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A 14500 Li-Ion battery has AA battery size and terminal layout and so it fits mechanically where an AA battery fits. AA battery has 1.5 volts nominal voltage and 14500 battery has 3.7 volts nominal voltage so most devices that use AA batteries will not work on 14500 batteries and will likely get damaged.

Some flashlights can use either regular AA alkaline batteries or Li-Ion 14500 rechargeable batteries - they identify the battery type electrically and just work. That's cool but it requires support in the device.

Surely it'd be much better to have a rechargeable battery with Li-Ion performance (very durable, low self-discharge, fast charging, high output currents) that fits into AA battery compartment. That would require adding a circuit into a 14500 battery casing that steps the voltage down such that it has 1.5 volts on terminals. Surely that circuitry would require some space so the "chemicals" part of the battery must be reduced. Other than that it looks like this is doable and may be implemented as a commercial product.

How practical would such adjustment be?

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Someone has done exactly this! Search Tenevolt AA Battery on Amazon.

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How practical would such adjustment be?

Using readily available circuitry: trivial?

Emphasis by me:

That would require adding a circuit into a 14500 battery casing that steps the voltage down such that it has 1.5 volts on terminals. Surely that circuitry would require some space so the "chemicals" part of the battery must be reduced. Other than that it looks like this is doable and may be implemented as a commercial product.

yep, you said it, you need a step-down voltage converter. There's literally thousands of ICs out there that do this job. You just throw in an inductivity (coil) and typically, one or two resistors and a smoothing output capacitor, and you're done.

All large semiconductor producers have such ICs in their portfolio – NXP, TI, Maxim, ST, Analog, Linear, ONsemi, infineon, Diodes inc.,… the list goes on forever.

Some companies even offer devices that integrate such an IC and the external components in one small module that you can directly integrate. So, I'm afraid, the room for margin might be rather small if you want to build something like that yourself. Here's how small Texas Instruments (TI) made their TPS82084:

TPS82084

You'd need to add 2 microscopically large (1mm x 0.5mm) resistors to set the output voltage to e.g. 1.5V, and a small SMD capacitor to stabilize it. Done.

The problem with integrating this into your battery will definitely be more one of the economy of scale and technology integration kind.

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  • \$\begingroup\$ What about standby mode - when the battery is not connected? Will the converter still run and drain the battery? \$\endgroup\$
    – sharptooth
    Nov 23, 2016 at 13:45
  • \$\begingroup\$ I can't explain the operation of a device in detail. You have the datasheet, which has a theory of operation section. Have fun! \$\endgroup\$ Nov 23, 2016 at 14:04
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    \$\begingroup\$ It says it has 17 micro-amperes power consumption when there's no load connected. Looks like it can run for five years continuously on a 750 mAh battery. \$\endgroup\$
    – sharptooth
    Nov 23, 2016 at 14:19

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