Charge Electric Bike Battery With Solar Panel




About: Geek

"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:

More details here:

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!



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    15 Discussions


    Question 1 year ago on Introduction

    Sir i am use 12v 20ah battery insereas for 48v ebike moter running i want to make solar panel for charging in running time pls give some idiato me


    1 year ago

    can i charge a 48v battery with solar panel of 36 v ?


    3 years ago

    Thank you for your time in documentation!
    I am curious what your final outputs are in volts and amps, and from how many panels, and what the specs are on those panels?

    I am trying to get an idea of how long it will take to charge a 20Ah batter that is 48v.

    I don't know how to calculate this yet.

    I have a few of these 100w panels, and could get two more for a total of 4 to give me 400w and 48v max:

    Specs are:

    • Maximum Power: 100W
    • Maximum System Voltage: 600V DC (UL)
    • Optimum Operating Voltage (Vmp): 18.9V
    • Optimum Operating Current (Imp): 5.29A
    • Open-Circuit Voltage (Voc): 22.5V
    • Short-Circuit Current (Isc): 5.75A
    • Output Cables: 4.0 mm2 (0.006 in2), 800mm (31.5 in)

    I was using them last year and on the charge controller I think I saw a total output of around 4.3 to 4.8A in full sun. I had three of them set up running through the charge controller to charge a 12V deep cycle.

    It seems like I would want a few deep cycle batteries to charge up from excess current, and then continue charging the bike overnight - I don't know if the bike batteries could handle a really high charge rate of many panels in a system. It's also important to ask the question - what is the max rate these batteries can be charged at? I see chargers on this site ranging from 72v 300w to 48v 300w, (amps 4.x to 6.x?)

    Given these numbers, maybe the batteries can be charged at 4 to 6 amps just fine. What panels can provide in terms of power? The panel site says they can be wired in configurations for 12v, 24v, and 48v. I assume to wire them for 48v I'd need to ... set them up in series? I'm not sure how this changes the amp output. The max I can get from my panels is, I'm assuming the 5.29A that they are rated at, even if they're set up in series.

    Clearly I'm confused:)

    I really like your project and want to figure out how to spec and build an optimal configuration. Thanks for blazing the way!

    1 reply

    Reply 3 years ago

    hi BeeeSB,

    Mine is 80W panel with 22Voc and with 17-ish Volt at MPP with 4.7Amps. Obviously, this is at ideal condition (which is never).

    Regarding calculation "How many hours to charge..." and so on, the short story is, multiply your panel rating by 4, and this is how much you will get out in Wh per day (this figure is conservative, in reality, you might get 5 to 6 times depending where you are). Obviously, if you want to be accurate, you need to consider where you are in the world. But hey, we don't want a thesis out of this right?

    So, let say with my panel of 80-Watt, I will get 320Wh per day (80 x 4). My battery is 416Wh. So, it's not enough to fully charge my battery using 80-Watt panel in a single day. In reality, I don't use full 416Wh out of my battery. So, my 80-Watt panel is sufficient to date.

    Now, let's get to your proposed battery:

    Your battery is 48-Volt 20Ah. This translates to 960Wh. If you want to fully charge your battery in a single day, you'll need 250-Watt panel.

    If you want to use my instructables as the MPPT, there is a problem here. My current component selection maxed out at 5-Amperes output (due to the blocking diode limitation). Also, since I'm using linear voltage regulator as an input, this limits maximum input voltage to 20-ish volt. Without being too technical, what I'm trying to say is, you only can use 22Voc panel (which you already found) and around 100-Watt maximum.

    So, without changing the schematic, you only can use 1-off 100-Watt panel (with 22Voc) with my instructables to charge your 48-Volt battery. You'll need 2 days to fully charge the battery from completely discharged state.

    Or, you can parallel them all. So, you'll end up with 4-off panels with 4-off MPPT. You'll probably end up cheaper in buying an already existing 48V MPPT.


    3 years ago


    great project. Thinking about how to do something like this myself. I've got a 12v car charger for my bike. Does that work with panels of lower voltage? Is that using the same principle as your design do you think?

    1 reply

    Reply 3 years ago

    Hi frith,
    My schematic is 'boost' topology (only works if the panel voltage lower than the battery voltage). So, just make sure your panel is lower voltage than your battery.

    If not, fortunately, there are lots of lots resources for buck topology in the internet. Even here in instructables have plentiful!

    Regarding whether you can do something like this yourself, this is a bit more tricky to answer. You do need to have some experience in electronics DIY. Lots of the components here are surface-mount only :(. The arduino bit is a bit easier, since all just plug and play :)



    3 years ago on Introduction

    I have an EW-36 Mobility Scooter which has a 48 Volt DC system. How can I build an inexpensive Solar Charger to charge my batteries? The off board chargers are very poor, I am on my third one already.

    1 reply

    Reply 3 years ago on Introduction

    Is your existing charger from solar as well? Unfortunately there are many el-cheapo solar chargers out there that don't do the job, and many that do the job well. It's tricky.

    To build one yourself, it's not cheaper than buy one unfortunately, unless your case is similiar to mine (panel voltage lower than the battery). This instructable has everything you need, except the making PCB yourself (covered by many other instructables).

    So, what is exactly you need help with? As this instructable is it (how to build your own).


    3 years ago on Introduction


    Congratulations for the project.

    I want to do the same with my recumbente trike but i´m putting the pannels in the top of the trike. But since my solar pannels are 22v 0,5amps i associate 2 to have 44v 0,5v and another set of two to reach 44v 1 amp, so total of 4 solar pannels.

    I connected a rectifying bridge with a capacitor to have stable current, and that makes the maximum voltage of around 43v and the minimum of 40v.

    I connected a battery of 36v 2ah and it did recharge it, but now i order one 36v 15ah ebike battery and i´m hopping it will recharge. My only doubt is that if a current between 0,5 - 1 amp is enough to start recharging a 15ah battery. Because if that work i wont need any MPPT RIGHT?

    The secrect, i think, is to associate solar pannels to not exceed 44v then we will have a much easy system without any chargers, my only doubt is if a small current can charge the battery. If yes that is the best way of charging because a small current means the battery will last more.


    1 reply

    Reply 3 years ago on Introduction

    Thanks for the compliment!

    What you're describing above is exactly the struggle of every one of us who try to charge our batteries using solar panel (including mine).

    Generally speaking, panels peak power occurs around 70% of open voltage (obviously, this depends on actual light condition). So, for 44V open voltage panel, you will have maximum power at around 30ish Volt. That's why the current is pretty low at 36V, just like you witnessed.

    Without MPPT, yes, you definitely can charge your battery. However, at a cost of much, much lengthened charging time. Also, do remember if you are charging lithium batteries, there is an absolute maximum voltage that MUST never be exceeded.

    So, the el-cheapo solution, from my experience:

    1. Ensure solar panel does not exceed maximum battery charging voltage (i.e. no electronics whatsoever) --> Consequence: SLOW charging time (as in your case here)

    2. Buy el-cheapo boost converter without MPPT --> Consequence: Manual adjusment ALL the time, or else the boost converter becomes unstable when the peak power is passed (I've documented this in my blog: )

    3. Buy MPPT --> Most MPPT in the market are buck type. So, your solar panel voltage MUST be higher than the battery voltage, ALL the time. This is the most cost effective solution at the moment.

    4. Buy boost type MPPT --> Very, very expensive. It's good if you are loaded (rich), and have panel that has lower voltage compared to your battery.

    5. Build your own. Just like my saga here.


    4 years ago on Introduction

    hi! this is so cool! i was wandering if this (with modifications) can be applied to a battery for a solar car?

    ps:excuse my english, my first language is french

    1 reply

    Reply 4 years ago on Introduction


    As long your solar panel voltage is lower than your load (motor controller), this will work just fine.

    You need to reduce the delay() in the software though, or else the MPPT tracking is too slow. The MPPT can not provide fast changing current. I mean, the software is deliberately slowed down not to supply 0 to 5 Amperes in less than a second. However, this is easily fixed by reducing the delay() function in the software.



    4 years ago on Introduction

    This is great! I've always wanted to do something with high powered solar panels.