Magic DeWalt BMS changing delivered voltage?

[ebinary]

I'd anecdotely heard that the integrated DeWalt BMS could control the
supply voltage of a DeWalt pack, but I had no concrete evidence of it.

However, I just disassembled a DeWalt 36V flashlight and did a few
measurements. It was interesting, but I'm not sure what to make from
it.

As you may know the two outer blades on a DeWalt pack are hot (pos
and neg respectively), and then there is a 2nd negative blade that
isn't energized until the proper network of components is hooked
across the low-amperage center pins in some combination.

Here are some measurements I took, all are measure from the positive
blade:

unconnected battery:
Voltage across to the outer neg blades: ~35
Voltage across the inner neg blade: 0

With tool attached and off:
Voltage across to the outer neg blades: ~34
Voltage across the inner neg blade: ~32

With tool attached and on:
Voltage across to the outer neg blades: ~34
Voltage across the inner neg blade: ~20
Voltage across the flashlight bulb: ~20

So, the weird part is the 20V. If this tool, like the drill, is
really being powered by the inner blade, somehow the tools is telling
the battery pack to deliver only a portion of the voltage.

Thats pretty sweet, if its possible.

All thats on the circuit board is a fuse, diode, two resistors, a
small cap and a small tranformer (with only one coil hooked up). So
I think it really is signaling the BMS to provide less than the
maximum voltage.

Of course, it may not be doing that at all, and I am fairly
electronically illiterate. I'm just reporting the behavior.

 

[efreak]

interesting !

i wish they sold those battery ports seperately as parts,
maybe they do.

i mean cheap

efreak

 

[jjackstone]

I ran across this info a while back. Might be what you're looking for....just not cheap.

"So, if you wanted to use unmodified DeWalt/A123 batteries with their original charger, you could purchase part #636747-00 (wiring harness, $39.90) and part #630141-00 (tool casing with battery latch, $28.76) to build something to properly interface with the battery. Other cases or harnesses might be cheaper; just poke around http://www.dewaltservicenet.com.

Hope that helps.
JJ

 

[ebinary]

I ended up buying four flashlights to steal the casings, at like $30 each.

BTW - Apparently this deWalt BMS pulse width modulation is documented a few places. Apparently, the pack can deliver a chopped output that appears on a meter to be lower voltage, but in reality is nearly the full pack voltage, chopped into an on/off duty cycle.

The "reduced voltage" could be used to run an inductive load (like a motor) but not a resistive load (like a controller), without some additional components.

I'd love to know what those additional components (cap/coil) need to be to provide <30V, which would allow dewalts to power most 24V nominal bike unmodified. Anyone? Anyone? Bueller?

 

[Malcolm]

If you replicate the voltage divider circuit found in the DeWalt drill you can draw up to about 20 amps from the pack at 33V. Doctorbass has done this.
There are some more links here: http://endless-sphere.com/forums/vi...&postorder=asc&highlight=dewalt+drill&start=0

 

[ebinary]

Thanks!

I'm aware of that post (although the guys who published the original .pdf on how to do it say that Doc messed up on his test with their published values). Apparently they have lots of bikes running off the original spec.

I am interested in a true voltage change (taking the PWM modulated 33V wave and smoothing it to appear as 29.5V to a resistive load). Similar to what it done to present 20V to the lamp, but it needs to be consistent enough to not blow up my controller.

Eric

 

[fechter]

I think you'd have to put a big inductor and a freewheel diode between the battery and the load if you want it to behave like a lower voltage sorce for a controller. The diode would go across the battery (in the non-conducting direction), and the inductor would be in series. The inductor would need to handle the full output current, as would the diode.

 

[ebinary]

Thanks fechter,

Can you give me an idea of what the specs of the "big inductor" would need to be, please? I've got some power diodes that should work, but I'll need to buy the inductor.

Eric

 

[fechter]

Hmm.... that sort of depends on the switching frequency. I have no idea what the switching frequency is. The wire gauge needs to handle the full battery current. I don't think it would hurt much if it was too big.

I'd suggest something like a 1" dia. torroid with 14ga. wire wrapped around it. You might be able to use a ferrite rod too. This is wild speculation, so results may vary. I don't think it's too critical.

Perhaps one of the real EE's out there would have a better idea. I suspect there's a formula to calculate it.

The diode should be a fast Schottkey, and should be able to take the full voltage and current you are working with.

 

[ebinary]

Thanks for the wild speculation! I've yet to find anyone who has posted the frequency of the PWM, but I'll keep my eye out.

Eric

 

[fechter]

Can you hear the motor whining at partial throttle on a DeWalt drill?

I don't think it's too critical. If the value is off, the efficiency will drop a bit. You may need to experiment. Inductors tend to be expensive unless you make your own. You might be able to use an iron core one. Those look like a transformer with only two wires.

 

[magudaman]

kind of off topic but the dewalt drills really suck power wise! I have an older 18v makita drill and had the 36v dewalt for a day or two. The 18v would out drill the 33v every time. It's really sad that they limit the current to 20a other wise I'm sure it could out drill even the plug ins.

 

[ebinary]

Re: the power of a Dewalt Drill.

I'm a little leary about drilling with 33V to begin with. Drilling multiple holes in plywood with a hole saw with only 16.8V drill often left my wrist ligaments/tendons badly torqued (whenever the hole saw would bind).

I wouldn't be suprised if drilling with unchecked amperage would simply snap a guys arm off.

Eric

edited: turns out my drill was 16.8 not 14.4

 

[TylerDurden]

I didn't use the side-handle on a 1/2" drill until my 24V Ridgid lithium drill.

Those suckers can really twist.