Recharge a 9V NiMh battery with a Solar Panel / Constant Current Generator LM317LZ

 
If you enjoyed the previous post solar-powered Arduino board, you should keep reading this one as well to improve your solar power generator. In fact, with that setup the 9V battery would decay too soon: A NiMh (Nichel – metal hydroxide) battery must be recharged with a constant current generator, a solar panel alone is not.

In order to do so, we can assemble a quite simple circuit using the following components:

    LM317M or LM317LZ
    A 200Ohm trimmer
    10Ohm resistance
    A LED to check if the current is actually flowing

The core of this circuit is the integrated three-terminal adjustable linear voltage regulator LM317. The ‘adj’ pin is a control terminal that is used to adjust the output current. So, the trick is to use a variable resistor (in our case, a trimmer with a small resistance put in front of it) to set the current at a definite value.

With an input DC voltage of max 24V, and the resistances shown in the diagram, one can obtain an output current of:

I_out = 1.2/(10 + R_trim) [A]

This current must be less than 1/10 of the battery output, e.g. for a 750mAh battery the output current must be less than 75mA. As usual, it is customary to insert a capacitor (100uF should be ok) in parallel at the input node to smooth out fast-varying voltages.

When PWM is turned on - IRFP450

During the ON time of the cycle, the MOSFET turns on and starts conducting. Since inductor poses no resistance to DC current, large current flows through the MOSFET. This large current then becomes constant at some point during the ON time of the cycle. Due to the property of inductor, the energy is stored in its magnetic field around it.

During the OFF time of the cycle, MOSFET turns off and no conduction can take place. Due to the property of inductor it blocks any change in current. Because of this, the energy stored in its magnetic field is released. This creates high current. But since the MOSFET is turned off, the large current flows through the diode and charges the capacitor.

When the whole process is done fast enough, we are able to achieve a large voltage at the output.

In this experiment, I have used an old inductor from switch mode power supply that I found in the computer’s monitor. So I don’t know the value, but it is definitely greater than 200uH. IRFP450 MOSFET (HEXFET as they call it) can carry 14 amperes of current, and has a very very low drain to source resistance. The reason for using such a large MOSFET is, it does’t heat up. If you use a MOSFET with large drain to source resistance, you are going to get a lot of heat, this increases the possibility of destroying the transistor (I already destroyed like 4 expensive MOSFETs)

MOSFETs are very sensitive to heat and over voltage, so please take care that you do no exceed the specifications. My advice would be to obtain MOSFETs from old power supplies. You can also get a good number of high voltage fast response diodes from those old circuits.

I suggest you to build this circuit and try out different values of inductor and PWM. Here is a small arduino code used for generating PWM with On time of 50 uS and OFF time of 15 uS.

When this PWM configuration is used with my circuit it can yeild upto 394 V. I did not go beyond that, because the capacitors I have used are limited to 400V and so is the diode. So it is better to be safe below the range.