I first estimated the charge currents I would like to have based on 3 battery setups (150mAh, stock 50mAh, and Layman's 40mAh):
(1) 500mA full charge rate from the adapter. This would be the charge rate when all resistors are turned off and I'm in a hurry to race NOW. Still safer than AA-based chargers at ~1000mA.
(2) 300mA, a fast but mostly safe charge for 150mAh batteries. Good balance.
(3) 150mA, the recommended safe charging rate for 150mAh batteries, or a fast stock/Layman's charger.
(4) 50mA, the safest stock/Layman's charge rate.
These rates of current would require the following resistances at 3V:
Code:
500mA - none needed
300mA - R = V / I
R = 3 / 0.3 (300 milliAmps = 0.3 Amps)
R = 10 Ohms
150mA - R = V / I
R = 3 / 0.15 (150 milliAmps = 0.15 Amps)
R = 20 Ohms
50 mA - R = V / I
R = 3 / 0.05 (50 milliAmps = 0.05 Amps)
R = 60 Ohms
Those were nice, whole numbers for resistance, but after looking at the same calculations using 4.5V and 6V (for 3 or 4 cell cars) I decided I'd want more max resistance, preferably in a range up to 100 Ohms. I checked Radio Shack's available resistors and found they carried resistors of 10, 15, 22, 33, 47, 68, and 100 Ohms (along with others not applicable to this project). I selected one
10 Ohm resistor, one
22 Ohm resistor, and one
68 Ohm resistor. Using three switches, that gave me eight different possible charge rates for each voltage. Note that individual resistors in series are added together for total resistance. (Ex: a 10 Ohm resistor connected to a 20 Ohm resistor is the same as a single 30 Ohm resistor.)
Code:
0 = resistor switched off
1 = resistor switched on
Pos Resist. 3v 4.5v 6v
------------------------------------
000 0* Ohms 500mA 500mA 500mA <--maxed at 500mA
100 10 Ohms 300mA 450mA 500mA <--maxed at 500mA
010 22 Ohms 136mA 205mA 273mA
110 32 Ohms 94mA 141mA 188mA
001 68 Ohms 44mA 66mA 88mA
101 78 Ohms 38mA 58mA 78mA
011 90 Ohms 33mA 50mA 67mA
111 100 Ohms 30mA 45mA 60mA
* This table doesn't count internal charger or PCB resistance
As you can see, the change in current is less dramatic towards the bottom, as mentioned in the potentiometer drawbacks, but it still hit pretty close to all the numbers I wanted at each voltage, and it should provide consistent results for each charge level.