Charge Electric Bike Battery With Solar Panel

"Oh no! Not another solar charger by Arduino!" I hear you say.

Skin depth, that is true. However, if you were like me, found an awesome deal of low voltage solar panel on the web (maybe second hand), and want to charge your e-Bike battery directly using this solar panel, then tough luck! There is no cheap boost MPPT charger out there, i.e. your battery voltage is higher than your solar panel voltage.

Most of MPPT charger out there (including almost all DIY arduino MPPT chargers) are buck type, that is, to charge battery with lower voltage compared to your solar panel voltage.

If that is you, then you have to resort to expensive Genasun boost MPPT charger, which at USD300 a pop! Well, if you are stingy like me, and already have arduino UNO lying around, why not make a boost MPPT charger yourself?

I've done this project last year, and been using it constantly since then. It has been awesome (commuting with my electric bicycle powered by the sun!!), so I guess it's a good time to write it down as instructables.

Step 1: Get Cheap Solar Panel

If you are lucky like me, then by now you have found a cheap solar panel already. I got mine second hand, so I got my 80-Watt solar panel for AUD120 (USD100) last year.

Today, you will very likely find a better bargain. The idea is, you need to find a solar panel with a price that is less than a-dollar-a-watt. For example, 100-Watt panel for less than USD100 is a good deal for today's price (most likely second hand).

I found lots of cheap solar panel with small rating that are 'cheap', but once you calculate dollar per watt, suddenly you'll find out they are not cheap. For example, 10-Watt panel for USD25 is not cheap, as that is USD2.50 per Watt! Ouch!

In short, get any panel larger than 60-Watt, for less than a-dollar-a-watt (if possible).

Step 2: Get Your Battery Details

Most electric bike batteries are 36V lithium-ion (exact Lithium chemistry is beyond the scope of this instructables). You will need to find out the maximum charging current. This information is very likely in the battery sticker itself.

The other information you need is the maximum charging voltage of your battery. You can find this in the actual charger of your battery. This is most likely 42-Volt.

In my case, my battery maximum charging voltage is 42-Volt with 2.35-Ampere maximum charging current.

Step 3: Assess the Requirement of Your MPPT Charger

Again, if you manage to find solar panel that has higher voltage compared to your battery, then you're in luck. There are simply too many MPPT charger out there already, and cheap too! Not to mention there are few instructables already how to make your own MPTT with arduino.

However, if you are like me, my solar panel voltage is lower than my battery voltage. You simply can't use those MPPT chargers. The only option that I manage to find on the web is Genasun Boost MPPT charger, and they are really expensive!!

For example, my solar panel open circuit voltage is 20-ish Volt. My battery is fully charged at 42-Volt. Now you see the problem don't you?

What you need is, any MPPT charger that can charge with Constant-Current-Constant-Voltage to match your battery profile (i.e. 42V max voltage) using lower panel voltage.

Step 4: Make Your Own Boost MPPT Charger

What will you do? Well, make one yourself using Arduino of course!!

You can find the schematic in the attached photo. I also included component details for your reference.

I didn't know how to use EAGLE Cad back then (still don't), so I used Microsoft Word drawn tracks, then use 'Press N Peel PCB Film'. There are already instructables to do this, so I'm not going to repeat. Hence, you can see my finished PCB doesn't look that pretty :(.

You can find the software here:

https://drive.google.com/file/d/0B_hfpbysZDTNMmZCU...

More details here:

http://epxhilon.blogspot.com.au/2014/06/bmppt-sola...

Everything in the software are user-settable. So, if your maximum battery voltage and maximum charging current is different than mine, well, simply change them in the software. Flexible isn't it?

Step 5: Enjoy Your Solar-Powered Ride

Well, that's it! Good luck!