DeWalt battery internal speed control?

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huntercook
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DeWalt battery internal speed control?

Hi everybody.

I'm new around here, so pardon my ignorance. That said, I did search around here for quite a while, and while I've seen a couple people allude to this possibility, it didn't seem like it has been fully explored, so I thought I'd ask around.

The basic question is this: the DeWalt DC9360 packs apparently contain some kind of speed controller, as witnessed by the fact that the variable-speed drill just has a potentiometer wired to the pack. So, could one build an e-bike with these packs without using a motor controller? What would be the drawbacks to such an approach?

Obviously the primary benefit to doing things this way is that you don't have to pay for a controller. This becomes a bigger benefit if multiple packs can be used in series with this method (thus saving a lot of money on a 72v controller, rather than a little on a 36v one), which strikes me as a much harder question to answer. Ideally I'd like to do a 72v (ok, really 66v) array with 6 or 8 batteries if this is possible.

If you like, head over to the motor forum where I'm about to ask whether a drill motor could be suitable for an ebike. Kinda looks to me like the Dewalt 36v drill (which comes with a battery and charger) is an ebike kit in disguise...just cut it up, add a rear wheel with an extra freewheel on the other side, a chain, some wires, and voila. Feel free to explain the huge things I'm overlooking.

Thanks

Hunter

LinkOfHyrule
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Re: DeWalt battery internal speed control?

Yes, I've heard about that too. I don't know much about it, but (assuming that you don't blow the fuse on the BMS) I think you could do it.

I think you might actually able to do it in series. Because you wouldn't by bypassing the BMS, you wouldn't have to worry about one pack cutting out before the other. You'd even be able to get the last bit of power from all the packs, since the others won't know if the other packs have cut out or not.

This does pose a new problem, however: The BMS fuse. This thing blows at something like 15A. If you sized the packs so that you didn't go over this, you'd be fine at first. But, as soon as the first pack cuts out, you're going to be drawing more amperage from the working packs to make up for it. You're either going to have to figure out a way to interface the packs so that they all cut out when one does, or fit every string of packs with (probably around 10A) breaker switches.

I don't know about the drill motor. Given that it runs at 33V and under 15A means it's a little low on the power. You might have heating problems too, if you remove the fan (I'm assuming it has a fan).

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huntercook
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Re: DeWalt battery internal speed control?

Thanks for the thoughts, Link. This seems very encouraging. Some of the things you mention do have me scratching my head, so here's some more questions:

First, concerning packs in series cutting out at different times. I don't see how this is going to change the amperage drawn from each pack; it just seems like the voltage would go down by 33v each time a pack dropped out. So I'd have much less power, but the same amp draw. Of course if I had them in parallel it seems like it would work the way you describe...or am I looking at this wrong somehow? I will say that if I'm really limited to 15A, it certainly seems like I'll want to have at least 2 packs, regardless of how I wire them.

That brings me to my other confusion. I thought that in most discharging scenarios the 15A fuse wasn't in the circuit. For starters, DeWalt is claiming their drill has 750W of power, which obviously isn't possible through that fuse. Can you clarify when that fuse comes into play and when it doesn't? I'm not sure I fully understood the other posts on the matter.

Also, concerning the motor, it should be higher power than 33v/15A, as described above. And I did not anticipate removing the fan or even the housing...really figured I'd just chop the handle off and mount it up. But I want to be sure not to muddle the issue here by attaching the motor question to this battery/controller question, as they are two very distinct issues. My primary concern in this thread is whether I could use the DC9360 packs to run "some other motor." The main reason is that ideally I'd like to use a 72v capable motor, which the drill doesn't seem to be (not that I'm not considering trying it ;-)) Anyway, for more on the drill motor stuff, be sure to take a look over here.

Thanks again for the response. I'm starting to get pretty excited about this project...

Hunter.

p.s. Zelda rules.

LinkOfHyrule
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Re: DeWalt battery internal speed control?

If one of the packs cuts out in series, you won't be getting any power at all. The pack simply breaks the circuit, and will cut off power from the second pack, too.

Regarding the fuse: In many cases where people need higher discharge rates, they simply bypass the BMS, taking it out of the circuit completely. This is pretty easily done by opening up the pack and adding leads to the + and - ends of the pack. Then they can discharge at whatever rate they want, but need a low-voltage cutout to keep the packs from being over discharged and ruining them. If you go through the BMS, you're limited to 15A per pack/string you put in parallel.

DeWalt is probably fudging their specs. Marketing and all. Either that or doing something sneaky like speccing the motor ITSELF and not the whole drill.

"p.s. Zelda rules."

Yes. Yes it does. 8)

The author of this post isn't responsible for any injury, disability or dismemberment, death, financial loss, illness, addiction, hereditary disease, or any other undesirable consequence or general misfortune resulting from use of the "information" contai

huntercook
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Re: DeWalt battery internal speed control?

First off, yeah, that scenario for series pack dropout makes a lot more sense. Next, I don't think DeWalt could really get away with fudging specs to the degree of claiming +50% power on their drill.

As to the 15A limit, I went a little deeper and ended up here where OneEye describes the two negative leads, one switched through BMS (and unfused) and the other fused (and not BMS-enabled except for LVC). This is without opening or modifying the pack in any way. Also, even if it were fused, since we are talking about using the internal BMS speed control, wouldn't that control keep the battery from discharging beyond its fused ability? In either case, at that same link above Doctorbass testifies to 20A+ discharges and low-voltage cutoffs, again (I think) without pack modification. I'm certainly far from sure, but I don't think this jives with what you're saying regarding this fuse.

Feel free to enlighten me...

Thanks

Hunter.

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