I want to use my 18VDC drill to convert my bike to E-assist. Has anyone done this?

 

There are a couple of safety circuits you will want to employ:
1. Current limiting. These batteries can't supply infinite current, but they will try to (in case of short or stalled motor), and they will hurt themselves in the attempt. The cordless tool is meant to do its own current limiting.
2. Undervoltage lockout. The battery charging dock will not charge a battery that has been discharged past a certain point. The cordless tool is meant to stop working before voltage goes below a certain threshold.

For these reasons I recommend using an entire cordless tool (drill, angle grinder, whatever) rather than just a cordless tool battery. It has all the smarts built into it already, plus it already has the socket to accept your battery. Maybe you don't want to look like you slapped a cordless drill on your bike, maybe you think that looks stupid, but I thing it looks smart. If you disagree you can take it out of the case and make a new case for it but you will probably wish you hadn't.

 

What is the power/torque rating of your drill? Is that a continuous rating (drills would normally only be used for brief periods at a time)?

 

What is the power/torque rating of your drill? Is that a continuous rating (drills would normally only be used for brief periods at a time)?

I used it (drill) for 15 minutes at a time, works well

 

I used it (drill) for 15 minutes at a time, works well

Under heavy load & full torque - I doubt it... the gearing on a drill is very different to that on an e-bike.

Anyway an e-bike battery is typically 36 - 48v @ 18Ah v your 18v @ 2Ah - not much of an e-assist there!

 

As a general case, the requirements for a drill and the requirements for a bicycle are vey different.

The motor inside the drill is already geared down to get the torque that appears at the chuck. An e-bike power unit needs to rotate fast enough—with sufficient torque—to drive the bike. It also needs to produce a lot more in the way of Wh (Watt hours) than a drill/driver does.

As an example, Bosch makes both e-bike drives and drill drivers. Their basic drive produces a maximum of 40Nm while their standard brushless drill driver has a rated peak of 45Nm. This seems to favor the drill, but it doesn’t when all is accounted for.

The drill is a low duty cycle device designed expecting a few minutes on (maximum) with at least a few minutes off. The cooling, gear train, and ESC (Electronic Speed Controller) are specified for this regime. On the other hand, the e-bike is expected to be continuous duty in the range of hours, not minutes. All the components are specified with this in mind.

The other issue is batteries. Sticking with Bosch, the smallest companion battery for the drive above is a 36V, 8.2Ah providing about 300Wh of power. The drill/driver is paired with an 18V, 4Ah battery which is about ¼ of the capacity. The very largest batteries offered for the drill/driver are 18V, 8Ah so you would need two of these to match the smallest e-bike battery.

Even if the drill/driver could sustain the necessary power for the bike, its run time would be at most 25% of the lowest end Bosch e-bike drive. Keep in mind, the Bosch drill/driver in this comparison is not very inexpensive. Many 18V drill/drivers would not even be able to perform this well.

This is also all without even accounting for the inefficiencies in the transmission system which will turn some (possibly significant portion) of the input power of the drill into heat rather than motion.

I suspect you would be much better off either buying a capable motor and gear box, or even better an integrated hub conversion kit (this one is premium, but you can find options on AliExpress, &c), and keep the drill in the toolbox using it to build the e-bike.

 

an e-bike battery is typically 36 - 48v @ 18Ah v your 18v @ 2Ah - not much of an e-assist there!

As a general case, the requirements for a drill and the requirements for a bicycle are vey different.

I made a bad assumption: that expectations were managed by TS. To me it seems obvious that a cordless drill is not going to be in the same class as a real e-bike capable of inner city speed limits and hours of run time, so my mind went to things like below, and I assumed that's what TS was asking about. I could have been completely wrong

 

I made a bad assumption: that expectations were managed by TS. To me it seems obvious that a cordless drill is not going to be in the same class as a real e-bike capable of inner city speed limits and hours of run time, so my mind went to things like below, and I assumed that's what TS was asking about. I could have been completely wrong

Yes, that's a possibility. If he means a novelty thing instead of a useful vehicle, a drill could be used in a limited way. I made the (possibly unwarranted) assumption he meant the latter—time will tell.

 

I've wasted a couple of hours on YouTube looking at various 'drill-bike' videos. I've yet to see anything that suggests more than a few minutes on a flat surface. My guess is that after a few short runs either the drill or the battery or most likely both dies... but they don't show that. Any sort of gradient will kill it quicker... its at best a novelty....

Simple numbers show the fallacy: power to climb a 5% gradient at 9mph (4m/s) on a bike + rider weighing 100kg:

m * g * h = 100 * 10 * 1m = 1000J​

1m climb @ 5% = 20m @ 4m/s = 5sec​

power req = 1000J/5sec = 200W out​

200w out at 80% eff = 250W in​

250w in @ 18v = 14A, which will fully discharge a 2Ah (20h/rate) pack in around 6min or less than a mile, assuming no pedal assist​

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A real ebike battery 36v @ 18Ah, 650W will give a 10 - 12mile range on moderately hilly terrain @ 10mph, ie about an hour assuming no pedal assist

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I've wasted a couple of hours on YouTube looking at various 'drill-bike' videos. I've yet to see anything that suggests more than a few minutes on a flat surface. My guess is that after a few short runs either the drill or the battery or most likely both dies... but they don't show that. Any sort of gradient will kill it quicker... its at best a novelty....

Simple numbers show the fallacy: power to climb a 5% gradient at 9mph (4m/s) on a bike + rider weighing 100kg:

m * g * h = 100 * 10 * 1m = 1000J​

1m climb @ 5% = 20m @ 4m/s = 5sec​

power req = 1000J/5sec = 200W out​

200w out at 80% eff = 250W in​

250w in @ 18v = 14A, which will fully discharge a 2Ah (20h/rate) pack in around 6min or less than a mile, assuming no pedal assist​

​

A real ebike battery 36v @ 18Ah, 650W will give a 10 - 12mile range on moderately hilly terrain @ 10mph, ie about an hour assuming no pedal assist

I agree there is not a ton of potential but I think there is (or can be) more potential than you're giving credit for. The 2AH from your math is a pretty small pack (all of mine are 5AH or more) and 200W mechanical is not exactly asking the world of a modern cordless drill. Check out this drill/battery kit and see if your opinion changes any. That drill is rated 1219 "watts" (UWO, DeWalt's measure of mechanical output power) and the battery is 60V/6AH. That's 9x more battery than your math was based on, and coupled to a drill that has as much power as the corded last-resort drill that most of us keep in a case collecting dust, reserved for jobs "too big" for a cordless tool. Also that drill has a high efficiency brushless motor. Also in the interest of fairness consider that not everyone will be driving perpetually up a 5% grade. If you do that, and use this (or similar high output "prosumer") drill/battery as your mathematical starting point, I think the numbers might point to something further from a novelty/fallacy and closer to legitimate e-bike but with less than normal range. And then consider that more than one battery can be on board and easily swapped out (if the bike is designed right, without even stopping) and then it could be that this is actually a legitimate e-bike solution that even manages to mitigate the hazard of charging DIY battery packs. Although admittedly it would cost a lot to do it this way and I have my doubts about the longevity of the drill's gearbox used in this manner.

 

I agree there is not a ton of potential but I think there is (or can be) more potential than you're giving credit for. The 2AH from your math is a pretty small pack (all of mine are 5AH or more) and 200W mechanical is not exactly asking the world of a modern cordless drill. Check out this drill/battery kit and see if your opinion changes any. That drill is rated 1219 "watts" (UWO, DeWalt's measure of mechanical output power) and the battery is 60V/6AH. That's 9x more battery than your math was based on, and coupled to a drill that has as much power as the corded last-resort drill that most of us keep in a case collecting dust, reserved for jobs "too big" for a cordless tool. Also that drill has a high efficiency brushless motor. Also in the interest of fairness consider that not everyone will be driving perpetually up a 5% grade. If you do that, and use this (or similar high output "prosumer") drill/battery as your mathematical starting point, I think the numbers might point to something further from a novelty/fallacy and closer to legitimate e-bike but with less than normal range. And then consider that more than one battery can be on board and easily swapped out (if the bike is designed right, without even stopping) and then it could be that this is actually a legitimate e-bike solution that even manages to mitigate the hazard of charging DIY battery packs. Although admittedly it would cost a lot to do it this way and I have my doubts about the longevity of the drill's gearbox used in this manner.

You make some fair points but my example was just intended to be illustrative. I agree there are some serious drills out there from DeWalt, Bosch and the like. But the pricing on those top-end drills ($300+) is on a par, if not more than, a cheap Chinese e-bike kit, so it makes no sense to assess the TS request with such equipment, especially as he already owns the battery and presumably the drill. And yes, there are bigger battery packs, but if you look at recent consumer 'best buy' reports on drills for the typical DIY/hobby user, which I suggest the TS is most likely, 90% come with a 2 or 2.5Ah pack as standard, so its reasonable to assume the TS' pack is of that ilk. The typical drill in that arena is rated at 300 - 500W but not continuous use so 250W is likely to overheat it if run on load for anything more than a few minutes.

The 5% grade calculation is not intended to be perpetual, but most places are not dead flat (if they were, why would you need an e-assist anyway?) and my point was to show how little distance you'll cover with even a light smattering of inclines - there'll be no regenerative braking here,.

 

I agree there are some serious drills out there from DeWalt, Bosch and the like. But the pricing on those top-end drills ($300+)

To your point, there is a very large gap between the drills you can get “cheap” and the ones that are used in professional settings. You currently have the classic choice of cheap, powerful, durable—pick any two.

Recently, low end tool sellers like Harbor Freight have been giving the more solid brands a run in the power department but they are no longer as dirt cheap as they were, and they aren’t built to last. You will pay a large percentage of a more reputable tool for one that sacrifices durability for power.

A tool from a manufacturer like Milwaukee is often among the most powerful but frequently not. However when power isn’t actually the concern for the job but durability is, you will find them among the most durable. A brand like Bosch is very other in the middle of the pack for power (though not always!) but like Milwaukee their professional tools are very durable.

The thing is, a drill is designed to do.. what drills do, and that’s not powering bicycles. So, while you would have to pay a very similar price to a bespoke e-bike drive/battery for a drill that might do sort of OK as a substitute, the same money used to configure the same materials in a different way will do much better. That’s why I used Bosch as an example, they make both.

Perhaps I should make an e-bike drive based drill/driver…