Friday, 15 February 2013

PWM charger works!

I soldered up the MOSFET circuit as per the last post and connected it up to my 'dumb' mains charger to charge one of my batteries overnight. You can just about see the MOSFET in the photo below sat on the bench with the wires coming in and out of it. The wires are red, but it is actually connected to the negative side of the battery.

I initially tried the MOSFET without the heat sink but it got finger-burningly hot. Attaching the heat sink meant you could touch it.

The Arduino was set to start the PWM pin at 0 and whilst the voltage was less than 14.4V increase the PWM output by 10, up to a max of 255 (100% duty cycle, ie. full on). If the voltage is greater than 14.4V reduces the PWM output by 10 to a min of 0 (0% duty cycle, ie full off).

Below is the graph of the current/voltage results:

PWM charging attempt, current and voltage
As you can see the charger only just got to 14.4V by the morning when I switched it off. But you can see it is holding the voltage around 14.4V. I'm not sure if the oscillation around the target voltage is due to my simple algorithm or due to the the fact the charger itself seems to pulse the current (as seen by the green band on the current in the graph). I could probably make my algorithm a bit smarted and get it to check the change in voltage between readings and not just the absolute values. This will probably allow it to converge on the right PWM value better.

My original intention was to hold the battery in absorption stage for a number of hours and watch what happens with the amount of current it accepts. It should get to a point where the change in the rate of current accepted flattens out. At that point I think that battery has taken all it can and the rest is just being converted to heat.

So I'll go out and switch it back on again so it can get some absorption charge. According to this page:

The Absorption time for my 110Ah battery being charged at 2A is 31 hours! That is way higher than I've seen elsewhere, but may explain why my batteries have not been getting enough charge in the past!

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