Simple 1 Resistor Battery Charger
9V Battery Charge Rate
Actual Power Supply Used
 20 Volt DC Unregulated
 400 mA
Fixed Resistor Calculation
The difference between the power supply voltage of 20 and the battery voltage of 9 is 11.
Divide 11 mA by 1000 to convert to amps.
This is enough information to use Ohm's Law.
; LEGEND
R = Resistance
E = Voltage
I = Amperage
P = Power
I = E / R
E = I * R
R = E / I
; RESISTOR VALUE
R = E / I
R = (20  9) / ( 11 / 1000)
R = 11 / 0.011
R = 1000 Ohms
; SHORT CIRCUIT CURRENT
I = E / R
I = 20 / 1000
I = 0.020
; SHORT CIRCUIT POWER DISSIPATION OF RESISTOR
P = (E*E) / R
P = 20 * 20 / 1000
P = 0.400
