How-To Geek

Maximize Battery Life with a Joule Thief

When your batteries are too weak to power your digital camera (or flash/game device/etc.) that doesn’t mean they’re totally dead. Make a Joule Thief to use every last bit of stored energy.

A Joule Thief is a simple circuit designed to draw the last bit of power out of a battery using a low load device like a low-voltage LED. Instructables user 1Up got the idea from Make magazine and then tinkered with and refined the idea. His design doesn’t include an enclosure but it’s a great start to a larger project–perhaps a “dead” battery powered nightlight?

Make a Joule Thief [Instructables]

Jason Fitzpatrick is a warranty-voiding DIYer who spends his days cracking opening cases and wrestling with code so you don't have to. If it can be modded, optimized, repurposed, or torn apart for fun he's interested (and probably already at the workbench taking it apart). You can follow him on if you'd like.

  • Published 06/2/11

Comments (14)

  1. Hatryst

    Nice way to torture a semi-dead battery :D
    Or perhaps, rising it from the dead :)

  2. Kevin


  3. Seasider2

    Use rechargables so never have dead batteries. Environmentally friendly too. Well a techie has to try a little bit.

  4. Yoshiyah

    Even rechargable batteries eventually run out of juice.

  5. ADWheeler Photography


    Devices that use batteries rarely outlive the typical life of a rechargeable battery. The results is much less waste even though the waste is just as dangerous to the environment.

  6. Mikkel

    Well… the real purpose for a joule thief has always been… at least in my eyes,
    to power components that otherwise would require a higher voltage.
    In this example… running a blue/white LED of an 1.5 volt battery.

  7. DWN

    Given the battery voltage curve is basically dwindling from 1.5-1.4ish until it basically drops off rapidly, I imagine this depletes the battery to death? I have a suspicion this probably isn’t good for a rechargeable battery – fully depleting a battery (well into the depleted zone of the battery) is considered dangerous for larger batteries when doing a recharge. Just a disclaimer of sorts for people who might be thinking of using this on rechargeable batteries; it’s probably worth reading more about if you’re thinking of doing it. :3

    @ADWheeler & Yoshiyah
    Not necessarily – it often depends on the type of rechargeable battery as well.

    For example, a typical alkaline rechargeable battery can be crippled as early as 2-3 charges into it, whereas NiCd batteries have a tendency to last upwards of the 100-1000 charge mark, and NiMH batteries in the 1000-10,000 recharge region (at least, that was the propaganda being pushed a couple years back).

  8. James

    I use a battery charger, that recharges standard Alkaline and rechargeable Nickel Cadium batteries. If you catch an Alkaline when it stills has about 30-40% of its charge left you can recharge it close to its original charge. I also have a battery checker that tests Button Cells up through D Cells of any type. I have found that an item requiring more than one battery does not always drain them all at the same rate, and only some of the batteries may need to be replaced.

  9. Vaidya

    James is right. I always check the cells when a device stops working. Normally, for a two cell requiring device, one cell is okay to be used for some other devices, requiring less power, like Wall Clocks. Luckily, I have one each room. And incidentally let me promise you, all of them show the same time.

  10. GLD

    If you’re bored enough to tackle this, at least you can get some satisfaction from the fact that you might be getting closer to your money’s worth out of a normally dead battery!! At any rate, have fun!!

  11. gabby Jim

    This might be just the thing for rechargeable Ni-CD cells (like in your digital camera, etc.). Just use a battery holder, so the individual cells can be easily swapped in and out.

    Contrary to popular practice, and the bogus messages that abound, Ni-Cd batteries SHOULD be INDIVIDUALLY completely drained before recharge. However, the problem is, quite often, they come in sealed “batteries” where the individual cell cannot be accessed – end game. Premature failure and otherwise un-necessary replacemnt is a vendor’s delight. Problem is, manufacturers often discontinue the device and battery pack to force you to buy the “New and Improved” model.

    The consequence of NOT discharging the cells is four-fold. First, a weaker cell will be reverse charged by stronger cells in a pack as the device continues to be used. Second, Ni-CD cells “remember” how much they are used – or not – and, like the human muscle, they tend to atrophy if not deep cycled often enough. Third, if the charge-discharge rate is incorrect (and it usually is, as most folks leave the battery on the charger long after full charge is reached – and few chargers have automatic shut-off features), crystals form between the “plates” and the cells become short circuited internally. Sometimes a high voltage “zap” charge (on the order of 12 volts from a car battery – no current limiter in line – for 40 to 50 MS for a 1.2 volt cell) will break down the bridge – like blowing a fuse – and restore the cell for a while. However, the residual material now provides a narrower gap for the bridge to re-form. Finally, a cell that has an internal short causes excessive drain on other cells in a battery pack, leading to more cell failures.

    Although I have professional training and moderate personal experience in this area, a decent “Google” search will confirm the above from many experts – I suggest the NASA study as one of the best articles.

  12. pyro226

    @gabby Jim
    The batteries in digital devices are normally either NiMH or Lithiums. NiMH are good to fully drain once in a while. Lithiums it’s not needed (lithiums just need to be used fully once every 6 months in order to prevent damage, charge before long term storage).

    In drills and hardware, NiCd are often used. Those may need to be fully discharged more often, but wikipedia states that there are varying opinions about NiCd memory.

  13. MFE

    I’ve been re-charging alkaline batteries for about 40 years – not the same ones, of course. In the early days they were rare and expensive, so if I could get them to last a bit longer, that was a bonus. The only rechargable batteries in those days were NiCd which had a lower terminal voltage. NiMH and Lithium hadn’t been invented
    To get the best results in charging the alkaline batteries, I used the un-smoothed rectified output from a bridge rectifier. The pulsed input seemed to work better than having a smoothed DC.
    As always, don’t use too high a current and don’t overheat the batteries.

  14. Grant Johnson

    Has anyone coupled a Joule Thief with a 7805 regulator to make a one battery Minty Boost that drains every last bit from the battery?

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