Power Tool 120V lithium-ion battery packs

[Weston]

I am working on a portable tesla coil (which I will hopefully be finishing and posting in the next month or so) that runs a boost converter to ~550V. Power draw is relatively modest at ~ 500W. I was originally planning on using a bunch of hobby li-poly battery packs in series to supply ~100V to the boost converter.

As the project got closer to completion I discovered that 120V lithium-ion battery packs for power tools exist and are a pretty good deal. None of the larger companies use 120V battery packs, but Redback makes a 120V line of power tools. The chainsaw kit, which includes a chainsaw, 120V 2Ah battery, and charger, is $200 new and someone is selling them for $100 as unbranded refurbs on ebay. I recently bought one of the refurbs to check it out and use if for my project. $100 seems like a great deal for a 120V ~240Wh battery and charger. I figured this information could be useful for other peoples projects. This find influenced me to try to fit my tesla coil built into the chainsaw, with the coil replacing the blade 

As a summary of the teardown, the battery pack contains 30 high quality Samsung INR18650-20R cells ( https://lygte-info.dk/review/batteries2012/Samsung%20INR18650-20R%202000mAh%20(Green)%20UK.html ), which are designed for powertools and have a 2Ah capacity and a 22A max discharge current. The battery pack contains some sort of BMS that I have not identified. The only terminals broken out on the pack are the power terminals, a 12V tap which is only used by the charger (if anyone has a clue what this is used for that would be great), a single thermoresistor, and a pin labeled "ID" which measures as 10K ohms. The company sells a higher capacity battery pack so I suspect this pin is just a 10K resistor and used to identify the pack to the device, I had difficult tracing it on the PCB without having to disconnect cells.

The refurbed chainsaw I bought arrived in barely used condition, except for a plastic grill which was crushed. The battery measured at 1.92Ah from a full charge to a cutout voltage of 3V per cell at a 1A discharge current. I have a second chainsaw kit arriving next week as it costs less than a single battery.

For those interested, here is the website listing for the new chainsaw https://redbackpowerusa.com/product/electric-chainsaw/ and here is the ebay listing for the refurbed chainsaw https://www.ebay.com/itm/Refurbished-UNBRANDED-120-Volt-Cordless-18-Inch-Chain-Saw-Kit/293255475559 .

18650 cells are ~ $3+ each in small quantities, so $100 for a 30 cell battery pack, chainsaw, and charger, seems like a great deal. My only complaint is that the battery pack seems to use a fuse instead of an electric cutout, but that is minor. I hope others can find these useful in their projects. Let me know if you need any more info. Here are some photos:

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