Could I double the batteries for double the usage time?
Basically Yes...

With large batteries, you have large energy and therefore SAFETY concerns (you might die).

Basic Theory

A battery is an energy store. Two (identical) batteries store twice as much energy.

However, a battery adds weight (most of a battery's weight (>99%) is inert mass -- electrons don't weigh much). So your improved range will be much less than 200%. In my experience with robots, range is typically about 160% for 200% battery (Ni-Cd).

Operations

How you install, manage, charge, and configure your batteries is application specific. 

In the case of your wheel chair, I would do the following:


Install a second set of batteries in an independent secured and ventilated compartment.
Wire the negative of these batteries to the same grounding point as the original batteries
Install a high-current rated, rotary, break-before-make, SPDT switch between the two battery banks


You can now activate this switch to select between the two batteries. This will allow the original safety and protection circuits to continue to work as designed.

This approach is basically just making the battery swap-out process really fast (sort of like carrying extra gas in a gas-can in the trunk (boot).









simulate this circuit &ndash; Schematic created using CircuitLab