Flashlight Batteries From Laptop Battery





Introduction: Flashlight Batteries From Laptop Battery

Use the cells from an old laptop battery to power a flashlight

Step 1: The Flashlight Battery

Step 2: The Old Laptop Battery

The old laptop battery. Open the plastic cover and remove the cells

Step 3: Cut the Cells

Cut the cells with wire-cutter

Step 4: Voltage Testing

Test the voltage of the cells and use them from the laptop battery to the flashlight. It works very well for me!



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    21 Discussions

    Hi, Thank your for sharing this Instructable, the batteries you desire are 18650 Lithium Ion cells. Their Voltage is supposed to be 3.7 V with a capacity of ~ 3500 mAh.

    Please keep in mind, that these cells ( as long as they where recycled from another device ) are unprotected, that means that in a short circuit case, they will discharge with a huge energy amount. That can cause an explosion, please take care of yourself.


    11 replies

    The protection discussed is a battery over voltage and under voltage circuit. Example the 18650 batteries get their name from the diameter and length of the cell. The Battery is fully charged at 4.2VDC and fully discharged at around 3.2VDC this is the safe operating range for lithium power cells. LiPo, LiFe, Li-Ion.

    Most devices that use this type of cell has a protection circuit built into either the larger battery pack or the charger. Laptops this is in the pack. R/C stuff the circuits are usually in the charger/balance charger (over voltage) and Electronic speed controller (under voltage).

    There are individual cells that are protected that are used in things such as Vape power supplies and flashlight/torches and they can be Identified as they are longer then 650MM due to the protection circuit disk being sandwiched on before the shrink wrap. Those however are not the types in laptop batteries.

    That circuit does 2 things, Limits the charge going in so that the battery never goes over 4.2v (over volt) and it prevents it from going below 3.0V (under volt) As like others have said a Lithium battery that goes above or below can have negative results (POOOOOF FLAME ON)

    how to protect your batteries? rub some lithium grease all over your body and pray to the heavens while counting backwards 7 times. some say this doesn't work, but I say try it and post your results back here.

    No, a fuse does nothing to protect from the two most common problems which are overcharge and over-discharge.

    You're looking for a Battery Management Circuit AKA "BMS". They do have them on a per-cell basis, where one circuit monitors one cell, but more commonly, you do a pack manager which ensures that the entire pack is properly charged/discharged and no voltages get too high or low.

    Search for "Battery management Lithium Ion" and it should come up with some examples. Finding an actual vendor may be difficult though.

    it would be nice to find a replacement for c123a batteries i tried this and noticed that most laptop batteries are a little too big for my surefire flashlight

    It's not just a fuse... lithium cells become dangerous if overly discharged too.
    The accepted normal low limit is 3.2v per cell.

    you will also need a li-po / li-ion charger too.

    That said... I use these in my torches too... normally with a plastic tube around to keep them central.

    I also use the AA sized lithium cells in stuff to.

    3 replies

    Careful with mixing up the "AA" sized lithiums with "Regular" batteries as they literally have twice the voltage of alkaline batteries.

    1) Not accurate. There are lithium primary cells that are the same size a AA and are directly compatible except higher capacity, but still 1.5V.

    2) Lithium rechargeable voltage depends on the chemistry but the most common 14500 (AA sized) have a nominal voltage of 3.6 to 3.7V which is over twice that of alkaline cells, and due to the discharge curve, are normally a substitute for 3 x alkaline cells.

    However there is a danger in substitution because practically no device designed to run off alkaline, or even NiMH or NiCd for that matter, have a necessary low voltage cutoff circuit so the cell itself should have that built in.

    fully charged li-ion cells are 4.1v so actually nearer 3 the voltage - definatly more than 3x the voltage of 3x ni-mh cells (3.6v)

    I tend to use them in place of the 3x aaa holders.

    I had discarded my old laptop batteries last week. It’s
    damn good eco-friendly idea to re-use old batteries. I just wanted to confirm
    if it’ll work when laptop batteries are no better enough to run the device
    itself, or else, why would I discard them.

    2 replies

    The requirements between a laptop and a flashlight are different. So the old laptop batteries can be perfectly used as flashlight batteries

    No, not really. If the capacity of a particular cell is too low to support good laptop runtime, that capacity doesn't magically go up when used in a flashlight. In that case, to achieve good runtime and brightness, it would be highly desirable to just use a new battery instead. Can you use one still and accept short runtime? Sure, if it's a toy. In real life it's better not to piddle with such things and have the one light you actually need and use, to work as well as possible. Backup lights... you rarely ever need them if your primary light works as expected.