Any recommendations? The brand one seems way overpriced and I hear they burn out a lot.

This past year I bought five of the cheapest ones I could find that came with the nylon case and Velcro strap that also had 1/8 inch mounting options. They are total no name brand models from amazon and I haven't broke one yet after dozens of uses. I cant link those as they are off thew market but I see no reason why this wouldn't do the same thing just as well.

https://smile.amazon.com/Altura-Photo-R … &psc=1

Godox makes a good unit, the BP960, which if you're scared off by the lack of warranty and local service, you can find rebranded by Cheetahstand, Adorama, B&H. You just need the battery pack and the correct cable for your flash. Links:

http://flashhavoc.com/godox-pb960-lithi … ck-review/

https://www.amazon.com/Propac-Battery-O … B00D06LUAA

https://www.amazon.com/Godox-PB820-PB96 … 00DGLTGBE/

http://www.adorama.com/fpppbp960.html

There is a removable cover to access the external power port

Manual --   Page  F-12    External power source
http://cdn-10.nikon-cdn.com/pdf/manuals … 900_en.pdf

OEM
https://www.amazon.ca/Nikon-SD-9-Perfor … B001CADR0W
http://www.bhphotovideo.com/c/product/3 … _Cord.html


aftermarket accessories
http://www.ebay.com/itm/JJC-External-Fl … 2478994651
http://www.ebay.com/itm/JJC-CABLE-BPNK1 … 2113840416
http://www.ebay.com/itm/JJC-BP-CA1-Exte … 2097808088


With the connector and specifications other power options are possible .  Simple 6v (5 to 7) volts is likely but it is possible the batteries have a center voltage tap connection -- you would need to know what the Nikon packs deliver.

I have seen power tool battery packs adapted and used to run cameras, etc.  Nothing magic.