want to check capacity. For this reason, many modelers resort to voltmeters of various types to try to figure out the remaining charge in a pack. Even cyclers have to measure the pack's voltage as they try to determine the end of a charge. NiCd's erratic voltage creates a lot of confusion when trying to determine the state of a battery, but this confusion can be minimised if you understand the causes of this erratic nature.
Voltmeters
As a battery monitoring tool, I find that a voltmeter is nearly useless. As described earlier, the voltage of a pack is a poor indicator of the pack's charge over about 80% of the pack's run. It will indicate the extremes, when the pack is nearly fully charged, and nearly fully empty, but is a poor indicator of charge state in the middle.
Safety-wise, there's nothing wrong with using a voltmeter to check battery status. If you discontinue using a four cell receiver pack when its voltage reaches 5.0 volts (1.25 volts per cell) or so, then there is little chance that a pack will be dead at the end of a flight. However, the pack gets to 5.0 volts within the first 20% of the battery's charge, which means that if you stop using it then, you're wasting 80% of the battery's capacity. You're basically carrying a battery that's larger and heavier than you need, and you would be better served picking one that was one-quarter the size.
Furthermore, without cycling, this high cutoff point tends to lead to a self-defeating condition where the pack reaches its cutoff voltage earlier and earlier each time the pack is charged. This is because the voltage of a pack is depressed in the absence of complete cycles. A single deep discharge cycle will remedy this, but many people don't cycle batteries on a regular basis.
A voltmeter with load does not cure the problems inherent with voltmeters, because the voltage is still constant over the majority of the pack's charge, even with load. A loaded voltmeter is just a little better at catching the extreme low condition of a battery, because it bypasses the battery's ability to recover voltage if left to sit unloaded for a time.
A better alternative to the voltmeter is a peak charger with a milliamp-hour display, taking advantage of the fact that most flights use nearly identical amounts of battery charge. At the end of a normal flight, the battery can be recharged, and the charger will read a certain milliamp-hour reading corresponding to the amount of charge used during that flight. Knowing the capacity of the battery, either by cycling or the battery specification, the modeler can determine how many flights can be flown before the battery needs to be recharged. For example, most of my 40 size planes use about 50mAh per flight, and use 600mAh batteries. If I'm going to fly within 20% of the battery's capacity (a safety margin is necessary), then I can use 80% of the battery's capacity, or 480mAh. At 50mAh per flight, this means I can fly 7 flights safely before recharging. Using this method, regular cycling is necessary to keep tabs on the total capacity of the pack, because the capacity will decrease as the pack ages.
Cycling
A cycler is the single most important tool in battery maintenance. A cycler provides the modeler with the information needed to make wise decisions about battery usage. By keeping track of the capacities of battery packs on a monthly or bimonthly basis, you can keep track of the relative health of your packs.
The only features necessary in a good cycler are a good selection of charge and discharge rates and a milliamp-hour display. It should also run off AC power in some way, whether directly or through a separate DC power supply. Other features are at the discretion of the user, like peak charging, PC interfacing, and high charge rates.
One should not make the mistake of viewing a cycler as a tool for fixing dead battery packs. If a pack has not been used for some time, all that is needed is a single discharge, full charge, then full discharge to check the battery's capacity. This will return the battery to near its best state by clearing voltage hysteresis (dare I say, "memory") caused by an atypical discharge cycle, and give an indicator of the pack's condition. A cycler will generally give better readings after the first cycle of an older pack, but repeated cycling of a pack gives limited returns, and needlessly decreases the life of a pack by exercising it. The variances in the repeated milliamp-hour readings are due to cycling the last of a pack's charge, which affects the voltage cutoff, and therefore affects the charger's decision to stop discharging. Unless you're actually using this last part of the charge, a practice uncommon with radio batteries, the extra cycles are of no benefit.
If a pack cycles less than 75% of its rated capacity, it is time to be retired. I usually cycle the battery twice to be sure that nothing went wrong during the previous cycle before making the decision to retire a pack. And remember, NiCd batteries should be recycled and not thrown away. I take my dead packs to my local Radio Shack for recycling.
Field Charging
I'm a big fan of field charging my batteries. I find that my batteries stay as healthy or healthier than they do on the standard wall charger, and I don't have to plan ahead when I want to go flying. Furthermore, when I used wall chargers and did manage to plan ahead, I often charged only to find that weather or appointments got in the way of flying, leaving the battery nearly full the next week, when I would have to charge again. Constantly keeping this full charge on a pack prematurely wears it down.
Modern field chargers are almost all of the delta-peak variety, which means they stop charging once the pack is full. This way, a pack that is properly field charged will never be overcharged. This is unlike the wall charger, which keeps pumping charge into the battery until you decide to take it off, which almost always results in a slight overcharge.
Fear of field charging stems from older chargers, which were high current timed chargers. These types of chargers would not only overcharge your battery, but do so at a high rate, creating a large amount of heat. High heat damages NiCd cells, and prematurely aged the packs. Modern peak chargers stop charging once the pack is full, so they do not overcharge at this high rate, and the batteries stay cool for all but the very end of the charge. For this reason, field chargers are now as safe as wall chargers, assuming the pack is fairly well balanced. If a pack is severely out of balance, however, a slow charge may be necessary to rebalance the cells to make them fit for field charging. A regular cycling regimen will take care of the occasional slow charge necessary to keep packs in good shape for field charging.
The single most important feature in a field charger is a milliamp-hour charge display, which tells how much charge was put into the pack while it was charging. An adjustable charge rate and cell count proves to be very useful, especially when charging different sizes of batteries. Also important, but fairly standard, is a delta-peak type peak detection. All field chargers are DC chargers, but a computer power supply can be found for under $10, and allows most chargers to be used off the AC power in the home. This is particularly useful for field chargers with cycling ability, because it allows you duplicate the functionality of two tools with only one.
Storage
Nickel-cadmium batteries are best stored in a fully discharged state. This poses one problem, in that discharging packs without special equipment can harm the battery. To be properly discharged, a NiCd pack must not be discharged past the point where any single cell reaches zero voltage. Doing so will permanently ruin the cell, and the pack in turn. To be properly discharged, most packs are discharged to a voltage cutoff point that is unlikely to result in any cell reaching zero volts. This voltage cutoff point is of some dispute, but somewhere between 0.9 and 1.0 volts per cell is typical. Any cycler can be used to discharge packs, and there are special discharge units that will perform this duty as well.
Discharging packs after a day at the field is something that takes a bit of time to do, however, and I usually don't bother. This is one of the positive side-effects of field charging. Because my packs are almost always at least half empty at the end of a day of flying, they are always stored in a mostly discharged state. This results in packs that are overall healthier than ones that I kept ready to fly on my radio's wall chargers.
Trickle charging is really effective for balancing a pack. While the full cells are maintained with a slow charge rate that does not heat them up, the other cells are slowly charged to their peak. Leaving cells on trickle charge for long periods shortens their life, however. The trickle charge maintains a high level of chemical activity in the pack, which tends to degrade the cells over time. Therefore, trickle charging should only be used occasionally to rebalance a pack, and not to keep cells fully charged for their next flight.
Final Comments
I hope you found this article on NiCd batteries useful. If you have any suggestions for additions or clarifications, or if you just want to flame me for my NiCd battery practices, send an email to web@jmack.net.
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