# 9 Volt Battery Chargers

## Simple 1 Resistor Battery Charger

• 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
```

Charge Rate

Wall Warts

Simple 1 Resistor Battery Charger

Constant Current Battery Charger from 2 LED's and a NPN Transistor

9V Battery Charger from 2 NPN Transistors

Adjustable Charge Rate Battery Charger from Op Amp

0 - 160 mA Adjustable Charge Rate Battery Charger

Test Results

741C Cheat Sheet

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