Advice on a bike-conversion project

Thread Starter

xox

Joined Sep 8, 2017
544
This past summer I bought an e-bike. I love riding the thing, but now the batteries have started to wear down somewhat and I'm lucky to get 15 miles out of a full charge. I'd rather not have to buy another battery. Besides the fact that they run around 600 bucks, I just want more range. So I'm thinking of mounting a gas-powered generator to it. The bike's nominal current requirement is 11.6 amps (~560 watts) so I probably need a generator that can handle at least 750 watts. Found one online that should work, but now I need something that can convert down from mains-level AC to 48V DC. I can't seem to locate anything that can manage that kind of power draw at 48V though. I really don't feel confident putting together a SWPS much less a linear regulator with my scant level of electronics experience if I can avoid it. Any recommendations on some brands/models I should be looking at?

Specs:
  • Bike: RadRover
  • Generator: HomeGear 950i
  • AC-to-DC Power Supply: 120VAC to 48VDC, rated at 20 amps or so, preferably a lightweight switched-mode supply with under/over current and voltage protection.
 

AlbertHall

Joined Jun 4, 2014
11,222
How about a transformer to get 48V AC and then a bridge rectifier? You probably wouldn't need to smooth the DC as the inductance and inertia of the motor will do that for you.
 

Thread Starter

xox

Joined Sep 8, 2017
544
How about a transformer to get 48V AC and then a bridge rectifier? You probably wouldn't need to smooth the DC as the inductance and inertia of the motor will do that for you.
Really, that simple? Okay, so assuming that would work, I just need to make sure the transformer and diodes can handle 20 amps or so to be safe, right? And what about heat dissipation, is that something to worry about here?
 

AlbertHall

Joined Jun 4, 2014
11,222
I'd probably look for a transformer that could handle 15A as they get bigger and heavier and more expensive for higher power. 20A would be good for the diode. The transformer is very efficient and reliable but it won't be cheap.

Incidentally, you should probably check the legalities of this in your region.
 

Thread Starter

xox

Joined Sep 8, 2017
544
I'd probably look for a transformer that could handle 15A as they get bigger and heavier and more expensive for higher power. 20A would be good for the diode. The transformer is very efficient and reliable but it won't be cheap.
Thanks, I'll start looking around for some hardware that can handle those current ranges. I don't want anything to burst into flames while I'm riding down the road either, so I'll be sure to stay clear of the cheap Chinese components and stick with the good stuff.
 

Thread Starter

xox

Joined Sep 8, 2017
544
The gas engine is a far bigger flame risk than the electronics.
Well, over the years I've had several experiences where the product was either downright flaky or just mislabeled in such a way that it could easily have lead to a catastrophic failure had I actually pushed it to the limits advertised, so just want to take whatever steps I can to minimize the risk of damage to the bike (or myself).
 

strantor

Joined Oct 3, 2010
5,534
That generator seems huge and clunky for a bike. If this were my project I would couple a chainsaw engine to an automotive alternator. The field current of the alternator can be regulated to output 48vdc instead of 12-14Vdc as in typical automotive applications.


Edit:
I take that back. If it were my project and I had $239 (plus tax, plus transformer, & misc) to spend on a generator, I would spend it on more batteries instead. You can buy hobby lipo from hobbyking.com for MUCH cheaper than $600.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
11,222
I just had a thought on the transformer and rectifier idea: That would work fine if it was connected directly to the motor but presumably there is some speed control between the battery and the motor and I don't know how that would react to an unsmoothed DC. If you need to smooth it then you need a lower voltage transfomer perhaps 35V (smoothed DC is 1.414 times the transformer voltage - 2 diode voltage drops) and it would take a large capacitor to reduce the ripple to a reasonable level.
 

Thread Starter

xox

Joined Sep 8, 2017
544
That generator seems huge and clunky for a bike. If this were my project I would couple a chainsaw engine to an automotive alternator. The field current of the alternator can be regulated to output 48vdc instead of 12-14Vdc as in typical automotive applications.
It's really not that bad. The whole thing weighs just 22 pounds, which is way less than I've carried before (at least 60+ pounds). It's pretty quiet too, which is nice. Plus interfacing a motor with an alternator requires a fair amount of precision and expertise. Not sure how successful I'd be at that.

I take that back. If it were my project and I had $239 (plus tax, plus transformer, & misc) to spend on a generator, I would spend it on more batteries instead. You can buy hobby lipo from hobbyking.com for MUCH cheaper than $600.
Thing about batteries is they take time to charge. If you want to jet around town all day like I do, you end up having to make long pit-stops at the local Starbucks, or wherever else you can find a free outlet. Huge pain in the arse. Filling up a 1/2 gallon gas tank is fast, cheap, and easy.

I just had a thought on the transformer and rectifier idea: That would work fine if it was connected directly to the motor but presumably there is some speed control between the battery and the motor and I don't know how that would react to an unsmoothed DC. If you need to smooth it then you need a lower voltage transfomer perhaps 35V (smoothed DC is 1.414 times the transformer voltage - 2 diode voltage drops) and it would take a large capacitor to reduce the ripple to a reasonable level.
Well, I would have to disconnect the battery anyway while the generator is running since the battery requires a special charging circuit (and the generator would be just dumping unregulated energy into it otherwise). Oh wait, or you mean the speed controller itself couldn't handle the ripple? If so, that's fine. How big of a capacitor? I've got a 50V 4000uF Sprague cap coming in the mail soon, do you think that one should work?
 

MrChips

Joined Oct 2, 2009
23,239
After it's all been said and done, I think it would have been easier to get a new battery. I have been commuting daily on an ebike for about six years now. The 36V lithium battery lasted 5 years before it needed replacing.
 

Thread Starter

xox

Joined Sep 8, 2017
544
After it's all been said and done, I think it would have been easier to get a new battery. I have been commuting daily on an ebike for about six years now. The 36V lithium battery lasted 5 years before it needed replacing.
I need more range though. Like 50+ miles and very little downtime. No battery is going to offer that. Anyway, even after the conversion I'll probably still replace the battery, though most likely I'll open up the existing case and just replace the individual 18650 cells (or whatever happens to be in there).
 

Thread Starter

xox

Joined Sep 8, 2017
544
What type of ebike is it, peddle bike or e-scooter?
What is the voltage and type of chemistry of the battery?
It's an e-bike, with "pedal assist" and regenerative braking. This one. Pretty sure it uses standard 18650 cells, but in any case the overall voltage is 48V.

Bike costs $1500 and you're looking to probably dump another $300 into it to essentially turn it into an underpowered gas motorcycle. Why not just sell it and buy a real motorcycle? assuming you get half price for it ($750) and then take that other $300 for the generator, that's $1050 to spend. or just a scooter like this
https://houston.craigslist.org/mcy/d/scooter-moped-2011-tao-50cc/6352984623.html
For one thing, you can't take a motorcycle or scooter everywhere you can a bicycle, like hike and bike trails, sidewalks, through the grass, etc. Second (and most important for me) I get a ton of exercise riding this thing. Anyway, I just really enjoy the bike, it's like a low-speed thrill. Hard to describe.
 

MrChips

Joined Oct 2, 2009
23,239
Good to know that it is a pedal bike and not an e-scooter.
It is rare to get 50-mile range on any ebike. With pedal-assist, yes, your range is unlimited.
The important thing is when you are out of battery power you can still make it back home. You cannot do that with an e-scooter.

afaik, ebike battery packs do not use 18650 cells. The cells are much bigger.
 

AlbertHall

Joined Jun 4, 2014
11,222
So how much voltage ripple is "acceptable" in most applications? Assuming it's something like 0.01V, I'm looking at round about the 20 Farad range. Holy crap!
It probably doesn't need to be that low but it is impossible to say what would be acceptable as it depends on what the control circuit can handle and we have no details of that.
 

AlbertHall

Joined Jun 4, 2014
11,222
For one thing, you can't take a motorcycle or scooter everywhere you can a bicycle, like hike and bike trails, sidewalks, through the grass, etc. Second (and most important for me) I get a ton of exercise riding this thing. Anyway, I just really enjoy the bike, it's like a low-speed thrill. Hard to describe.
If you fit the gas engine are you still allowed to ride it in those places? Have you checked the rules?
 
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