(updated, June 2005)
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NiCd's are used almost exclusively in RC applications, for powering Rx's and Tx's and for electric motors, glowplug drivers and electronic ignitions. In short, NiCd's are everywhere in RC!
NiCd's are manufactured in a number of flavors - Sealed Cells and Vented Cells being the most common. RC NiCd's are of the Sealed Cell variety, so the following discussion refers to them exclusively.
NiCd's are ideal for RC, due to their l-o-o-w internal resistance. In layman's terms, that means that under heavy load, their terminal voltage does not drop significantly. Also, NiCd's have good energy density (i.e., they pack a lot of z-a-a-p for their weight). Sealed Cell NiCd's are relatively easy to charge, are robust (they withstand vibration (and some crashes!) very well). Finally, they have been around for a good, long time, so they have become very economical as industry has gained lots of experience manufacturing them in very large quantities for power tools, appliances, toys, and yes, RC!
NiMH - Nickel Metal Hydride
NiMH's have much high energy density than NiCd's (smaller and lighter for the
same capacity as NiCd, or more capacity for the same size) but:
- They have slightly higher internal resistance (heavy, sudden servo loads like your
"corner-the-sticks" snap rolls can cause your Rx to see a sharp voltage drop.)
But, recent technology has improved this especially in the larger size
packs (Sub-C's and up), so that this is no longer a serious disadvantage.
They must be carefully charged (or they could vent corrosive liquid).
We used to be wary of NiMH's, but in the last few years, the technology has
improved to the point where we can now recommend their use in RC equipment, with
the restrictions that they should be charged at the "overnight" rate, and
never fast-charged at anything higher than the 1-hour rate, and then only with a
charger that is designed to handle NiMH.
NiMH's may, for most applications, be used as "drop-in", higher-capacity
replacements for NiCd's. NiMH's share the same many of the characteristics
of NiCd's, so most of the procedures used on NiCd's apply equally to
NiMH's, with the exception of fast charging.
In contrast, other battery technologies have their shortcomings:
Lead-acid "Wet Cells", "Gel Cells"
- Very cheap but:
- Care must be exercised when charging (they vent hydrogen gas!)
- They don't withstand high frequency vibration (our screaming motors up front!) very well.
- They like to fail open-circuit (i.e., the battery goes "dead" if not "topped up" regularly).
Lithium-Ion, Lithium-Polyner (LiPo)
- Light, high-density, but rather expensive.
- Extreme care must be exercised
in use, and especially when charging. They cannot
and must not be charged with NiCd or NiMH chargers, there is a fire and
- Proper attention must
be paid during installation to avoid puncturing or deforming them when in use,
the "guts" are flammable.
- They are NOT
for plug-and-forget charging, there is a possible fire hazard.
So, we recommend that modelers stay with NiCd's and NiMH's for now...
Why are NiCd's "easy to charge"?
NiCd chemistry generates oxygen during charging. The oxygen generated by one electrode in each cell is absorbed by the opposite electrode in the same cell, so that if the battery is charged at the C/10 rate, all the oxygen generated is totally absorbed internally! What does it mean? It means, you can have a truly plug-in-n-forget charging system! PROVIDED, of course, you are charging at the C/10 rate.
What's a C/10 rate? C refers to the rated capacity of the NiCd battery, in mAH (milliamp-Hours), and C/10 refers to the number of milliamps (mA) you get if you divide C by 10. E.g., a "standard" battery that comes with most radio systems is rated at 600mAH. The C/10 rate is then 600/10 = 60mA.
By the way, it's incorrect to refer to a 600mAH battery as a "600 milliamp" battery. Milliamps is a measure of the flow of electrons or current in a circuit (kinda like the number of gallons per second coming out of your faucet), while milliamp-Hours is a measure of the total number of electrons or capacity (kinda like the total number of gallons you got out of your faucet after leaving it on for a few minutes). So... please, mAH and mA are totally different beasts!
NiCd's are rated in mAH. I.e., the number of hours during which you can draw a fixed current, or the number of milliamps you need to draw to exhaust the battery in one hour. E.g., a 600mAH battery is rated to run for 10 hours at 60mA draw, or to deliver 600mA or one hour. Or, 200mA (your typical 4-channel Tx) for 3 hours. You get the idea, Capacity (in mAH) = Load (in mA) x Time (in Hours).
What's this about "plug-in-n-forget" charging? Well, if you have a 600mAH battery, and you charge it at 60mA (C/10 - most "wall cube chargers" do this), you can fully charge the battery in 14 hours.
Whoa! Wait-a-minnit! Why not 10? Doesn't 60mA x 10 Hours = 600mAH? Unfortunately, the charging process is not 100% efficient, so we need to put in 140% to get 100% back.
Here comes the dirty little secret: If you leave the darned thing on charge (at the C/10 rate), you can leave it on for 90 days (yes, Bubba, THREE WHOLE MONTHS!) after that, WITHOUT DAMAGING the battery! Yep, there's no need for fancy trickle chargers!! (See, you learn stuff here!)
This does not
apply to NiMH, they will get quite warm on overcharging, we recommend taking
them off the overnight charger after 16 hours,
this is where a fancier charger with an automatic trickle function is a good
Are there any caveats (restrictions, etc.)? Yes, don't charge your battery at below 50 degrees, or above 100 degrees. Otherwise, have at it!
Are we suggesting you leave your batteries on the wall cube charger for 3 months? Not really... (We don't trust the AC power line that much!) But, we are saying that there's no need to be paranoid if you "forgot" and left the charger hooked up for a week!
Having said that, them fancy trickle chargers are nice to have anyway, mainly 'cos they blink at you when they are done, reminding you that your batteries are ready, and they do offer a some measure of protection against the elements sending your NiCd's a nasty surge during a thunderstorm.
Next time, we'll talk about what's voltage and current, their effects on your NiCd's, servos and radio equipment, and how to test NiCd's. And, we'll answer FAQ's that'll surely result from this TOTM (Tip Of The Month)!
Till then, ciao. Make all your landings soft ones.
Third Order Intermodulation (3IM)
De-Mystiflying NiCd's Part 1
De-Mystiflying NiCd's Part 2
De-Mystiflying NiCd's Part 3
Installing RC Equipment in Larger Aircraft pdf file (250kB)