Here is a crude diagram of two methods to charge and discharge a supercap bank for a battery spot welder.

In fig 1, the supercap bank is between the mosfet‘s source and ground. The bank is always connected to ground and the mosfet controls the availability of charging power to it (via a CV/CC source). Since it’s always grounded it’s easy to measure the voltage across it for sensing. However, I believe the charging mosfet‘s gate would have to be at Vgs + the supercap bank voltage. In this method, is the discharging mechanism correct? Once charged would the caps slowly discharge?

In fig 2, the bank is between the power supply and the mosfet’s drain. This, I believe, is the way to connect a load when controlled by a N-mosfet. However, how will I sense the voltage across the supercap bank? When the bank is neither being charged or discharged I wouldn’t be able to see the voltage across it since the negative end of the bank is floating.

Any thoughts would be helpful and appreciated.

 

It would appear to me that you have not done enough research on the
discharge-capabilities of your Super-Caps, ( part-number and/or Spec-Sheet please ).

Throwing a direct-short across your Super-Caps is very likely to
damage them, or destroy them completely.
Also, a FET to Ground the second Electrode ???
The Current can get extremely high,
and not many FETs will be able to withstand the severe abuse.

A "DC"-Spot-Welder ???
Have You looked into how this may affect Electrode-Life ?

I would do just like everyone else and use a MOT, (Microwave-Oven-Transformer),
it's a great deal less expensive too.
If you're not prepared to do surgery on a MOT,
You can simply purchase a large, standard,
Toroidal-Power-Transformer, and wind your own Secondary on it.
It won't require "Charging" or "Discharging", or anything other than a simple Power-Switch.
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