The Battery Clinic - 2006/05
Red Scholefield | [email protected]
Red answers Allan Schneider’s and Dell Erikson’s excellent battery-related questions
I've had several requests for a means to monitor field-box or electric-flight charging batteries. My first recommendation is to use that inexpensive digital voltmeter I keep mentioning that you should have if you are going to mess with batteries. Some have expressed an interest in something such as a Voltwatch device we use for receiver packs. With a little help from Google I recommend that you visit: www.uoguelph.ca/~antoon/circ/batmon12.htm.
Readers Ask: Allan Schneider
Allan Schneider has asked some basic but important questions—particularly for those of you who are getting your models back into the air after a long winter.
Q: Is there a finite number of times Ni-Cds can be recharged or recycled? The guys at my club believe it is between 700 and 1,000. Thus should one count or keep track of such things?
A: Many things come into play in the life equation, including:
- temperature
- vibration
- depth of discharge
- the design of the cells themselves
Cells usually die of separator deterioration (shorts) long before they die of cycle life. The mean time to failure at room temperature is roughly five to seven years. I doubt many packs will see that many full charge/discharge cycles in that time frame.
The easiest way to keep track of your packs—unless you're the bookkeeping type—is to get a decent charger/cycler and check the packs occasionally. This should be every month or so, depending on your flying frequency. Write the capacity and date on the pack. By the time you have no more space to write, the pack will probably have dropped to less than 80% of rated capacity and should be replaced. And no, writing smaller doesn't make the pack last longer!
Q: Does recycling have the same effect on battery life as recharging a partially discharged battery?
A: A full discharge has more impact than a partial discharge, although this doesn't have great significance for most users. If an individual deliberately wanted to wear out packs, he or she could cycle them several times a day. That would take many hundreds of cycles to reach the end of battery life.
Q: How do you know when it is time to replace a Ni-Cd? When it cycles 10%, 15%, or 20% lower than the originally rated capacity? Is there a capacity decrease that automatically dictates a dead cell? I guess 25% for a four-cell battery. Is this correct?
A: The accepted "throwaway" point is when packs will no longer deliver 80% of the rated capacity. Another good indication is to charge the pack, let it sit for a week, and then discharge it. That will reveal any pending separator failure (soft shorts) by yielding far less than 80% capacity.
A dead cell will give you about 1.2 volts less in the voltage reading, or in the rare case of an open or severely dried-out cell you will get no voltage across the pack. Always look for corrosion around the connector and the soldered joints on the pack.
Q: How many consecutive cycles do you do to get an accurate or usable reading: two, three, or more?
A: For packs used frequently—at least once a week—one cycle will give you a reliable reading. For packs that have been stored for longer periods, two or three cycles may be required before you get a stabilized reading.
Q: What is the best "load" to use in cycling mode to get the most valid or usable reading?
A: Packs are rated at the C/5 discharge rate, so that would be the one to use. You should not see much difference with other reasonable loads, however.
Q: What are the risks associated with using a battery that cycles 20% lower than the originally rated capacity? Do they fail? Do they not hold a charge? Will they fail without warning? Is the only risk its diminished capacity?
A: An expanded-scale voltmeter reading before flight will let you know if a cell has died. Cells that read low in capacity usually have a high self-discharge rate too, so if you don't check them before flight they could self-discharge in a couple of days, leaving you short on capacity during a flight.
Packs don't just die suddenly, and they rarely fail in the air unless you count running out of capacity as a failure. I put that in the pilot-error category.
Q: Does lower capacity affect receiver or transmitter performance (lower sensitivity or range)? Those who say yes point to voltage drop; those who say no claim the equipment is designed to work across the battery's discharge range.
A: Receiver and transmitter performance is not that sensitive to the slight voltage drop you might see with a pack near the end of its life. I did a range check using three alkaline cells (4.5 volts) for the receiver and couldn't detect any deterioration. I found the same with eight primary cells down to 1 volt each in a Hitec three-channel transmitter.
Dell Erikson (Minnesota)
Dell Erikson wrote: "After having my digital camera stop working when it gets cold, I thought about what may happen when I fly RC airplanes. When I have flown in the winter the lowest temperatures were in the teens or twenties and I did not fly for long.
My guess is that, like my camera, control will fail. And then possibly 'fly away' or be locked into the settings (turn, up, down ...) when loss of battery power occurred. I hope I am wrong."
Other than diminished capacity of the battery pack under severe cold conditions, I have not heard of any problems with radio systems. Where am I going to get 0° F conditions to test radios here in Florida?
When I lived up north I flew frequently in the wintertime (which seemed to be 10 months of the year), but never when it was 0°—maybe 25°–28°. My thumbs didn't work that well at those temperatures either, and I was flying reeds back then.
It shouldn't be hard for you to do a normal range check when it's extremely cold, just to see if you notice a reduction in range or loss of control. Let me know if you do it and at what temperature.
Otherwise, think warm thoughts. It only got to 60° here today (January 6). Does anyone else have anything to offer regarding cold-weather flying, other than sympathy?
Slow Stick camera model (Mike Robinett)
I thought Mike Robinett's ([email protected]) Slow Stick camera model might be of interest. His objective was to get a low-cost camera airplane in the air—and the results were outstanding. A photo shows an aerial view of his club field.
Specifications:
- Model: GWS Slow Stick
- Motor: E-flite Park 400 outrunner brushless motor
- ESC: 20-amp ESC
- Battery: E-flite 3-cell, 1800 mAh Li-Poly
- Prop: 11 x 7 APC Slow Flyer
- Camera: Aiptek 1.3-megapixel modified for RC with a "backpack" switch to trigger the camera
A photo shows the model's camera installation.
Electric Control-Line (Bob Frogner)
Now that I've drifted out of my battery-column charter I'll share Bob Frogner's experiments with electric-powered control line (ECL). He wrote:
"I have been experimenting with ECL for a little while now and I think there is a definite place for electrics in control line flying. There has been a lot of talk about the stunt fliers and some articles on their airplanes. I have taken a little different approach that I think might get a few more people trying this scheme out.
"I have built a couple of small, 1/2A-sized airplanes. They do not require a large (expensive) motor and many batteries to fly.
"The latest is a foam-wing 1/2A Combat airplane. The wings are Styrofoam (from Hacker blanks available from the Core House) and the rest is just simple building similar to the flat foamies that RCers are used to. It has a 4mm Depron and 1/32 plywood nose section with a motorstick attached, a carbon-fiber boom for the tail, and lightweight covering.
"The motor is a HiMaxx 2812-850 with a Phoenix-25 ESC. I use a Z-tron timer (which works great) and run a Thunder Power 1320 Li-Poly battery. I fly on 38-foot .008 lines and believe it would fly well on 42-foot lines as well. It has plenty of power and can do any maneuver you could do with a gas-powered model. The total cost for the power system, Z-tron, and battery is about $200, which makes it much more economical than the full-sized stunt airplanes.
"You don't need a large airplane to enjoy the benefits of electric power for control line. It's really great to have consistent power all through the flight and have a timer so you know exactly when the motor will stop. I think in the future there will be many more ECL airplanes and I would like to promote the keep-it-simple idea to get more people involved."
Big Stuff
As electric power continues to get more sophisticated, so does the equipment for testing batteries. I got a prototype "monster" on loan to do some high-rate lithium verification (up to 100 amps), so I can tell you what these continuous C rates on Li-Polys that are being touted really mean.
West Mountain Radio is also promising a high-power unit as an add-on to its Computer Battery Analyzer II. I hope to have more information about these units in a month or so.
Product Alert
I have received information that misrepresentation of true cell ratings destroys the cycle life of the lithium pack and costs the customer more money because it needs to be replaced in a few dozen cycles. The aggressive ratings promoted by some vendors also put the modeler at risk; these packs will puff up and possibly blow when used at the promoted current levels for extended periods.
Until next month, safe charging and safe flying.
You can write me at: 12219 NW 9th Ln. Newberry, FL 32669
Please include an SASE if you want a reply.
M/A
Transcribed from original scans by AI. Minor OCR errors may remain.
