Tuesday, 25 February 2014

Solar Charging for Electric Bike (2)

The cheap-skate version of my lithium battery solar charger was a success IF (there is a big IF) I monitor it continously, i.e. the output power of the converter (and losses) is less than the solar panel output power (Watt). I had to do this by changing the CC (Constant Current) setting.

The problem is, everytime the solar panel power (Watt) drops below the required output power (let say due to passing clouds), the DC-DC converter (CC-CV) becomes unstable. I'm guessing this is because the converter is trying to suck out even more current out of the panel. For example:

At 9 o'clock in the morning, I set the CC of the converter so that the output power of the solar panel reaches 39.4W (out of 80W rated Wp). When I tried to increase more power, the converter suddenly became unstable (making humming noise) and output of the solar panel drops to around 15 - 20W (jumping erratically). I had to decrease the CC setting all the way down in order to re-stablise the converter, and then turning it back up. unplugging and replugging the load didn't stabilise the converter.

Later, I found out that installing huge capacitor (I tried 15,000uF, definitely can do with less) at the converter input can help to stabilise the converter. Although, you still have to unplug the load and replug it back in.

So, in short, I was the slave labour to mimic the MPPT. By 12 mid-day, I managed to squeeze 67W out of the solar panel. The converter is pretty hot at this point of time (still touch-able) even with extra heatsink that I installed.

From my test, between 9am to 3pm (this time of day of the year), without any cloud, the panel can comfortably supply at least 40W (out of the specified 80Wp), so I don't need to fiddle much with the CC setting of the converter. Obviously, in practice, even the slightest cloud passing will throw this converter out of whack. So slave labour still required to monitor.

Oh, converter effeciency is around 89%, using power consumption figure from my in-line power meters. Not too far from the acclaimed 92% from the product website.

Next step, is to design and build my own BMPPT solar charger, as there is no way I can do this MPPT manually all the time. The current one in the market that fits my battery pack (36V 11.6Ah) simply doesn't exist. Either Genasun or GSL don't fit my charging profile. From googling the web, the Tim Nolan's one inspired me to make mine from Arduino Uno. Watch this space!

No comments: