What coil/ferrite core for battery output?

ebinary

ebinary

100 W
Joined
Dec 12, 2007
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Some folks have suggested a ferrite core wrapped in insulated 12 guage wire to reduce the onrush current on my new-style dewalts. The new-style DeWalts are very sensitive to draw, and seem to shut down at a any draw whatsoever (thought they still work fine for the circular saw, for example.

Anyone know if this would this part be an appropriate choice for that job - or could anyone please put a link to an appropriate radio-shack part to use for this?

http://www.radioshack.com/product/i...&sr=1&origkw=choke&kw=choke&parentPage=search

Thanks!
 
dirty_d said:
its probably bad idea to put additional inductance in the battery wires.
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Can you explain why?

I now have three DeWalt batterys (thats $450) - all bought in the last month - that won't power my bike through their internal BMSes because the BMSes shut down as soon as the controller is turned on. They will power DeWalt tools, however.

I have four older models that all work fine.
 
DirtyD is right about inductance being a bad idea, so instead try using a pre-charge resistor to charge up your controller's capacitors before directly connecting it to the batteries. Maybe try a 100 ohm resistor (or similar) between controller and batteries for a few seconds, them bypass it with your regular power switch when the controller's caps have charged up to a reasonnable enough voltage level to avoid tripping your battery's BMS.
 
ZapPat said:
DirtyD is right about inductance being a bad idea.
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I believe you guys, I'd just like to qualify the "why" its a bad idea.

The guys who originally hacked the DeWalt battery resister network and designed the series/parallel circuit that many have copied once mentioned that they also put an inductor on the output. So it doesn't seem like its a terrible idea.

Does it rob power, generate heat.... whats the issue?
 
wrobinson0413 said:
Inductance acts like a really big resistance in series with the battery when pwm switching is present. A battery can be considered to act like a capacitor in some ways, so the shorter the wire is to your controller the easier it is for energy to flow into the controller. If you have the inductor present, during switching, the energy has to come predominately out of your ripple capacitors that are inside the controller. So if there is not enough ripple current rating or they are poor quality capacitors, you will let the magic smoke out of them. ZapPat is correct about using a precharge resistor. Simple precharging the caps will eliminate the surge current that you are seeing. This is what they do in most battery vehicle controllers out there, except the ebikes.
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Pefect - thanks. I'm not sure if this is even the real issue with these packs, because they would shut down even if I had three of the older-style-bms packs (paralleled) from DeWalt.

You'd think that the other three would help the new-style unit not get swamped. One old dewalt could - poorly - power my bike. Two was great, four was awesome. These new ones piss me off.

I wonder how dewalt does it. They don't shut down when a circular saw starts up, and there can't be a precharge resistor, or the battery would be constantly drained, no? Or would the drain stop completely once the capacitors are charged?
 
The issue of inductance interfering with PWM output is a very good point, but it may not apply to the wiring between the batteries and the controller. There should be some significant capacitance on the DC input for the controller, and assuming that the capacitors used are large enough to load level the draw on the batteries, there should only be a simple DC current between the batteries and the controller. If that's the case, adding inductance to the input on the controller won't interfere with the PWM output that drives the motor.

However, there are other potential down sides of adding inductance to the DC input of the controller. When you go from full throttle to nothing, the biggest change in current in the shortest amount of time, that inductance is going to kick up the voltage on the input side of the controller. You'll end up with voltage spiking, and the size of that spike will depend on the size of the inductor and how fast the change in current occurs. It's a tricky thing, so be sure your math is sound before actually trying anything out. Too much inductance and you can easily blow out the input capacitors or other components in the controller.

I think the best way to address the problem you're having with too large of an initial draw on the batteries would be adding a "soft start" function to the controller. The controller's logic can control the output to the motor such that the power has to ramp up, rather than immediately going to maximum current. If you don't have a programmable controller, though, this probably isn't an easy option.

I'd avoid adding actual resistors at any point between the motor and the batteries. That's really just a band-aid that will overall reduce your system efficiency.
 
lepton said:
It's a tricky thing, so be sure your math is sound before actually trying anything out. Too much inductance and you can easily blow out the input capacitors or other components in the controller.
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I didn't have any math. I was going to wrap as many wraps of 12g wire fit around the rectangular "core" I linked to in the original post. I'm thinking this is a bad idea now.
 
ebinary said:
Some folks have suggested a ferrite core wrapped in insulated 12 guage wire to reduce the onrush current on my new-style dewalts. The new-style DeWalts are very sensitive to draw, and seem to shut down at a any draw whatsoever
Click to expand...

I would suggest a different sort of circuit. Perhaps an N-channel MOSFET on the negative side with a pullup to V+ (use a resistor and zener to limit voltage on the gate to about 12 volts.) Add a small capacitor from gate to source to slow down turnon. This will allow a ramp-up of current instead of an instantaneous connection. It's also a pretty good switch.
 
wrobinson0413 - I agree with all of that, I think we may be saying approximately the same thing. What will happen certainly depends on the value of the capacitance and inductance in the system, but in the small capacitance / large inductance situation, I think you're dead on.
 
Yeap, the high current battery discharge that results from initially connecting charged batteries to a controller (that has discharged capacitors) can definitely cause problems.

it seems like having a battery management system / controller that handles charging, balancing and discharging might make a lot of sense to deal with these sorts of problems. It's difficult to expect an unpowered controller to limit current before it's even turned on, whereas it many battery systems there's already a need for battery management circuits.

wrobinson0413 said:
I was just thinking on the way home that there was another case of hot plugging the controller into the battery pack that would also generate an voltage overshoot at the controller. We had a forklift application that was blowing up displays that we manufactured because they would hot plug a freshly charge 48V lead acid into the truck, and the surge current charging up the caps would cause a voltage overshoot because of the line inductance, and blow up a 73V transorb and a bunch of other things in our display.
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On your way home from work!? Are you, too, an engineer that spends company time posting on Endless Sphere?
 
Here's a quick update:

Using one of the older DeWalt pack, I am able to power up the controller, snap in two new-style DeWalts, unsnap the original, and use the bike at full power on just the two new-style packs.

But if I simply turn off the main power switch for even a second and try to turn it back on, the BMSes shut down. So obviously the capacitors discharge quickly... possibly through the CycleAnalyst.

I think I will add a momentary switch next to my power switch that that hooks full power through a 100 ohm resistor. If I press that switch for a second and then turn on the main switch, things should be ok.

Comments?
 
ebinary said:
I think I will add a momentary switch next to my power switch that that hooks full power through a 100 ohm resistor. If I press that switch for a second and then turn on the main switch, things should be ok.
Comments?
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ebinary: That's the way to hook up your pre-charge resistor mentioned before, and from the sound of it, it should solve your problem.


wrobinson: I use united chemi-con KZE series caps that have low ESR, and are not too expensive either. I wish digikey had the KZH's from them too, but have never seen them. The VZ's have no ESR rating, so might not be the best for this type of application. From what I see in the datasheets, the KZE's can handle about 3 times more ripple current than a similar rated VZ, so I would guess the ESR is also about 3 times better for the KZE. The VZ's have no life span data over 1000 hours; the KZE's live over 5000 hours (the bigger ones at least). The price is fairly similar for 100V 1000uF/820uF caps of each brand. The only small hic is that the KZE's are not made in big formats nor with higner voltages - but for us I don't think this is a big problem.
Inexpensive low-ESR capacitors: United Chemi-con KZE's
 
ZapPat said:
ebinary said:
I think I will add a momentary switch next to my power switch that that hooks full power through a 100 ohm resistor. If I press that switch for a second and then turn on the main switch, things should be ok.
Comments?
Click to expand...

ebinary: That's the way to hook up your pre-charge resistor mentioned before, and from the sound of it, it should solve your problem.
Click to expand...

Thanks ZapPat and everyone else who helped me figure the right way to go to solve this.
 
Reporting success: adding a momentary charging switch through a 180ohm resistor did the trick (btw, a 480 ohm resistor did not work).

I can start the bike on a single new-style-bms dewalt battery pack now, by pressing the momentary switch until CycleAnalyst is booted up, then switch on the main power. I positioned the switches so I could do both with one hand.
 
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