Here is the low cost universal charger circuit diagram for NiCD – NiMH batteries. This battery charger circuit is ideal for car use. It has ability to transform a mains adapter in to a charger . This one can be used to charge cellular phone, toys, portables, video batteries, MP3 players, … and has selectable charge current. An LED is located in circuit to indicate charging. Can be built on a general purpose PCB or a veroboard.

Battery Charger Circuit Diagram

Universal Battery Charger Specifications

  • Ideal for in car use.
  • LED charge indication.
  • Selectable charge current.
  • Charges Ni Cd or NiMH batteries.
  • Transforms a mains adapter into a charger.
  • Charge cellular phone, toys, portables, video batteries

Low Cost Charger Features

  • LED function indication.
  • Power supply polarity protected.
  • Supply current: same as charge current.
  • Supply voltage: from 6.5VDC to 21VDC (depending on used battery)
  • Charge current (±20%): 50mA, 100mA, 200mA, 300mA, 400mA. (selectable)

Determining the supply voltage:
This table indicates the minimum and maximum voltages to supply the charger. See supply voltage selection chart below.

Determining the charge current:
Before building the circuit, you must determinate how much current will be used to charge the battery or battery pack. It is advisable to charge the battery with a current that is 10 times smaller then the battery capacity, and to charge it for about 15 hours. If you double the charge current , then you can charge the battery in half the time. Charge current selection chart is located in diagram.

Example:

A battery pack of 6V / 1000mAh can be charged with 100mA during 15 hours. If you want to charge faster, then a charge current of 200mA can be used for about 7 hours.

From http://www.extremecircuits.net/2009/10/low-cost-universal-battery-charger.html