Here's a mock-up of Ron Brandt's dual-battery circuit (he got it from his friend Nikola Tesla) that he put into a small car, reporting that it greatly extended the range of the vehicle. You'll note it looks (and operates) a lot like the Dickson charge pump I detailed in a prior post.
My mock-up is set initially so the bulb (representing the load, whether it be an actual bulb or a motor) is set to take ~1 HP worth of power, but the slider allows it to go up to 270 KW, or approximately what a Tesla automobile would take at full throttle. The total battery voltage delivered to the load is approximately that which a Tesla uses, as well.
I've created a separate circuit beside the first, with the same resistance and load, to show the power draw from the batteries for a direct-connected circuit, versus the battery-switching circuit. The resistance in the second circuit is an approximation of the resistance in the first circuit. Once the light bulbs representing the load level out, you can see that they're both drawing identical power.
The direct-connected circuit draws 192.7 watts from each battery. The battery-switching circuit draws an average of 77.08 watts from each battery. Part of this is due to the fact that the battery-switching circuit has twice the number of batteries than the direct connected circuit, and accounting for this gives 154.16 watts if the battery switching circuit had the same number of batteries as the direct-connected circuit.
This leaves 38.54 watts per cell which is recharging the cells. This means that with double the number of batteries, the battery-switching circuit is sending half the power (38.54/77.08) back to the batteries, without impairing the resultant voltage at the load.
Here's a screenshot of the circuit:
And here's the falstad.com circuit emulator code:
$ 1 1.0000000000000001e-7 1.9265835257097934 11 5 50
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v -128 480 -128 384 0 0 40 96 0 0 0.5
d -128 352 -128 384 2 default
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159 64 192 64 224 0 0.001 10000000000
159 192 192 192 224 0 0.001 10000000000
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w 304 320 448 320 0
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d 576 320 576 352 2 default
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181 720 272 720 320 0 300 745.7 375 0.001 0.001
w 656 272 720 272 0
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r 656 400 720 400 0 1.182
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o 136 64 1 28931 0.0001 0.0001 2 1 0.0001
38 131 0 745.7 270000 Nominal\sPower
38 136 0 745.7 270000 Nominal\sPower