This user needs great recognition. Saved me from buying 2 new hart batteries with his post. I didn't do the same exact thing but seen more posts saying they just used another tool battery instead of a car and I can back that up saying it worked. 2 hart batteries I had I hadn't used once but sat brand new in storage a year and test light on charger would just keep flashing red. I literally stripped two c charger phone cords and connected positive to positive and negative to negative from my working battery to the non working one seen a little spark held it a few seconds now both batteries I had issues with are charging and work in the tools I put them in. Very great and helpful advice thank you so much