ElectroDynamics: Tip of the Month, October 1997 
      Electronics and MORE for the Discerning Modeler! 


Demystifying NiCd's
Part 1

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NiCd: The chemical name for Nickel-Cadmium rechargeable cells. 
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!
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 .60's up front!) 
  very well. 
  They like to fail open-circuit (i.e., the battery goes "dead" if not "topped 
  up" regularly). 

NiMH - "Nickel Metal Hydride": 
  Have much higher energy density than NiCd's (you get a lot more bang for your 
  buck) but: 
  They have higher internal resistance (heavy, sudden servo loads like your 
  "corner-the-sticks" snap rolls can cause your Rx to see a sharp voltage drop - 
  some Rx's just don't like that and "go to sleep"). 
  They must be carefully charged (or they'll vent corrosive liquid). 
  They're great for computers, etc. that draw a constant load, but RC models 
  draw a "spiky" high-current, intermittent load. 
  RC Tx's would be an ideal place to use a NiMH battery, only you cannot use 
  your regular "wall-cube" plug-n-forget charger, for fear of accidental 
  overcharging, which could corrode the insides of that $$$ bells-n-whistles 
  lovely Tx! 

Lithium-Ion: 
  Wonderful stuff, but you gotta have a budget like NASA to afford 'em! 
  So, it's the trusty old NiCd 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!) 
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. 
On that note, we'll leave you to absorb this stuff, and write us a load of Email 
(support@redstickrc.org).

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.
Andy Low