Spot welder

[Newbgene1591]

Hello all, 

I’m fairly new to electronics and I know the basics but I’ve always been intrigued. Well I’ve been wanting to build my own spot welder for lithium batteries to fix my power tool batteries. I plan on using the transformer out of a microwave, now the part I’m confused about is the relay timer. How can I go about doing that and wiring it up? Also could I use the board from a microwave being it has the relay and the timer already built in? 
another thing I’m curious about is I have an old Tenma soldering station which inside has 2 transformers which are small. Is there anyway I could use it and just reverse certain things? 
thanks