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Simple 1 Resistor Battery Charger


9V Battery Terminal Polarities

9V Battery Charge Rate

  • 9 Volts
  • 11 mA
  • 14 hours

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

Back Next

9 Volt Battery Charger

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