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My collection of partially spent batteries has increased, and I would like to use a battery holder to build a power bank/supply which will provide power for a low voltage light (to be attached to my garage bicycle pump), but am confused as to what kind of voltage control I should best use to adjust the voltage.

From what I read on this site, It seems a potentiometer is not what it best, and that instead I need some sort of simple (cheap) voltage divider. The alkaline batteries would range in voltage from 1v. to 12v (23a: doorbell type), and the light would likely be 3v.

I have used a DC motor speed PWM to control the speed on a 12V DC motor, but I do not know if that is what I should use on this project.

I am aware that both the battery voltage and discharge rate would vary, but unless there is a real problem with that then I rather face that then see these batteries go to waste when they still have some some left in them.

So if someone can tell me the name of what I am looking for I would appreciate it. Thanks in advance.

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The higher-voltage batteries should go through a buck converter down to 3v.

The lower-voltage (1.5v) cells can utilize a Joule thief circuit to run a 3v LED directly.

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  • \$\begingroup\$ Thanks, you mean like <a href="ebay.com/itm/… ">this </a> And why would not that work for the the lower voltage bats if I ran them in series? You mean there is no basic voltage control that will do both? \$\endgroup\$
    – user263693
    Feb 19, 2018 at 20:24
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    \$\begingroup\$ You could use multiple cells in series (a battery is composed of multiple cells, only the 9v/12v are "batteries", fwiw), but the performance will be restricted by the most-dead cell. A Joule-thief is a well-known circuit that can be built very simply and will extract all the remaining power out of a cell. There's no reason the buck converter and joule thief outputs couldn't connect to the same LED. \$\endgroup\$ Feb 19, 2018 at 20:33
  • \$\begingroup\$ The <a href="en.wikipedia.org/wiki/Joule_thief">Joule thief - Wikipedia</a> is very interesting. "Clive originally named the circuit "Vampire Torch", because it sucked the last remnants of life from a battery." \$\endgroup\$
    – user263693
    Feb 19, 2018 at 23:58
  • \$\begingroup\$ I originally though I would just stack cells together and control the voltage, but I <a href="forum.allaboutcircuits.com/threads/… ">read </a>, Stacking a pile of worn out cells to get the original voltage won't work - the cells will all have varying amounts of internal resistance, the one with the highest resistance will be reverse 'charged' - that is a reverse voltage will be impressed across its terminals by other more lively cells. That can cancel out the voltage supplied by some other cells that are actually doing something. \$\endgroup\$
    – user263693
    Feb 20, 2018 at 0:05
  • \$\begingroup\$ I am not sure how to make this work for multiple cells, but thanks for pointing me in the right direction. There is an instructable here: instructables.com/id/Make-a-Joule-Thief \$\endgroup\$
    – user263693
    Feb 20, 2018 at 0:22

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