Q-92: I have two kind of related questions. 1) Assuming there is no 'memory' effect in Ni-Cd or NiMH batteries used in RC systems, why should they ever need to be 'cycled' or 'discharged'? 2. What would happen to RC system Ni-Cd or NiMH batteries if they were charged at a 5C or even 8C (very high) rate as opposed to the recommended 2C or 3C fast charge rates?
A-92: I never really believed that the so-called "memory" effect ever
existed. It may have in the older Ni-Cd batteries. It was certainly never a
problem with NiMH, and it is definitely not a problem with new Li-Poly
batteries.
In my way of looking at it, "cycling" and "discharging" are different functions.
"Cycling" to me is taking a battery down and then recharging it. Quite often,
and especially when flying in contests, I use an old motor to run down my
battery pack. I let it cool for a few minutes and then recharge it. Doing that,
I obtain the best possible flights from that pack, from the first flight of the
day. When I "cycle" this way, I'm not trying to determine the pack's capacity.
NiMH packs have poor charge retention. In a week's time they might lose 20% of
their capacity. My first flight of the day with NiMH cells is usually "soft."
The model will be sluggish in flight, and the run time will be less than normal.
But after that first fast charge at the field, subsequent flights will be up to
par (for that particular pack).
"Discharge" to me is when I actually test the battery for its capacity. I use
any of the testing devices on the market and apply a known load, allowing the
pack to go down to approximately 1.0 volt per cell on Ni-Cd/NiMH and 3.0 volts
per cell on Li-Poly batteries. The amount of the load and the time it takes to
get to those minimum voltages allows you to calculate the capacity in mAh.
I do this type of discharge testing on all of my RC transmitters and four-cell
receiver (airborne) battery packs. I never cycle or discharge-test the battery
packs I use for electric-powered flight because they are essentially cycled
(taken all the way down and then fully recharged) on each flight.
Capacity can simply be judged by the aircraft's performance. If the model
doesn't get off the ground or is sluggish in flight, you either have poor
capacity or you improperly charged the battery.
Now to your question about charge rates of 2C or 3C vs. 5C or 8C for RC-system
batteries. Most of the batteries supplied with RC systems (transmitter and
airborne packs) are not intended for fast charging. It won't be stamped on the
battery pack, but believe me on that point. Those batteries are generally the
cheapest available on the RC market. They should be charged only at the
overnight rate (C/10), which you get when you use the supplied RC-system
dual-output wall-plug chargers. Feel free to charge overnight, one, two, or four
times a week, and still get 500-1,000 cycles from your packs.
Also consider that RC transmitters usually have an internal fuse and relatively
lightweight wiring. When you go to 2C or 3C, you can easily burn out fuses and
wiring. Going to 5C or 8C, you could really damage the equipment.
This all comes down to why you would ever want to recharge your RC batteries at
2C, 3C, 5C, or 8C. Today you can purchase AA-size NiMH cells with capacity
ratings of as much as 2,300 mAh. If you switch to those types of cells, you
could literally fly all day. There should never be any need to fast charge at
the field.
These AA NiMH cells are inexpensive, so having a second charged pack to
substitute during the day is a practical solution. However, the only person who
would need that might be a Sailplane flier who is going for the League of Silent
Flight Level V!
Regular electric-flight batteries are the ones intended for fast charging. But
even then the charge currents should be limited to 2C (30 minutes) for NiMH, 3C
(20 minutes) for Ni-Cd, and 1C (one hour) for Li-Poly. More than that is
possible, but only by sacrificing battery life. It simply isn't worth it. I hope
this covers your questions.