Battery configuration

I have a system that uses 4 Lion bats. Three of the batteries are used constantly (12.6v) the 4th bat does nothing, most of the time. Occasionally I need to add the voltage from the 4th battery to boost the output to (16.8v).
The way I plan to accomplish this is to have all of the bats connected all of the time, but only using the 4th bat when I press a moment switch between the 4th bat’s + & the 3rd bat’s – obtaining the 16.8v I briefly need-a power surge.
When I tried it nothing happened. I did not get the 16.8v. Does anyone out there know why I did not get 16.8v and, in baby terms, how to get the 16.8v. My hope is to set this up as simply as possible. I would very much appreciate help. Thanks a bunch IA.
Cgc210

Can you sketch how you had them connected? Did you use a diode anywhere? A DPDT switch? Something else? What is your current draw at each voltage?

With the information you provide, no one can guess.

John

are you using a SPDT Push Button Momentary Switch? with the centre connected the the load, and one contact to the 3 batteries, and the third connected to 'top' of the batteries. Assuming all 4 batteries are in series.