Nicad Battery Chargers
The proper care of nicad batteries includes regular charging, visual inspection and periodic discharging. There are many products on the market that are capable of carrying out some or all of these tasks and in different manners. Some work better than others and some can be over complicated depending on the user requirements. I hope to convey different charging methods, general product limitations and how best to live with them. These products can be called battery cyclers, chargers, analyzers or conditioners.
First a short note on battery memory, verses voltage depression. Most laymen confuse the two or package them together in the category as nicad memory. Originally, memory effect was experienced in orbiting satellite batteries subjected to a repeated discharge/charge cycle that was a fixed percentage of total capacity. This was due to the earth’s shadow. After many revolutions around the earth or cycles, when called upon to provide the full capacity, the battery declined to do so. The above is not really applicable to model helicopters since our batteries are not subjected to these same conditions.
In our case what is relevant is voltage depression. Voltage depression can be attributed to physical changes in the negative or cadmium plate. Charging involves converting Cd (0H) to Cd metal. Usually under moderate charging currents, the cadmium that is deposited is made up of very small crystals. Grain boundaries between the crystals given time, have the tendency for the grains to come together and form larger crystals.
This is not good for the battery since it makes the cadmium harder to dissolve during high current discharge, which leads to increased internal resistance and voltage depression. Battery capacity is reduced. To avoid voltage depression means avoiding the formation of large crystal cadmium. Very slow charging is bad, since slow crystal growth aids large crystal development. High temperatures are bad, since the growth of crystals is increased by elevated temperature.
The problem is that given time, one will get a growth of cadmium crystals nevertheless, and so the material needs to be broken down into smaller crystals. Partial cycling of the battery means that the material deep within the plate never gets reformed. This leads to a growth of the crystals size. By administering a correct discharge/charge cycle, one breaks down the large crystal cadmium into a smaller microcrystalline form which is best for discharging.
There are two popular methods of charging a battery, one is called constant current charging and the other constant voltage. Although both work, the constant current method is the best. A less understood third method is called negative pulse charging or reflex charging.
As the name suggests the constant current method insures that throughout the charge the same current is flowing into the battery. This is regardless of a changing terminal voltage at the battery as its state of charge alters. The constant current can be measured over a time period to ensure that the required energy is put back into the battery, indicative of a fully charged state and without excessive overcharging. A fully discharged nicad battery needs 140 percent of its amp/hour rating inputted to become fully charged.
The other benefit of constant current charging is that excessively high current will not be forced into the battery at the early stages of charging. Battery temperature will be consistent and lower throughout the charge as will the peak stress level. Excessive heat will damage the battery over time. Furthermore, if a current is high enough and the battery is fully charged, this same constant current may heat up the battery causing it to vent electrolyte. A good constant current charger will sense the battery is reaching full capacity and then drop to a lower charge rate thus preventing a damaging overcharge.
It does this by sampling the battery voltage because nicads have a noticeable peak rise in voltage as they reach full capacity, before dropping off somewhat. Hence the term peak voltage detection is used do describe this method of terminating a main charge cycle. Some cell types can reach over 1.6 volts towards the end of the charge. When full sized aircraft nicad batteries are serviced they use the constant current charging or reflex pulse methods just like our models.
Constant potential, also called constant voltage charging is the other method of charging nicads. Cars, aircraft, boats and motorcycles normally use this method. As indicated the charger voltage remains relatively constant. As the battery charge state rises the charging current will be reduced. Much higher current will initially flow when the battery is charged in a depleted state. Again as the battery comes up in capacity this current will gradually drop. In this manner we have no way of knowing exactly how much energy has been supplied to the battery.
Granted full capacity will be reached eventually if the battery is serviceable and left on charge long enough. If the charge rate is high enough overcharging will occur, so again an unattended fast charger should have a peak voltage detection feature to drop to a lower charge rate.
Reflex chargers can be purchased for model use. The asserted advantages they have are, extended battery life, and the prevention of any so called memory issues. This does not remove the requirement for periodic capacity checks though. They have the ability to fast charge with less heat build up in the pack. Since routine discharging for conditioning purposes is not usually required, less stress is imposed upon the battery.
Claims by some charger manufacturers are made of 5-10 times the battery charge cycle life. Make no mistake these are good chargers if you can afford one. It is hard to find a combination of this charger type with an integral discharger/analyzer feature (in the hobby industry) so a separate discharger might have to be purchased. TRC Engineering make a full functioned hobby type reflex analyzer unit called the “Impulse” and there may be others.
Picture# 1 A typical reflex charger designed for the field with voltage peak detection.
Here is how the pulse charging basically operates. Positive pulses are supplied to the battery for charging it. In addition shorter duration higher peak negative discharge pulses “may” also be furnished during the charge cycle. The reflex charging method specifically employs positive current pulses alternated with these shorter negative current discharge pulses. The battery is claimed to be conditioned each and every second by redirecting the gas particles which migrate to the battery plates, eliminating the damaging effect of charge plate oxidation. The discharge pulse helps to reabsorb the oxygen produced during the high charge current phase.
The negative pulse charging method is said to reduce or continuously break down the crystalline size on the plates thus lowering the battery internal resistance. It reconstructs the older cells by reducing the crystals in size that grow too big during normal use. These crystals decrease the reaction surface of the chemicals inside the cell, reducing the ability to transfer energy back and forth. Furthermore large crystals can eventually grow through the separator located between the plates and cause the cell to short internally or to have a high self discharge rate.
By using the reflex pulse charging method batteries low in capacity can sometimes be rejuvenated. This is said to eliminate the requirement to deep discharge a battery before recharging to erase the impending “memory effect” (voltage depression). Read more on this voltage depression matter earlier in the article. Charging can be very rapid due to high efficency, low heat, and many nicad types can be charged safely in as little as 30 minutes.
The question is which “pulse” chargers employ the negative pulse to make them similar to the reflex type. Due to generalities sometimes found in advertising specifications, I suggest proper research before you buy. For extensive technical information pertaining to this charging method on the web try the following sites:
“The heart of Christie’s unique system is the ReFLEX charging method. With this design, sharp, negative-current, discharge pulses are alternated with positive-current charging pulses. This process prevents the developement of gasses across the plate area of NiCd batteries, and rejuvinates the crystal structure of the cadmium anodes. Charging efficiency is improved greatly, and the battery actually undergoes a electrochemical restoration.”
The reflex charging method has its proponents and adversaries, but I do know one thing…. they work very well as hobby chargers, all hype aside. Most people in the aviation community have already or are presently considering switching to reflex charger/analyzer type simply because they work so well, extend battery life and save servicing time. These are the reasons why I became so interested in this charging method to the point of actually purchasing a reflexed hobby unit.
The pictures numbered 1 and 2 depicts negative pulse reflex chargers manufactured by hobby companies. While both have no discharge measuring feature (cycling) they are able to operate from a 12 volt field box or automobile electrical system. It is not possible to overcharge since the battery voltage is fully monitored. The neat thing about some peak detection methods is that it is done between pulses (after the discharge pulse) and in the quiet time when the charger output has no masking effect on the measurements.
These two can charge one receiver and one transmitter pack at the same time. Some chargers automatically go into a pulsed maintenance mode to maintain full available battery capacity. Full reverse polarity protection is provided with 750ma fast charge rate. While the charge rate is not the highest considering some of the bigger packs, remember when a pack is topped up it needs less charge time than a fully depleted pack. For use at the flying field it works great over lunch time.
I have even plugged into the cigarette lighter during the drive to the flying field to top up the battery packs. Diagram 2A shows what the typical reflex charging wave form basically looks like throughout the charge.
Many constant current chargers have a selection switch for the number of cells in a pack while some have auto selection. Charging at a higher cell count selection than what the pack has will eventually cause battery distress. Many will have a charge rate select switch for the main/topping charge. This selection will depend on the size of the battery since a larger battery will handle higher current flow. Selecting too small a value for a battery size may well reduce capacity. The other consideration is time…. how long do you want to wait for the battery to charge.
If you don’t mind replacing the packs more often, much shorter or fast charging may be performed. I have seen packs charged at the half hour rate using peak detection. In this case keep in mind that since the pack is easily subjected to strain, closer periodic attention to its condition is necessary. All batteries are under stress but it is strain which causes physical damage, in addition to the cycle life fatigue. Some cells types are better than others for high discharge/charge rates. A practical example would be Sanyo SCR fast charging cells.
An entry level constant current charger employing a timer instead of peak voltage detection. This one incorporates a discharge feature for measuring battery capacity. Sometimes called a ‘battery cycler’.
A battery cycler cannot make up for a defective pack. If you have a cell shorted in a pack then its terminal voltage will be much lower. This is normally how a nicad cell fails. If the charger relies solely on peak detection to prevent overcharging the battery, it will continue to supply higher current. This could cause more damage to the pack, or possibly worse in the case of a very high and continuous charge rate.
Having a timer feature in addition to peak detection is a good idea if things are to be left unattended. While very rare, the battery could enter a thermal runaway and explode. I have seen the nose of a Bell 206 blown off from this destructive condition. Many model applicable transmitter battery packs now come with a thermal protector which will actually break the circuit at extreme temperatures. Reputable battery pack re-builders usually reinstall said device when rebuilding a transmitter pack.
While the constant voltage charger that came with your radio system works fine for charging the supplied battery it is not meant for battery conditioning or inspection. It is also not meant for the bigger packs. Leaving the thing on slow overcharge for a long time, and often, encourages the development of larger crystals and voltage depression. For battery conditioning and inspection you will need the specialized charger.
The only way to accurately know if a battery meets a minimum established requirement of 85 percent capacity is to discharge it at a known current over the required time period. Cycling a battery low in its specified capacity can restore it to a serviceable state by breaking down the larger crystals into smaller ones. I’d recommend battery cyclers due to the ease of use, confidence and the safety factor they add to the hobby. Shop around since you requirements may well be lower or different than those of someone else flying electrics or those with a large fleet.
Battery cycling is nothing more than fully charging followed by discharging down to an average cell voltage to about 1.1 volts. One or two full cycles should bring the battery to full capacity. As a last resort the cells can be deep cycled. Deep cycling, a less understood method, breaks down the crystal structure deep within the plates at the advanced stages of voltage depression. Deep cycling means that the battery cells are taken below 1.1 volts, then shorted individually, left to set for several hours, then full cycled by the normal method.
A 1.5 volt light bulb helps to bring down the individual cells and can be used as a shorting resistor. Shorting without a resistor should be done at .5volt and below otherwise expect the shorting strap to overheat. You will obviously need four alligator clip assemblies in conjunction with shrink rap removal. These methods will remove any voltage depression or cell imbalance occurring in a good battery pack by breaking down the crystalline size deep within the plates. Deep cycling is harder on the battery and should only be used when absolutely necessary if other methods fail.
Remember that it is harmful for a multi-cell battery to be discharged at too low a level because of the danger of individual cell polarity reversal. You may cause more harm than good by driving cells into reverse polarity and damaging the internal structure. For this reason, manufactured discharge equipment usually and should offer an automatic discharge cut-off, by sensing a predetermined voltage level.
My recommendations to the average person operating glow or gas powered machines is to initially purchase a good quality constant current charger/discharger with peak detection. It should have 4 and five cell select for the receiver packs and at least two charge current rate selections applicable to the packs in use. Two or more discharge rates are desirable in case you want to fully certify a battery pack overnight. It should also charge and cycle the transmitter pack at a slow and fast rate. It should have a timed shut off feature to prevent excessive overcharge should the peak detection fail in the case of a shorted cell.
Personally I prefer the unit to operate off the line AC in my home and off the 12 DC system of my car or field box. The unit should record the discharge energy extracted in milliamp/hrs to save you figuring out the capacity using time and discharge rate. If you have a cycler without peak detection built-in as in picture 3, a peak detector can be added in line, at a cost less than that of replacing the charger. These “add on” peak detectors are however getting harder to find these days. You may also want a reverse polarity feature built into a charger at the DC input and output.
A caution with many cyclers is to keep the charge leads short. If you must use longer leads, then install heavier wire to reduce the voltage drop. The smaller gage wire of the plug can be spliced in at the end. Some chargers have banana plugs much like the ones on our field boxes. Using this connection method sometimes offers Murphy a seat at the dinner table since reverse polarity can ruin a battery charger. A charger with a 12VDC input has double the chance for a screw up. Even with color coded leads mistakes can happen. Personally, I hard wire the charger output leads for this reason. Let me put it another way, how many people have blown a perfectly good glow plug by inserting the glow driver into the electric starter sockets of a power panel!
Most any tool is only as good as the man holding it. Discharging a battery at a rate higher than it is designed for can indicate low capacity when in fact the battery is perfectly fine. An example might be discharging a 500ma pack at the one hour rate of ½ amp (500ma). Selecting the 250ma discharge rate could well result in full capacity results. Excessive cycling of the battery will lower its lifespan since you are after all using up valuable cycles. Capacity checks need only be carried out every month to every couple of months.
Here are some other companies that supply cyclers of various types such as ACE, Novak, Hangar 9, Astro Flight, Hobbico, FMA Direct, Hitec. Ace seems to have a very wide selection of different chargers. Quality but expensive cyclers can be purchased from non hobby charger specific companies like Christie.
The next time you read a review of a specific battery cycler or investigate a purchase I hope you will now be better armed. If a charger manufacturer cannot answer you basic needs look elsewhere, since there are many fish in the sea at reasonable pricing. All I can say is buy right and buy once. Your expensive helicopter and the safety of others depends on the condition of your machine as a whole. At the very least I hope you’ve come to appreciate the importance of periodically checking a battery pack for adequate capacity through measured discharge.
Remember again, cycling batteries too often causes more harm than good so use your better judgement depending on the operating situation. All batteries wear out eventually so careful consideration to age needs to be factored into battery pack rebuilding. The information contained is meant to be of general interest in nature as much more depth to the subject exists.
Here are some basic terms and established values:
– Trickle charge: The rate at which a battery can be left on charge indefinitely without distress and usually is at 1/100th of the packs capacity rating.
– Overnight charge: The rate for a slow full charge between 10 to 14 hours in duration at 1/10th of the battery packs capacity rating.
– Fast charge: The high current charge rate specification traversing from a few hours to as little as 20 minutes. This will vary with cell type and manufacturer.
– Timed charging: The duration of the charge is dictated by a clock or timer and not by the battery state of charge.
– Peak voltage detection charging: The main charge is terminated by peak battery voltage detection so overcharging is prevented.
– Battery capacity: The maximum amount of electrical energy a battery can supply expressed in miliamphere/hours (ma) and specified at a specific discharge rate and temperature.