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If I were to double the exact same battery of a particular device (noted below), could/would that effectively ~double the device's usage time (more or less)?

For reference --

There's a motorized wheelchair I'd like to increase usage time of before needing to recharge. I'm curious if doubling the exact same batteries in parallel could be an option for roughly doubling the wheelchair's use time -- perhaps enabled by custom circuitry for cell/charge balancing/monitoring.

The wheelchair uses lead-acid 12-volt/70 Amp-hour batteries (sealed).

If doubling batteries is not feasible (particularly in this case), how might one go about increasing the usage time of a device, without neutering/trimming already present functions/features of such device?

(Note -- I'm asking this question in the feasibility of such approach, and not in a "I'm about to do this willy-nilly and I'm waiting for either thumbs-up or thumbs-downs". I understand that, if feasible, such a route would likely require more than simply "stacking" batteries in parallel.)

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  • \$\begingroup\$ In theory, wiring the batteries in parallel would double your usage time by doubling your available current. However, there could be potential caveats to attempting a sort of thing. As PeterJ mentioned, the battery type is important, try reading up and learning about the type. \$\endgroup\$ – Nick Williams Jun 30 '13 at 3:16
  • \$\begingroup\$ @NickWilliams Understood, thanks. And I'm asking this in general principle, and not in the sense that I'm looking for quick approval on whether to do it or not. If it's a feasible possibility, I understand it's a route that would take much further research, and particularly into device specifics (wheelchair, batteries, recharger, etc.). \$\endgroup\$ – Coldblackice Jul 1 '13 at 19:28
  • \$\begingroup\$ @PeterJ Good point -- question updated, battery type noted (12-volt/70 Amp-hour lead-acid batteries). \$\endgroup\$ – Coldblackice Jul 1 '13 at 19:37
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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).

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ +1 This is an excellent idea because it allows the existing charging system to be used with no worry about its compatibility for charging two batteries at once. \$\endgroup\$ – Michael Karas Jun 30 '13 at 5:21
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    \$\begingroup\$ +1 about doubling battery adds weight and reduces lifetime \$\endgroup\$ – Andy aka Jun 30 '13 at 10:38
  • \$\begingroup\$ @MichaelKaras Thanks for the answer. So would it not be feasible to use the existing charging system on two parallel batteries? My (very limited) understanding was that most charging systems today are "smart"-chargers, and recognize where the batteries are in the recharge process, adjust recharge accordingly. \$\endgroup\$ – Coldblackice Jul 1 '13 at 3:43
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    \$\begingroup\$ @Coldblackice - Like I said before - Not nearly enough information available to make any sort of valid effort at providing guidance. \$\endgroup\$ – Michael Karas Jul 1 '13 at 19:43
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    \$\begingroup\$ @ColdBlackIce -- You can connect the two batteries in parallel and use an ammeter to detect any current flow between them (there should not be any). Alternatively, you can compare the open-circuit voltage to the voltage with the two batteries paralleled. It should be the same. As long as the batteries are roughly the same (capacity, age, chemistry, state-of-charge), cross-charging isn't a problem. A small difference between them will result in one battery briefly charging the other until the two batteries equalize at the same voltage. The real consequence is reduced lifespan. \$\endgroup\$ – DrFriedParts Jul 4 '13 at 4:45

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