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Thread: How to calculate battery's sustained current

  1. #1
    Flashaholic
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    Default How to calculate battery's sustained current

    How do I calculate or determine if a battery can sustain current amps of 3.5 Amps?

    What words do I look for when looking at battery specs or what math is needed?

    According to google the formula is Watts / Volts = Amps.
    But I'm not sure that applies in this instance because the head is made for a 1x16650 2500mAh.

    Thank you

  2. #2

    Default Re: How to calculate battery's sustained current

    Cannot determine from vendor claims.

    Need to measure using accurate testing gear

    or accept reco's from trusted members.

    Good analysis of the testing process here

    https://www.rcgroups.com/forums/show...91&postcount=1

  3. #3

    Default Re: How to calculate battery's sustained current

    3.5 Amps for protected 16650 is too high and can't do the job, unless battery is not protected or your device has some kind of current control or slow current ramp up.

  4. #4
    Flashaholic*
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    Default Re: How to calculate battery's sustained current

    ^^^ignore that crap.

    The spec you're looking for is "maximum continuous discharge", which will often be expressed straightforwardly in Amps, but may also have a "C-rate"(1C = 1hr to discharge the battery's rated capacity).

    "Made for 2500mAh 16650" spells Keeppower, and that cell is going to say "2C" for it's discharge rate.
    2C means half an hour to discharge the rated 2500mAh, so 5A.
    From what I recall of seeing a test of that cell, it can get close to 8A before the protection circuit trips.

  5. #5
    Flashaholic
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    Default Re: How to calculate battery's sustained current

    Thank you, all.

    now I can connect the dots and see how the C-rate and mAh compute together to make a max continuous discharge.

    the particular head with the battery warning is the Malkoff E2XTD

  6. #6
    Flashaholic* DIWdiver's Avatar
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    Default Re: How to calculate battery's sustained current

    I doubt you are limited to Keeppower, but the rest of what Owen says is correct.

    You want a cell rated for 3.5A (or higher) continuous discharge. For a protected 16650 this is high, but not unreasonable. If a cell doesn't have a max discharge rating, you don't want to use it in high discharge applications.

    The maximum discharge is a current. A funny thing about rechargeable batteries is that there are two ways of talking about current. There's amps and milliamps (A and mA) which every field of electrics and electronics recognize. But in rechargable batteries we also have Capacity (C). The simplest way to think about C is to take the hours off of the capacity rating. So for a 10 amp-hour battery, C would be 10 amps. For a 350 mA-H battery, C would be 350 mA. 0.5C would be half the C value, 10C would be ten times the C value.

    So if you had a 2000 mA-H rated cell with a max discharge of 1C, that would be 2000 mA or 2.0A. This cell would not be suitable for the 3.5A application. A 1500 mA-H cell with 3C discharge would be 1500*3 = 4500 mA or 4.5A rated, and thus good for this application.

    You also want a quality, reliable cell. Bargain cells typically fail to meet claimed performance measurements, among which would be safe discharge rates.

    EDIT: It appears you already figured this out. Maybe this will help some who haven't.

  7. #7
    *Flashaholic* idleprocess's Avatar
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    Default Re: How to calculate battery's sustained current

    It's been my limited experience that Li-* chemistry manufacturers will state a "maximum continuous discharge" or similar, and some will specify a "maximum peak" or "maximum pulse" discharge (the duration of which is hopefully defined) - both expressed in unambiguous mA or A figures. Note that your realized capacity will tend to decrease the harder the cell is discharged - drastically in some cases - thus it pays to note the test conditions for determining the cell's faceplace Ah and plan accordingly.
    I apologize that this letter is so long; I did not have time to write a short letter

  8. #8

    Default Re: How to calculate battery's sustained current

    And it is very rare for the nameplate to be honest, if within 10-15% of actual that's pretty good.

    With very high quality LFP prismatics it's often the reverse, rated for 180Ah may actually test out at 185-190Ah

    but different from this market niche

  9. #9
    Flashaholic
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    Default Re: How to calculate battery's sustained current

    Thank you. This is all great stuff. I'm trying to learn more about batteries.
    Right now I just trust the sellers that I buy the lights from and order whatever batteries they sell.
    But eventually I'd like to maximize output by trying out different batteries, which is tough to do when one doesn't have a good battery understanding.

  10. #10

    Default Re: How to calculate battery's sustained current

    KD, when I lurked at a vaping forum lots of members there would mention this fellow named Mooch. Mooch this, Mooch that, when it came to making sure that super vape modular box (called mod by the cool cats) would not explode. Some of those things draw way more amps than a flashlight will. It was mostly unprotected cells they were using with claims of being able to draw 40 amps for a few seconds.
    Well I wasn't into all that but I did want to know some stuff like if the brand I saw on Amazon was any good or how many amps brand Q could really handle so I looked up some Mooch battery reviews and learned enough to have a basic understanding of protected and unprotected cells. Enough to know that both fivemega and owen are correct depending on the battery you are looking at acquiring.

    Good thread by the way. Very relevant for those just getting started down the rabbit hole of 65mm long batteries for 2x123 flashlights.
    John 3:16

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