I've restored seemingly dead li ion power tool batteries. You'll need a good battery of the same voltage (or marginally higher, e.g. bad battery 12v, good battery 12, 14, or 18v). Connect a jumper to both negative terminals. Connect a jumper to the positive terminal on the good battery ONLY. With the other end of the jumper quickly sweep the positive terminal on the bad battery two or three times. Put on charger and see if it charges. If not, throw it away. Of course it's a good idea to wear safety glasses and gloves when you do this.