DeWalt questions

[huntercook]

Hi everybody. I've been lurking for quite a while, but this is my first post, so go easy...

I have a couple of related questions regarding the DeWalt 36v gear that now seems to be all the rage in ebikedom. The first one seems less complicated, but the second may be cooler.

First, it seems to me that a DeWalt DC900 drill is an ebike kit in disguise. It's supposed to be 750W at 33v, and variable speed with the controller in the battery. Is there any reason this motor/battery/controller would not work (or work well) for this application? It seems to me that one could simply cut the drill up into 3 pieces (battery holder, handle/trigger, and motor/housing/fan), mount the pieces, wire them together, get a rear wheel with a second freewheel on the other side, chuck a sprocket into the drill, chain it up and ride off. I'm only wary of this because it seems too easy and too cheap...you can get a battery, charger, and drill on ebay for about $200 total, and 36v ebike kits are over $400 near as I can tell. So if it's this easy why isn't everybody doing it?

The other question centers on the DC9360 battery pack and its internal speed controller. Could these batteries be used to run a normal ebike motor without a controller? Seems one could save a bit of money that way, particularly if they could be used this way in series to run a 72v motor. I'd love to have a 2s3p setup running a crystalite if that would really work...seems like by skipping the controller you end up with LiFePO4 weight/performance at a lot closer to SLA prices. Of course, I haven't the first inkling of an idea as to how to wire such a thing up. Any gurus out there want to give me a whack with the cluestick?

Thanks

Hunter

p.s. Doesn't it seem strange that the DC900KT drill kit (drill/battery/charger) runs $450 on ebay while you can get the parts separately for under half that?

 

[kbarrett]

With enough gearing, anything can be made to run anything.

I would be a bit concerned about the drill motor. I don't think it is rated for continuous use. Might get a bit hot. But don't be discouraged ... if you want to sacrifice the drill, get a whizzer ( gas motored bike ) rear sprocket kit from spookytooth cycles or some such, and have at it.

 

[Drunkskunk]

Welcome to the forum!

You have an intresting idea there. It might just work, too.
But I see 2 big problems right off.

The first is the heat. The motor is rated for 750 watts peak, but the tiny can motor they use can't disapate 750 watts worth of heat. it would cook quickly, even with a cooling fan under hard running. If you wanted to use it limited to 200 watts or so, it might work.

The second is the efficancy. The motor in a drill is ment for high torque for short bursts, not sustained efficancy. it would suck the jiuce out of a battery pretty fast.

There's also a problem with the gearing. those small can motors turn around 30,000 RPM at 33 volts. a bike wheel goes around 150rpm. gearing from 30,000 to 150 isn't easy, and the more you reduce rpm in gearing, the less efficent it is. you would need around a 200:1 gear reduction. An alternitive might be a friction drive, but those have issues, too.

The controller, Now there's an idea. I think its been mentioned before, but I don't know of anyone who's tried it yet. one battery might work pretty well, but running a 2s3p set, you might spend more money developing the system to link the batteries for there internal PWM than a prebuilt DC brushed controller would cost.

 

[Toorbough ULL-Zeveigh]

Volcano challenger.

 

[fechter]

The DeWalt drill 'controller' would work for a brushed motor only, not a brushless. It should work quite well with the right motor though.

 

[Dr. Shock]

I thought you all would like to see this, speaking of drill powered bikes.

http://www.youtube.com/watch?v=y8abvlYHK3Q

 

[huntercook]

Thanks everybody for all the replies.

Dr. Shock, that minibike is definitely a proof of concept, and shows that this thing will work, to some degree. Unfortunately I guess it also defines the limits of the system: even with a tiny all-aluminum clown bike, we're looking at 4 miles and 14 mph. On a full-size bike it doesn't even seem worth pursuing...even if the duty cycle was fine and the packs could be parallelized, it would still be too low-end to justify component cost. That lack of efficiency/performance for the motor, combined with a suspect duty cycle, means I'm done with that idea. Thanks to all you guys for putting it to rest. I may still get one of those drills (hey, I like tools, what can I say?) but thankfully it won't be ritually sacrificed in vain.

So, that still leaves the idea of using these packs to run a brushed motor without a controller. Fechter, I was happy to see your endorsement of the basic idea; could you elaborate at all as to what you would consider "the right motor?" If not a particular motor, then maybe you could describe the criteria I'd be looking for?

As to your concern, Drunkskunk, that the development of a system to use multiple packs would be arduous and expensive, let me say that it looks to me like the hard work has been done. First off, the famed neodynamics resistor array (http://www.neodymics.com/Images/V24ProtoSwitch070818E.pdf), according to (it's author's?) post (http://visforvoltage.org/forum/batteries-and-chargers/1699) "looks like a DeWalt drill with the trigger pulled in fully." Now, he goes on to say that the hub-motor he's using requires PWM which the BMS does not provide, but this is leading me toward something like "I need a motor that can be controlled voltage-proportional (fechter is that why you said brushed-only? again, sorry if I'm dumb...i know very little about these motors really)...then I'll put these resistor arrays on the batteries and add some other potentiometer that "looks like" the one in the trigger of the drill." Then I see a post (http://endless-sphere.com/forums/viewtopic.php?f=14&t=2050&st=0&sk=t&sd=a&start=15 ...look about halfway down at the post from Fri Sep 07, 2007 4:23 pm)that I think has Doctorbass claiming to have done just such an arrangement. Did I read that wrong, or does this problem look solved as long as one has "the right motor" ??

I think this looks super-cool. I think if we figure out the right cheap motor and iron out the wiring we could have a real price-performance winner here. Am I crazy?

Thanks again for all the insight.

Hunter

 

[Link]

The drill motor just doesn't cut it. Not enough POWAH!!

PWM is a way of controlling the speed of a motor. It's basically like switching the power on and off thousands of time each second, and leaving the switch on a certain amount of time on each pulse. This is used on brushed and brushless motors.

If the packs really aren't using a PWM system, they might be functioning as a switching power supply, which might not be as efficient as a PWM controller. Might not make much of a difference.

The pot is basically a throttle.

If you can get the BMS controller thing to work, any brushed motor (as long as you don't draw more than 15A for every parallel) should work.

 

[huntercook]

Link, it still doesn't look to me like that 15A fuse comes into play here. See the first post (by OneEye) on this page: http://endless-sphere.com/forums/viewtopic.php?f=14&t=2050&st=0&sk=t&sd=a&start=15 ...where it says:

There are two ground paths to the cells. The "main ground" with the 15A fuse is the outside spade connection. The BMS does not provide any control over this path. The inner spade connection runs through the 60V 80A mosfet as pointed out by Doctorbass. This inner spade is not connected through the 15A fuse.

Later in the discussion Doctorbass tests the packs (still unmodified, I think) at around 20A, and the BMS still is providing low-voltage cutoff. So, isn't it the inner spade that I'm looking to pull from in this case? Sorry if I'm totally reading this wrong. And of course it's pretty well academic since I'm talking about going 66v with at least 2 strings in parallel...that's near 2kw at 30A, which I'm confident is way more than I need. Depending on the motor I'd probably end up fusing them lower than that anyway.

 

[kbarrett]

You might want to look through the forum archives.

A lot of folks have had some success running e-bikes on un-modified dewalt packs ... especially with brushed motors, but with some success with hall effect motors as well.

They all run into the same problem that dewalt tool owners run into ... great batteries, but a BMS that really sucks. Add that to putting 10 cells in series to get a 33V battery, and you have a balancing nightmare.

You can run a bike on unmodified DeWalts for a while ( five in parallel make a damned fine 33V11A LiFePO4 battery ) ... but you will start to lose the packs one at a time when a single cell in the pack dies, and the recharger just refuses to charge it.

When you have a bunch of these collected, you then need to break them into single cells, trash the dead ones, and start reading the A123 threads in the "Batteries" forum section to learn how to make your own better batteries out of these cells, and build better BMS's to put on them.