The Battery Clinic
Red Scholefield, 12219 NW 9th Ln., Newberry FL; E-mail: [email protected]
OLD COLUMNIST Resurrected: I guess MA Technical Editor Bob Aberle and Aeromodeling Editor Bob Hunt wanted someone with which to share the misery. They figured that after a quarter century as manager of battery design and application for General Electric, Gates, and Energizer, and after more than 65 years as a glue dauber and balsa butcher, I might have some useful information to pass along.
Through The R/C Battery Clinic (www.rcbatteryclinic.com) and as Battery/Charger Forum moderator on RC Universe (www.rcuniverse.com), I thought I had educated every modeler who had ever chewed Ambroid off of his or her fingers. They assured me, however, that there are now readers to whom batteries and chargers are as much a mystery as Ambroid is to them.
Once faced with your first deadline and the suggestion that you should have a name for the column, reality sets in. To keep with the general editorial trend established by various modeling magazines, I considered the existing column names. Ah, "The Engine Shop"; how about "The Battery Clinic"?
Although engines are deemed important to at least part of the RC fraternity, you can fly without them. Batteries, on the other hand, are mandatory for RC activities.
How many engine analyzers do you see offered in the hobby magazines? The proliferation of battery analyzers is evidence that batteries get much more attention. The engines have enough problems of their own without compounding those by trying to "analyze" them with a black box with its own problems.
So welcome to "The Battery Clinic."
Flying season is here; wake up, battery pack! For best results with NiMH and Ni-Cd batteries—those that are new or that have not been used for several months—you should "condition" the packs. Lithium batteries neither need nor require conditioning before use.
As shipped from the factory, cells may be in the "uninformed" state; that is, they have never been given a full charge to convert all of the active material to the charged state. In this condition they may not respond well to fast charge and may not give their rated capacity.
It is best to give these cells one or two cycles of slow charge—something on the order of C/10 (a charge equal to 1/10 of their rated capacity). This first "formation" charge, as it is called, may be beyond the capability of the wall-wart charger that came with your radio if the pack capacity is 1000 mAh or more. Most have some other charge device at hand.
Many of the microprocessor-controlled chargers should be set as close as possible to the desired C/10 current. And if there is the ability to set the peak sensitivity, it should be opened up as much as possible to prevent the charge from being terminated early.
Some chargers have an input-capacity setting, while others have a time-limit to 16 hours.
Discharge rates on these "wake-up" cycles are noncritical. Anything between C/5 and C is okay. Discharging at the C/5 rate is what the manufacturers use to rate the cell capacity. Higher discharge will give you a slightly lower reading on capacity.
Requiem for a Dead Battery: People are becoming more and more aware of our environment and frequently ask where and how to dispose of battery packs that are no longer serviceable. They should be turned in for recycling.
You can learn about disposal of all battery types we encounter at the Rechargeable Battery Recycling Corporation's Web site: www.rbrc.org/. For those without Internet access, RadioShack, The Home Depot, and other similar consumer-product outlets usually accept these returns.
You can find some good battery "deals" by sorting through some of these returned batteries. An early survey showed that there was nothing wrong with a significant number of the batteries returned by consumers—just a charger problem, a device problem, or a user problem.
Knowing how to sort the good cells from the not so good helps. Foremost, look for the pack voltage. If it reads close to the label voltage, there is a good chance that there are some usable cells in the pack. These are the ones to take back to your shop for a closer look.
Open the pack, remove the cells, and check individual voltages. Also look for any discoloring, white powder, or evidence of leakage. Discard any cells that are significantly lower in voltage than the others in the pack. Don't pull off the interconnecting tabs; cut in the middle because the tabs are needed to solder to later when you make a pack. Avoid soldering directly to the pack, particularly the positive end. Charge and cycle the individual cells to check capacity.
The final test is to discharge the cell completely—down to 0.9 volt or less—and then short it out for 24 hours. Remove the short and let it sit for another 24 hours. If the voltage comes back to more than 0.5 volt, the cell is good for service. These are "used" cells; how long they will last is anyone's guess, but for noncritical applications you can't beat the price.
Shop Safety: Have you ever forgotten and left your soldering iron plugged in? To avoid this concern, use a timer or a timed outlet that will conveniently shut your iron off after a time you crank in.
These outlets are available at most hardware stores and Lowe's or The Home Depot. A good choice is the Intermatic 20-amp, 125-volt, AC, 60-minute, spring-wound time switch (model FD60MW). It can be mounted in a standard receptacle box and control several outlets. Just plug in your soldering or covering iron, and set the timer for as many as 60 minutes.
Getting the Most From Lithium: By now it is well established that Lithium packs are different from Ni-Cd packs; we can abuse those to no end and they keep on ticking. A prominent Li-Poly-pack source has said that one can nearly double a pack's cycle life by limiting the charge and discharge.
So rather than pack it to the limit with a 4.2-volt/cell voltage cutoff, back it off to 4.15 volts/cell, which equates to 90%, give or take a few percent. Some of the Lithium-capable chargers I have tested at The Battery Clinic do this already—and I downgraded them a bit for not fully charging the pack. Try to keep the discharge to something less than 80%. A cutoff at approximately 3.1 volts does that.
So you end up sacrificing a bit of flying time for a significant bonus in the number of flights. If you must have more flying time, look at the economics of running higher-capacity packs or running packs in parallel.
FMA Direct sells an Automatic Cell Detect Low Voltage Cutoff (item AVC1A1R) for use with ESCs that do not provide proper cutoff voltage for Li-Poly packs. The device will work with any ESC that provides motor cutoff on loss of signal.
It is imperative that Li-Poly cells not be allowed to discharge lower than the manufacturer's recommended voltage. To do so can permanently damage the cell and potentially cause a hazardous situation.
Until recently, not many ESCs were designed for use with Li-Poly batteries. Many can be programmed for a cutoff voltage, but the FMA Direct unit does the programming automatically. It senses pack voltage and automatically determines the cutoff voltage. To set a more conservative cutoff voltage to enhance cycle life, the low-voltage cutoff point can be programmed by using a voltmeter and flipping a tiny switch on the printed circuit board.
The Automatic Cell Detect Low Voltage Cutoff installs between the receiver's throttle connection and the ESC. Installation requires that you connect a single wire to the battery pack + connector terminal.
When the pack reaches a safe cutoff voltage, the device terminates the throttle signal to the ESC, and then the ESC cuts off the motor while retaining adequate battery power to operate the receiver for safe landing. The unit includes multiple-restart capability found in most ESCs by retarding and then advancing the throttle. It can be used on brushless and brushed ESCs.
The Importance of Cell Balancing in Electric Power Packs: This issue becomes more critical with Lithium technology, primarily because Lithium packs should not be subjected to overcharge. On the other hand, Ni-Cd and NiMH packs usually see some degree of overcharge in the form of a topping charge, which assures that all of the cells reach the same state of charge, so it never became a big issue.
Lithium packs require essentially a constant potential charge regime as opposed to the constant current we use with the NiCd systems. As we start adding cells in series, it becomes increasingly difficult to assure that all cells reach the same voltage.
In a three-cell Li-Poly pack, for instance, where the end-of-charge voltage would be 12.6, which is the sum of the individual cell voltages, the individual cells could be out of step. One cell would be at 4.05, another would be at 4.15, and the other would be at 4.40—way beyond the safe limit.
If any cell measures more than 10 mV (.01 volt) different from another, they should be charged individually to bring them back into balance. How the cells get out of balance is unimportant; it could be the electrochemistry, subtle differences in the cell construction, or a number of other reasons.
Some pack assemblers have gone to great pains to give us means to counter this problem. They have provided taps to each cell and a device that monitors all the cells during charge and terminates the charge as soon as one cell reaches 4.2 volts. These taps to each cell in the pack enable one to charge individual cells if any unbalance is found.
This technique does not balance the cells. That must be done by charging individually through the adapter cable supplied with JST connectors to access the individual cells.
Lacking this, you should find a means to tap the pack so you can actually measure the individual cell voltages to check the balance after every half-dozen flights. If you assemble your own packs from individual cells, you can run a short wire from the connection point, tin the end, and use that to measure the cell voltage.
I soldered a small piece of tab material to the end and covered the termination with masking tape, leaving a small space open to probe the cells. On another factory-assembled pack, I found that I could access the intercell connections by simply cutting away a tiny bit of the shrink sleeve over that area.
My low-voltage alarm is beeping; it's time to recharge. I'll be back on alternating months at "The Battery Clinic." I hope you will not be shy about hitting me with your questions; I'll answer them personally or in the column if they are of general interest. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
