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I have grip warmers on a bike. basically just heat tape. I would like to reduce the supply voltage from lithium battery from 18 vdc to 12 vdc. My goal is to reduce warmer element temperature and increase battery life. What is the simplest way to do this? thanks!

Adding a resistor half the resistance of the elements will get you down to 12V, and increase battery life by 50%, but you end up wasting a third of the energy in the battery (unless you use that resistor to keep something else warm). There are cheap DC-DC buck converter modules available, that will regulate down to 12V, that then gets your battery life to more than double - power at the element falls by 56%, power consumed at the battery falls by that less the efficiency penalty of the converter.

A resistor dropping 18 down to 12V wastes $1over3$ of the energy.

A DC-DC buck converter can be about 90 percent efficient, but the cheap ones typically get about 85% - wasting about $ 1over 6$, but PWM can improve on that by removing the inductor and capacitor losses inherent in DC-DC converters.

Look for a PWM dimmer module that meets your requirements or build one from one of the many designs available on-line.

for heating applications a lower modulation speed can be used than is used for lighting, but lighting modules are more common.

If you don't need that last fraction of the available energy you can just go with a cheap DC-DC converter, the cheap XL4015 modules run well at up-to 2A and the good ones will go higher.

Adding a resistor at the input will lower the current and the wattage this should help make the battery last longer but the heaters will not get as hot