Battery powered DRSSTC

[TMaxElectronics]

I just rewatched the video of Steves tesla gun and was thinking about making my own battery operated coil. The issue however is finding a cheap but safe source of decent LiPos or LIIon cells. Has anybody here done something like this before and, if so, which batteries did you use / how much power did you get out of them?

I kind of had the idea of using a whole bunch of 18V makita packs, since they have all the protection needed built in and can deliver quite a bit of power. I can't seem to find any official spec for max. current or anything like that though. My calculations are as follows:

Number of packs I could get: 3 (mine) + 1 (from Netzpfuscher) + 4 new, costing 150€ (quite a bit but I could of course use them for other stuff too) => 8 total
Current (best estimate I found): 15A peak per cell, 2 cells in parallel = 30A per pack
Voltage: 8 x 5 series cells = 40 cells => 148V
Peak power: 148V @ 30A = 4,44kW
Runtime (at peak power, so probably longer): 3ah per pack, 30A discharge => 1/10th of an hour => 6min

With those figures I'd probably go for a 30cm-ish coil, maybe a bit less.
As for IGBTs, I recently got an entire 400a VFD output stage on ebay for 5$ that had 6 skm300 bricks in it which I'd probably use. Not sure if they will be fast enough for such a small coil though (or if I could get FRes down far enough without making a bazillion turns). Any guesses about maximum freq?

I also guess I'll need a boost converter since 120V (worst case) on the bus is a little boring. If I go for 350V I don't have to but caps in series, so that is probably what I'll go for (plus, it's the rectified single phase voltage ).

I think I'll try to make this a build log post if I remember to take enough pictures, but no guarantees

[Mads Barnkob]

Looking forward to the results of this project

SKM300 maximum frequency, without doing any math, is properly closer to 100 than 200 kHz, which will properly result in you redesigning your secondary or go with smaller and faster IGBTs.

Did you consider a high voltage battery pack that would go directly on the DC bus? Like LOD did with electric vehicle packs for some 800VDC in his travel trailer compact cone design DRSSTC? https://www.flickr.com/photos/bellmine/sets/72157706088757021/

[Weston]

I built a small battery powered QCW coil last year, but I never really finished the integration of it into an enclosure.

I found a chainsaw that had been discontinued that had a 120V battery pack and picked up a few for cheap. The packs had 20A rated powertool cells, so it was a pretty good deal. https://redbackpowerusa.com/product/electric-chainsaw/

I never ran the coil at continuous high power, but technically I could have pulled ~ 2.4kW from the battery. I used a boost converter to go from the 120V battery to a ~500V DC bus.

If you find a teardown of a battery pack you should be able to find the model number of the cell and look up the maximum current rating. Although the BMS might have some additional limit.

I believe Steve Wards coil used lithium polymer hobby/RC car packs and a minimal (none aside from under-voltage lock out and  a fuse?) BMS. For something running at such a short duration its probably not worth worrying about having a full BMS or anything.


Also, just a heads up, you are going to get significantly less energy out of the cells at a 10C discharge, so the true runtime values are going to be even worse than what you calculated.

[TMaxElectronics]

Did you consider a high voltage battery pack that would go directly on the DC bus?

I thought about it, but that would have to be some pretty custom thing that I probably wouldn't be able to use too well for other stuff. The benefit with using power tool batteries (of the power tools I regularly use) is of course that I'd be able to just pull them from the coil and use them the way that they were initially intended. And if I dump a bunch of money into batteries I'd rather use ones that didn't go to waste afterwards

I found a chainsaw that had been discontinued that had a 120V battery pack and picked up a few for cheap.

Interesting, didn't know anybody made powertool batteries with this high of a voltage. But unless I find them very cheap those aren't too useful for me either I think. Simply because I don't have the tools that it would be used with

I believe Steve Wards coil used lithium polymer hobby/RC car packs and a minimal (none aside from under-voltage lock out and  a fuse?) BMS. For something running at such a short duration its probably not worth worrying about having a full BMS or anything.

Those were my initial plan too, especially since hobbyking had an offer on 10c 4ah battery packs for 30€ a piece. But my issues with those are the same as with any other battery I wouldn't use otherwise, and also the fact that they tend to be a lot more sensitive than LiIon.
I had quite a few so far and none of them lasted me longer that a year without even using the damn things much, and nowhere near their maximum ratings (and storing them at 50% charge). Some of them even turned into spicy pillows all on their own. So if I put them into a huge big bank of cells I feel like I'm basically building a bomb (hello FBI, nothing to see here. just a joke ).

With those tool batteries all of the protection would be handled internally. All I need to do is to prevent reverse voltage when the mosfets open (which is just an inverse parallel diode) and to sense once a pack turned off (for which an opto isolator per pack and a resistor would suffice).

I think I'll do some high load tests with my old homemade electronic load, and see how far beyond 30A I can go before the OCD of the BMS kicks in. Or before my load goes up in smoke