Storage of lead-acid batteries

Battery University is a good starting point. A vast amount here Battery University

Particularly:

Charging Lead Acid

Based on the following, it would SOUND wise to charge fully for "a while" under trickle and then to open circuit the charger until Vbattery fell to approaching 2.1V/cell. A controller to implement this could be extremely simple.

Dangerous in isolation, but, they say:

• Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.

  Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.

  Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation.

  Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.

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:

1. Install a second set of batteries in an independent secured and ventilated compartment.

2. Wire the negative of these batteries to the same grounding point as the original batteries

3. 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 – Schematic created using CircuitLab}