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Thread: Having conceptual trouble with charging speed.

  1. #1

    Default Having conceptual trouble with charging speed.

    I figure this is one of those simple things where I am missing a fundamental understanding. I have a Nitecore D2 that charges 2 channels at 500mA each. When I put 2 AA 1900mAH Eneloops in it takes them 5.5 hours each to charge. Is it not as simple as dividing the capacity by the charging rate? I expected 4 hours max given the math. Is the extra hour and a half some ramping up and down thing that the charger doesn't display?

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    Default Re: Having conceptual trouble with charging speed.

    Yes, "CC/CV"(-ish) algorithm largely accounts for the difference

    Check out its charging curves, here ...

    https://lygte-info.dk/review/Review%...20D2%20UK.html

    ... if you like
    Last edited by archimedes; 03-16-2018 at 09:51 AM.
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  3. #3

    Default Re: Having conceptual trouble with charging speed.

    Thanks. I toiled over what charger to get and chose this one over the Xtar VC2 Plus due to the built in power supply to save on packing space but it gets so warm I am wondering if I made a mistake especially given how long it takes to charge 2 cells. I really just need a reliable, compact nimh charger for eneloops but liked the ability of the D2 to do other chemistries should I ever need (unlikely for me but still a selling point). At 15usd or so I am not sure there is a better choice.
    Last edited by hap124; 03-16-2018 at 10:07 AM.

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    Default Re: Having conceptual trouble with charging speed.

    Yes, everything is a trade-off ... features, flexibility, power, heat, reliability, cost, etc, etc
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    Default Re: Having conceptual trouble with charging speed.

    Isn't CV/ CC for charging li-ion ....shouldn't the eneloops be charged at .5 amps the whole time?
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    Default Re: Having conceptual trouble with charging speed.

    Quote Originally Posted by jimbo231 View Post
    Isn't CV/ CC for charging li-ion ....shouldn't the eneloops be charged at .5 amps the whole time?
    A lot of it depends on how full the batteries are when you place them in your charger.

    If they're partially full, whether li-ions, or NiMH, they might be in the slower stage of the charging algorithms.

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    Default Re: Having conceptual trouble with charging speed.

    Oops, didn't notice you were referring to 'loops, sorry

    Still, looking at the graphs at the link is instructive ... looks like that charger is generally delivering a bit less than "rated" current.

    There is also a fair bit of "noise" / pulse / fluctuation that may be slowing things down some.

    And, of course, heat losses and other inefficiencies.
    Last edited by archimedes; 03-16-2018 at 08:41 PM.
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  8. #8

    Default Re: Having conceptual trouble with charging speed.

    I haven't tested anything of the kind, and haven't seen tests done, but I presume, perhaps in error, that the losses in the system as a whole would increase with increasing charging current as well. Whether it's linnear or following another scale, I don't know.

    Almost tempted due to lack of nothing better to do, to do some tests with charging a cell on my C9000 starting with very low charge current, and ramping it up to see the various time differences as compared to the ideal situation with no losses.

  9. #9

    Default Re: Having conceptual trouble with charging speed.

    I'm sorry to tell you. . . In order to get 1900 mAh out of a battery, you always have to put more than 1900 mAh in. We're bump up against the laws of thermodynamics here.

    c_c

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    Default Re: Having conceptual trouble with charging speed.

    Quote Originally Posted by Curious_character View Post
    I'm sorry to tell you. . . In order to get 1900 mAh out of a battery, you always have to put more than 1900 mAh in. We're bump up against the laws of thermodynamics here.
    This is indeed true for NiMh chemistry, i.e. unlike Li-ion, its charge (Coulombic) efficiency is not 100%, so you need to put more charge in than you get out. Actually they are 100% charge efficient below 90% SOC, e.g. see AA Cycler's NiMH Charge and Energy efficiency graphs for Eneloops.

  11. #11

    Default Re: Having conceptual trouble with charging speed.

    Quote Originally Posted by Gauss163 View Post
    This is indeed true for NiMh chemistry, i.e. unlike Li-ion, its charge (Coulombic) efficiency is not 100%, so you need to put more charge in than you get out. Actually they are 100% charge efficient below 90% SOC, e.g. see AA Cycler's NiMH Charge and Energy efficiency graphs for Eneloops.
    Thanks, I stand corrected. The energy efficiency must be less than 100% but in some cases as you say, the Coulombic (amount of charge) efficiency can be 100%. At 100% Coulombic efficiency, energy efficiency is less than 100% because the average charge voltage is greater than the discharge voltage.

    c_c

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    Default Re: Having conceptual trouble with charging speed.

    Quote Originally Posted by hap124 View Post
    I figure this is one of those simple things where I am missing a fundamental understanding. I have a Nitecore D2 that charges 2 channels at 500mA each. When I put 2 AA 1900mAH Eneloops in it takes them 5.5 hours each to charge. Is it not as simple as dividing the capacity by the charging rate? I expected 4 hours max given the math. Is the extra hour and a half some ramping up and down thing that the charger doesn't display?
    No battery expert but the rate of charge seems to slow down near the end. Maybe those electrons are scrambling to reach their nose bleed seats. So the math never works out in my experience. I have tested this with my USB detector and it also applies to powerbanks, lithium ion batteries and other USB chargeable devices. Guessing the same is going on there as well.
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    Default Re: Having conceptual trouble with charging speed.

    Quote Originally Posted by jimbo231 View Post
    Isn't CV/ CC for charging li-ion ....shouldn't the eneloops be charged at .5 amps the whole time?
    Seems most of my chargers are doing 1/4 amps for NiMH. Some even lower. Again no battery expert just going by what the readings say. Some pulse charge NiMH. For example the O-light UC charges all Lithium ion batteries at a max of .75 amps but pulse charged eneloops at .25 amps. The Lii200 does .5 amps for 2 LSD NiMH. The goal zero guide 10 plus does around .80 amps for 4X NiMH.
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    Default Re: Having conceptual trouble with charging speed.

    Chargers slow down as they approach the max battery capacity, both for safety and also to fit as much energy as possible in it.
    You can get faster chargers, such as the new BQ-CC55 from panasonic which is a lot faster than most nimh chargers I've seen around.

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