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9V Battery Charger from 2 NPN Transistors


Schematic of 11mA Constant Current 9V Battery Charger from 2 NPN Transistors

9V Battery Constant Current
Charger on Breadboard

This circuit uses 2 NPN transistors to provide a constant charge rate of 11mA suitable for charging a 9V battery in 14 hours.

The LED serves 2 purposes

  • It lights up when the battery is connected to the charger.
  • It stops the battery from discharging when the power is turned off.

Volt and Amp Meters

The ammeter shows the charging current.

The voltmeter shows the charging voltage across the battery.

A battery with a charging voltage that differs greatly from it's peers has something wrong with it.

A battery with a high charging voltage also has something wrong with it.

Open Voltage

An important measurement is the open voltage. This is the voltage reading with NO battery attached.

When the charging voltage gets close to the open voltage, the charger will have difficulty supplying enough current.

Results

A B C D E
15V Supply 10.05 11.32 NG 11.32 11.31
19V Supply 11.42 11.43 NG 11.44 11.43
Readings in mA.

Note the highlighted cell where the charger could not supply enough current to charge battery 'A' when using the 15V supply.


References
Back Next

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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