At Luna we get asked this a lot. So here is some info. I have added this to the knowledge base for Fixed for future reference.

Considerations for adding second battery for more range

Generally speaking this is not advisable. It would require extensive modifications and could compromise the battery. Please note that Luna can not take responsibility for the consequences if you break your bike or battery. There are a few separate proposals, we will discuss each below.

Manually switching batteries: Physically unplugging battery 1 from the controller, then plugging battery 2 into the controller. This would be the most straightforward, and eliminates a lot of potential danger in doing the other methods incorrectly. As long as the second battery is a normal 36v nominal battery that should be fine.

Parallel wiring: The idea here being that you would put another battery in parallel. Generally this is strongly discouraged on bms protected packs unless you are unplugging them from eachother when charging, and are making sure both batteries are identical, and both batteries are same voltage at time of connecting them. Given that this would be difficult to meet these requirements there is another way you might go by using a diode to block current from flowing back into the other battery. This page goes over that in more detail. For a working example of this idea in practice, see this page.

Theoretically you could use Luna remote switches on each battery in that parallel circuit, and switch back and forth by turning the one battery off and then turning the other battery on. This would not be advisable due to the possibility of accidentally not turning the other battery off, which would lead to a massive influx of power which could cause a fire. It might be possible to modify a couple remotes to use the same button with a circuit to control making sure that one remote is always off before another one is turned on, but this is outside the scope of what we can advise on.

Charging integrated battery via the charge port: This is unsafe for multiple reasons. Primarily because any second battery you want to wire up will have a discharge rate ten times higher than the safe charge rate on the charge port. This will burn out the charge port instantly and is also likely to damage the integrated battery on the bike. The port expects something like 2-3A, and another battery can put out 20-30A, maybe higher.

Additionally, as with any BMS-protected battery the charge port is how the battery bms is reset if something trips it. So if you are charging the original battery while in use, if something like a short happens in a cell, (which normally the BMS would protect against by shutting down the battery) you may be forcing it to reset despite it not being safe to do so.

In theory you could limit the current of the discharge on the secondary pack using some custom circuitry, so when you plug into the charge port it is not charging it at some massively high rate of current. For example something like this, which can also do CC/CV charging, an important safety consideration. However, you would still be resetting the BMS on the original battery whenever you are charging it. Do so at your own risk.

I've done it. I used a single 100w panel hooked to an MPPT charge controller with variable output voltage. It would only charge higher voltages than 18v, but that was okay since my bikes are 24, 36, and now 48v. Obviously, it was slow going. But i was able to put in about 50wh over an hour. Not the best efficiency, but it did work.

Out of curiosity is the boost converter actually putting out the voltage it's saying when under load? When I was experimenting on making one for my XR I found I'd set mine to put 63v out, but when I measured it under load regardless of the voltage of the battery at the time it was pumping 68v! I was using one of these mind you so it might just be an inferior product compared to what you're using.

I've since opted for a solar charger that has been working fine. It will start charging in a constant current state at the same voltage of the battery at that time up untill it reaches the max voltage then charges in constant voltage mode to top off the cells.

References

[0]: 1st charger
http://imgur.com/gallery/SFL5Gz3

[1]: Docooler® MPPT Solar Panel Battery Regulator Charge Controller 10A https://www.amazon.com/dp/B01HCL7LEW/ref=cm_sw_r_cp_apa_i_wZQ7CbFHZ58AP

[2]: laptop supply
http://imgur.com/gallery/EeTMH4O

[3]: battery charging dongle and spare parts
http://imgur.com/9etsFlw

[4]: 48V 17Ah (13S5P-35E) https://em3ev.com/shop/em3ev-48v-13s5p-jumbo-shark-ebike-battery/

Is it possible? Yes. Is it a good idea? You're an adult, you be the judge!

Flexible 100w solar panels weigh about 5 lbs and are about 2' by 4'. Their maximum power is produced at about 17v, so you'll need a CC/CV MPPT charge controller that can boost the voltage into the 50v range. You can find the Ming He mpt-7210a for $25-35. You'll need one of these for every 10 amps of electricity produced by the panels, but you can easily wire 2 panels in series to double the voltage/halve the amperage, I think you could wire four panels in a 2p/2s configuration and get 300 watts into your battery in full sun. Here are a couple of videos you'll want to check out, since the instructions are inadequate:

https://www.youtube.com/watch?v=mM6JTquNSZU

https://www.youtube.com/watch?v=RVkehQ_RypI

I wouldn't bother charging while riding, I would take a 2-3 hour noon siesta and get a bit more charging in the evenings. I would get 3 or 4 panels and stack them up on a bike trailer, and then lay them out to charge. In the evenings, you can prop them up with a couple of tent poles or bamboo stakes. Tie them all together and then ram a few pegs into the ground to secure them. You could charge while riding but you probably won't want to lay out 16 square feet of solar panels (wind and all that) and you won't be able to position the panels accurately.

A 48v 10ah battery pack has about 500 watt-hours of energy. With 400 watts of panels, in peak sun, after losses from boosting the voltage, maybe you could get 300 watts into the battery per hour. In theory you could charge the battery in a couple of hours. Solar energy peaks at ~12-1pm, there will be twice as much power (or more) at solar peak as there would be about 2 hours before sunset.

It'll cost you around $200 to get one panel, one charger, and associated wires. Try it and report back!

Parts:

Panel

Controller

Controller to XT60

Panel to controller

Extra wire

The monetary cost to do pedal recharging is minimal, you would need a regenerating motor controller which is less than ten dollars more than a non-regenerating one and you would need a way to prop the rear wheel off the ground so it can be spun with the pedals.

I built a solar recharged ebike about five years ago with very similar parts to what you see on these bikes. The problem I had was I was running 50V of lithium cells and the only solar charge controller I could find for lithium at the time had to be custom ordered and cost more than the rest of the bike put together. That has changed now, you can get a high voltage solar controller that would be suitable for lithium from 24V to up over 60V for less than fifty dollars.

The average person can't turn the motor fast enough pedaling as a generator to get it into a reasonably efficient range or at least not for longer than a minute or two.

Of course you could hire a few kids to pedal your bike to recharge, they take turns and can pedal fast for a few minutes each and then swap out.

I've been studying up on the same topic, and there are a few ways to go about it.

I think the best option for you might be this particular MPPT controller, which you could between the panel and battery, or the battery and a higher voltage battery if need be.

following that link I found one, that specifically say: user can set the output voltage and current according to actual need.
https://www.amazon.com/Docooler%C2%AE-Battery-Regulator-Charge-Controller/dp/B01HCL7LEW/ref=sr_1_7?s=lawn-garden&ie=UTF8&qid=1506314112&sr=1-7&keywords=mppt+charge+controller

but it seem is for 24v system and up. not for 12v.
btw when using MPPT controller do you need to have 24v panels?