I took the leads of my Harbor Freight spot welder and spliced some heavy gauge copper wire to them with spliced heavy gauge copper points. When I go to make a battery spot weld using nickel battery strip it just kind of arcs across the leads as opposed to spot welding the nickel onto the battery?

 

Welcome to AAC.

That should work, except for one problem. In fact, that effectively is how I made my first tab welder. It was based on a microwave oven transformer (MOT) welder I had built.

The big problem is controlling the welding time to avoid too much heat. I used a Variac to control the MOT voltage and a 555-based timer connected to a solid state relay to control time. I could get that time to about 16 ms (one cycle @ 60 Hz), but even that was too long. The strips got red hot. I didn't pursue that path any longer. One might be able to make it work.

I switched to a capacitor discharge design, and that works well.

 

When I go to make a battery spot weld using nickel battery strip it just kind of arcs across the leads as opposed to spot welding the nickel onto the battery?

To give another point to what jpanhalt said, are you putting enough pressure on the electrodes? Spot welding isn't like other types of electric welding where you need a small gap between electrodes and the work. Spot welding needs pressure between the work and electrode to keep the "spot" small and in direct contact with the work.

 

Welcome to AAC.

That should work, except for one problem. In fact, that effectively is how I made my first tab welder. It was based on a microwave oven transformer (MOT) welder I had built.

The big problem is controlling the welding time to avoid too much heat. I used a Variac to control the MOT voltage and a 555-based timer connected to a solid state relay to control time. I could get that time to about 16 ms (one cycle @ 60 Hz), but even that was too long. The strips got red hot. I didn't pursue that path any longer. One might be able to make it work.

I switched to a capacitor discharge design, and that works well.

Thanks, I originally built a spot welder using the solid state relay and 12v Battery and such, saw it on YouTube and it worked okay but came across the Harbor Freight spot welder for cheap and wanted to convert it. I wanted to know if maybe converting it was not do-able and wrong but it seems like the internal timing is just not set up to do batteries.

 

Here's the schematic for one of the HF spot welders:


That is not much different that using a rewound MOT. If it works OK for you, fine. There is also a very lengthy thread here that started out using simply lithium ion batteries: https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857

1) You need high current for a short time. Open circuit voltage can be fairly low. I get best welds at about 17V.
2) Because of the nickel strips are very thin, any arcing at the contact points risks burning a hole in the strip or the battery case itself. You need good contact and pressure with the electrodes. I forgot to mention that and thank @shortbus for pointing it out.

I wanted to know if maybe converting it was not do-able and wrong but it seems like the internal timing is just not set up to do batteries.

I believe that's correct, but have no idea how the timing is done. If you try to base it on appearance and manual on/off control, it will not be very repeatable.