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I need some advice / info on solar charging an eBike battery. We have two RadRovers from Rad Power Bikes. They are powered with a Panasonic Dolphin 48V, 11.6Ah Lithium-Ion battery https://www.radpowerbikes.com/collections/replacement-parts/products/radrover-battery-pack.

We are planing a road trip from the North West US down south and would like to extend the range of the bikes. The specs say that the range is between 20 and 40 miles depending on how you ride. We are planning on using the low pedal-assist setting as much as possible, so we should be able to get to 40 miles or above.

We are bringing a second battery so we should be able to count on 70 to 80 miles a day. With that said, I’d like to rig up a solar set up to extend our range even further. Ideally, we’d have it set up so that the solar panel is plugged directly into the battery being used to extend the range.

I have found these 100w folding solar panel that will work for us https://www.amazon.com/dp/B071YP7X4F/ref=psdc_2236628011_t3_B01M28W7KR.

We can attach this to the rear of the bike with a frame. However, the panel will be fixed in a flat horizontal position, and will not always be in direct sunlight, but it’ll still be better than nothing I figure. If we can get just a little extra boost in between plugging in for a recharge, its worth it. My question is, is it possible for us to plug directly into the battery that is currently powering the bike? If not I’ll just charge the spare battery while in transit, but the first option is preferred. Also, what additional equipment do I need to achieve this? I know I need a controller, but I am not familiar with how this works exactly. A link to an ideal controller would be much appreciated.

Finally, if I then want to change the solar charger over to this laptop battery to charge it on occasion https://www.voltaicsystems.com/v72, would this same controller be used? How would this work.

I really appreciate any advice and info you can give us.

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    If you're moderately fit, throw away both batteries, get a decent touring bike and your range could exceed 100 miles/day. You'll eat a lot more though... Aug 25 '17 at 19:10
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    As this comes up regularly I feel a beirf response is require.
    – mattnz
    Aug 25 '17 at 22:33
  • Vague memory moz had a story about someone touring the great Australian Outback with a big trailer, whose entire flat topped surface was a solar panel. It was easily a metre wide and 4 metres long. That solar panel was a fixed roof at about waist-height and charged some batteries all day. I don't recall if the bike was powered.
    – Criggie
    Aug 26 '17 at 2:42
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    @Criggie this is a solar powered bike on the Canning Stock Route. Was that what you were thinking of?
    – Chris H
    Aug 26 '17 at 6:33
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    @chrish Yes that's him, with a solar array about 1.2 metres by 4 metres long, scontent-syd2-1.xx.fbcdn.net/v/t31.0-8/… So OP is looking at 55 cm a side panels at 2.5 kilos each. That's roughly 16 of the linked solar panels for the same surface area, and would weigh 40 kilos (80ish pounds) plus a framework to hold them. That's roughly $3000 USD worth of solar panels too. Better to carry 2-3 batteries for each bike, and have some way to charge them all overnight at stops.
    – Criggie
    Aug 26 '17 at 10:54
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A 100 watt solar panel is nominal watts under ideal situations. You'll be lucky to get 25 watts tied to the back of your bike - and this would be only for a few hours of the day, from 11a to 3p.

Given that your bike is likely using at least 100-200 watts even on low pedal assist, you're only going to gain 5-10% more range over the entire day. From experience, even with low pedal assist, its highly unlikely you'll get anywhere near the maximum (or even middle) advertised range of the ebike.

There is a further complication that your battery management system won't allow you to charge while biking. Many if not most don't. So you can only charge the spare pack and you'll have to figure out how to convert the solar panel to the right D.C. voltage and amperage for your charger. If you overcharge or overvolt, a poorly design lithium pack can go up in flames.

It'd be much easier and safer logistically to just bring another spare battery pack or two. I assume you'll be pulling trailers and as a result your mileage will be even further reduced but it means you can carry more. Just load up your trailer with spare packs and bring multiple chargers. Forget solar, it's not worth the hassle. The one exception would be to use solar to charge your iPhone/iPad and other electronic devices -- it's relatively easy to find small solar chargers that have USB leads.

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    I agree with this, and would add that setting up a solar rig will impose a weight & aerodynamic penalty that will exceed the energy captured.
    – Adam Rice
    Aug 26 '17 at 14:06
  • Thanks a lot, RoboKaren. I'll have to rethink this approach. Perhaps just carrying extra batteries is the way to go.
    – Chris
    Aug 27 '17 at 2:40
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    100-200 watts/hr? Oct 22 '18 at 21:26
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Those are 48V batteries, which you want to charge with a 12V (nominal) solar panel. That's not going to work directly.

So we look for an indirect solution. At the very least some form of boost convertor would be required. I won't go into details as we're now on bicycles.se not electronics. But designing a charger for lithium batteries isn't for the uninitiated as a significant failure mode involves flames and not easy ones to put out. Therefore unless the manufacturer makes a 12V input charge we're still not getting very far.

The solution that could work would be a 12V solar panel, connected to an inverter, feeding the original mains-powered charger (or the laptop charger but that's another issue). I'll assume for the moment that the input voltage of the inverter is forgiving (some will run on anything from 12V nominal to 24V nominal without a break). You'll need the solar panel to put out enough power in real use to enable the inverter to work properly. That means some margin over the input power requirement of the charger. Test it

A 100W solar panel is rather large. If you're riding during the day, then to catch much sun you'll need to ride with the panel, as you've spotted. That means extra drag and extra exposure to catching the wind. But if you're towing a trailer anyway and use the solar panel on top, this is less of an issue.

I also wouldn't advise charging the battery while it's in use. The control electronics might not like it. The e-bike I'm familiar with specifies that the main switch should be turned off before charging, to avoid damage.

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  • Thanks for the advice, Chris. Its looking like this approach might be more trouble than its worth. I'll keep researching it. I appreciate the feedback.
    – Chris
    Aug 27 '17 at 2:41
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All great responses above, particularly regarding the true efficiency of charging the batteries via solar. In an ideal world, this would be so effin cool, to theoretically ride across the country without ever plugging in.

I have a 1400w Li-ion solar generator from goal zero and fabricated a trailer to tow behind my Radwagon. I intend on getting an extra Rad battery to go a little further.

To start with, the battery must be turned off in order to charge. So you could charge a second one while riding, however, the problem is that the charge rate is not equal to the depletion rate... in other words, you will deplete one battery, replace it with the second and then plug in the first while riding on the second charge. By the time you have depleted the second battery (30 miles or roughly one hour, at best), the first would not be anywhere close to completely charged. Diminishing returns...

You could possibly get away with it using 4 or 5 extra batteries, keeping them in rotation, but at that point, your weight would be significant.

You'd be better off bringing 1 or 2 extra batteries and plan your trip around 50-80 mile increments, stopping for the night and recharging.

Enjoy the ride, and also enjoy the scenery :)

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    Good points - but note an electric bike is an assist, not a replacement for the rider's power. You can totally ride a bike with a flat battery - its just less fun. There's an implication here that a discharged battery stops the ride, which is not true,
    – Criggie
    Dec 14 '17 at 0:45
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My e-bike battery of 500 watt-hours lasts at 24 km/h around 120 kilometers. Thus, it lasts for 5 hours. This requires charging of 100 watts if I used solar panels to charge the batteries. I estimate that about half of the power is produced by the battery and the rest of the power (the second half) by me, the rider. So it's 200 watts total.

Solar constant is 1000 watts per square meter, if the sunlight travels through minimal air mass. Common silicon solar cells have about 20% efficiency (those multi-junction cells used in space applications have over 40% efficiency but they are very expensive). Thus, near the equator during midday, you get 200 watts out of a square meter of perfectly oriented panels, so for 100 watts you need half a square meter.

Anything not perfectly oriented, any location away from the equator, not during the midday? You need more panels.

Let's say you are able to have the required power with 0.7 square meters of panels. Let's also say the sun is at a 45 degree angle. Those panels increase your frontal area by 0.7*sin(45*pi/180) square meters or about 0.49 square meters.

A road bike has air resistance coefficient times area of about 0.4 square meters. According to Wikipedia, adult human has coefficient of air resistance of about 1 in upright position so this is about 0.4 square meters of area.

The solar panels powerful enough to provide 100 watts are going to DOUBLE your air resistance! This is going to nearly double the power needs. So instead of 200 watts, you would need nearly 400 watts to go at this speed to haul not only the bicycle and the rider but also the solar panels. This means 300 watts more power needed, but the panels produce only 100 watts.

Forget it! The poor aerodynamics are going to kill the idea.

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Well it is the end of 2020 and now there are solar panels Panasonic VBHN325KA03 for around $350 that will do

Electrical Watts (STC) 325 W Max Power Voltage (VMPP) 59.2 V Max Power Current (IMPP) 5.50 A Open Circuit Voltage (VOC) 70.9 V Short Circuit Current (ISC) 5.94 A Max System Voltage (UL) DC 600 V Warranty 25 Year product warranty, 90% output 25 years, 90.76% output

In 2013 a 7800 KM solar bike race took place from France to Kazakhstan with only solar power and pedaling. So we should be able to power most bikes with solar now in 2020.

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  • Even with 2020 solar panels charging a retail e-bike battery with solar panels is a very different question from the viability of purpose-built solar bikes.
    – Affe
    Oct 19 '20 at 20:03

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