|
Demystifying NiCd's
Part 2
Copyright, ElectroDynamics, Inc.
This page may be printed and distributed freely, only if it includes the ElectroDynamics
Logo, this copyright notice, Website link and an acknowledement of its
source.
http://www.electrodynam.com
Hi! This is the second part of our "put-it-in-layman's terms" articles on NiCd batteries.
Voltage, Current, Capacity and What's in a NiCd battery
What's this Volt / Amp thing anyway?
Many people are mystified by things electrical, but readily understand things mechanical
(at least modelers should!). We will use a simple mechanical analogy to explain these
electrical concepts.
Electricity behaves somewhat like water... Water flows in pipes, and it can have pressure.
Electricity? Why, it's composed of itty-bitty things called electrons, which also flow in
electric pipes (called wires), and can possess electrical pressure.
Water flow is measured in gallons(or litres)/minute, and water pressure is measured in
Pounds-per-sq. in, PSI (or kPa). Similarly, electron flow is measured in Amperes, or Amps
for short, and "electron pressure" is measured in Volts.
OK. How do we make water flow thru a pipe and do useful work (like run a water wheel,
turbine, etc.)? We connect pipes to the water wheel, pressurize a supply of water (using
a pump), and voila!... The water wheel turns.
How do we make electrons flow and do useful work (like run a motor)? We connect wires to
the motor, pressurize a supply of water (using a battery) and... huh, this analogy works
great!
So... what's in a battery? A supply of electrons ("fuel"), and an "electron pump"!
Simple, eh?
OK. we need our motor to put out more work. What to do? Increase the electron pressure,
of course! We connect two (or more) electron supplies/pumps back-to-back... This type
of connection is called a "Series" connection.
How much long will our motor run? Why, as long as the "fuel" supply lasts, of course!
If we increased the pressure in the system, what's gonna happen? Why, that's easy, the
motor will run faster, and use up the "fuel" quicker! What if we changed the motor to a
bigger one that takes more electron flow... why, the "fuel" will run our quicker!
How do we measure the amount of "fuel" in our battery? A simple way would be to measure the time it takes to exhaust the fuel under a given flow rate. I.e., draw a controlled number of Amps, and measure how many ours it takes to run the battery down. The battery "fuel capacity", or just plain "Capacity" would be expressed in Amp-hours.
What's a milliAmp? that's 1/1000 of an Amp. I.e.,
1000mA = 1A.
Our radio equipment actually use a small amount of electron flow, so using plain Amps would result is lots of funny fractional numbers, mA makes life easier... A typical Rx
draws 0.030A, or 30mA. A typical servo at rest draws 0.020A, or 20mA. A standard servo
under load can draw as much as 0.25A or 250mA.
So... how long will a 600mAH battery last on an airplane with 4 servos? Let's see...
1 Rx 30mA
4 servos @ 20mA 80mA
Total 110mA
Time to exhaust 600mAH = 600mAH / 110mA = 5.45 hours!
Wait-a-minnit! That's with servos AT REST! What if ther servos were under load 50% of
the time?
1 Rx 30mA
2 servos at rest 40mA
2 servos at load 500mA
Total 570mA
Time to exhaust 600mAH = 600mAH / 570mA = 1.05 hours.
Hmmm... B-I-I-G range between an airplane at rest and an actively flown one!
A bit more on batteries, in particular, NiCd's.
Batteries are actually composed of "cells" in series. A cell is the basic chemical package that makes up an "electron pump - fuel" unit. Technically, the term "battery" is only correctly applied to an assembly of cells, not to individual cells. A NiCd cell has a nominal voltage of 1.2V, at whatever capacity, e.g., 600mAH, 2000mAH, etc. A typical Rx battery pack (4.8V/600mAH) is made up of four 1.2V/600mAH cells.
NiCd's are rechargeable. I.e., if you run the "pump" backwards, it'll actually refill the "fuel tank"!
Of course, as with amny things in nature, this process is not 100% efficient, you typically need to put in 40% - 60% more than you take out. I.e., you need to put in at least 600 x 1.4 = 840mAH to fully charge your 600mAH pack.
From our discussions last month, we know that we should charge our battery with C/10 = 600/10 = 60mA. So how long to we need to charge it?
Charge time = 840/60 = 14 hours!
Ahah! So that's where they get the charge at C/10 for 14 hours deal!
The NiCd's pump behaves like a small engine. As long as the tank has sufficent fuel, the pump works pretty much at constant pressure. As the tank goes down, there's very little loss of pressure, until the tank gets nearly empty. Then, the engine sputters as "air" enters the fuel line, and finally, after a very short time "sputtering", it just quits.
The NiCd battery puts out nearly constant voltage as long as it is not exhausted. Then, near the end of its charge, the voltage takes a dip, and falls off rapidly.
Another thing... if you run the NiCd's engine until it's "dry", and keep it "spinning" without fuel (using an external pressure source - like another battery), the little engine doesn't like to run dry, and its "bearings" will fail.
When does this happen? Consider the following:
3 nearly full cells (#1 - #3) are run in series with an almost exhausted cell (#4), into your radio. Eventually #4 goes "dry"... but, #1 - #3 are still good, and continue the current flow in the same direction. #4 sees, not a "backwards" charging current (which refills the tank), but a "forward" discharging current, which is "running dry" like we described above... it's going to get ruined!
Heyyyy... didn't I start with 4 good cells? Yes, but like all things natural, they are not 100.0000% matched, and after a little use, there will be the weaker cells and the stronger ones. If we discharge our battery to a very low voltage, it is likely that one or more cells will go "dry" before the others. If we continue the discharge, we will damage these cells.
How to prevent this? NEVER discharge NiCd batteries below 1.0V per cell. I.e., a 4.8V Rx battery must never be discharged below 4.0V, and a 9.6V Tx pack should never be discharged below 8.0V, or irreversible damage will occur. NEVER, NEVER, NEVER run batteries "flat" for ANY reason!
On that note, we will leave you till next time, when we will discuss charging, storing and cycling NiCd batteries.
May all your landings be 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)
| 
