Brake pedal went to floor when I put key fob in. (key fob is dead; been dead for years) Problem???

'08 43615 miles

 

And, how would I do that?

 

Looked on line for a digital multimeter, there are Many. Have no idea how to find "jump point in the fuse box." I'm about 40 minutes from the dealer. I have changed the small battery twice over the years. Perhaps I can check charge of all batteries and change?

 

I have a slow charge battery charger which I've used on the other 2 batteries. They recharged rather fast so I wonder??? Perhaps I can recharge (am now).
Can I check the battery that is in the car with the recharger without disconnecting? Can I recharge that one without disconnecting?
Really Appreciate your response and patience.

 

Have already recharge older disconnected battery.

The only way I can check battery voltage is with the connected (to electric) battery charger.

 

I have the charger connected to car battery/ plugged in. Charger indicates it is charging. Is this connected ok on top of what is there or do I need to disconnect what is there connected to battery?

We are getting rain/freezing rain tonight into tomorrow so Have to disconnect over night.