NiMh 12V,1.2Ahr Battery Charger

Difficulty Rating: 4/10

Exactly 5 months after my Internship/project  at M.A.P.L. Pune, on Lead acid SMF Battery charger,  my knowledge on Battery charging was put to test by a Mechanical Engineering friend. I was asked to charge a 12V 1200mAh NiMh battery. Now, before getting into its technical aspects, a little overview about battery chargers.

General battery charging techniques-

Trickle chargers

This charger provides a constant charge rate of ~ 0.1C. The general idea behind keeping the charge rate low is to compensate for the batteries self-discharge. The charge rate being low, the battery can be left unplugged for days!!!The charge rate of the battery should be equal to self-discharge of the battery. Such chargers are generally used in industrial applications , where less of human intervention is required.

Float chargers

The battery and the load are constantly connected to the source. The float chargers maintains full charge & provides a protection against overcharging. The charger senses when the battery voltage is at maximum level and floats the voltage to minimal value.

Fast chargers

Fast chargers, as the name suggests, charges the battery at much higher rate (~1C).The one obvious advantage is the reduced charging time. A full charge indication is like 'a must' for this charger. As the battery approaches full charge, charger switches to trickle or floating techniques.

Intelligent chargers

 Intelligent chargers charge the batteries according to their requirement.

After the charging technique is selected, next part is selection of voltage regulator. There are many types of voltage regulators, like fixed & adjustable regulators, step up & step down regulators, positive & negative regulators etc. 78xx ICs are series of fixed regulators.Eg. 7805-5V regulator, 7812-12V regulator...L200 is a
5 terminal, voltage and current regulator. Most commonly used adjustable voltage regulators used are LM117, LM217, LM317, LM350 etc. I had a couple of spare LM317's at my home, so i thought of using one of those thereby preventing unnecessary visit to Lamington road. LM317 is a 3 terminal adjustable positive voltage regulator capable of supplying greater than 1.5A current over an output voltage range of 1.2 to 37 volts.

Implementation on veroboard

Testing the circuit

The best way to check the circuit is to check the regulating action of LM317. If the reference voltage is close to 1.25V, then the circuit is working as desired. Also check the voltages at input as well as output pins of LM317. The regulating action can be observed.Now output voltage can be set as per requirements of the battery to be charged, by adjusting the trimpot. If you think the current is too much for the battery, then add a high watt (3W-5W) low value resistor in series with the present one( this will consume a bit more energy, hence needs to be avoided). The two red LEDs serve as visual indication. For successful operation of the charger both the LEDs should light up. The upper LED is for input section & the bottom one for output section.

While checking the regulating action of LM317 using multimeter, see to it that you don't short the adj. & Vout pins. If you do, you will see smoke rising from your BC547!!! Because of which you will have to change both of these components!!!
(Try if you want)



Final Testing:

To conclude, in this evolving field of electronics, this model of charger would find no place owing to its inefficiency. As an budding Electronics Engineer, its very much essential to cope up with the cutting-edge technologies, and develop them. So its time for me to move on & design a more cost, energy efficient charger for my battery.

Here's the Circuit diagram. Click the image for enlarged view.

Drop a comment if you need any help with the circuit.
Thank you.