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Charging a Laptop with 20000 mah battery

Discussion in 'Power Electronics' started by pally, Nov 17, 2013.

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  1. pally


    Nov 17, 2013
    Hey everyone,

    My question is about capacity and my confusion with why a 20000mah battery gets completely drained when charging a 3750mah battery

    I just bought an external battery Anker astro pro2 which has 20000mah capacity

    My laptop has a 3560mah battery inside.

    I just don't understand why it would eat up the entire external battery when charging my laptop.

    I leave the laptop off and the max my external battery can deliver is 3amps at 19V

    I'm not sure how much current my laptops trying to draw when its off I only know the adapter it comes with is 19V 4.57A

    Any advice or explaination? I want more mobile life so I can stay out longer and use my laptop

    Thank you!
    Last edited: Nov 17, 2013
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Hi pally
    welcome to the forums :)

    what have you got between the external battery and the laptop battery that is limiting the current to 3A ?
    if nothing, then there is no 3A current limiting as you suggest
    the battery is capable of delivering 20 Amps for an hour

    a bit of clarification on how you have it all hooked up would help!

  3. pally


    Nov 17, 2013
    Thank you Davenn,

    Simply that the back the Anker Pro2 says this
    Battery Type: Li-polymer Battery
    Capacity 20000mAh/74Wh
    Input: DC 15-19V/1.3A(MAX)
    DC 12V/4A(MAX)
    DC 15V/3.5A(MAX)
    DC 19V/3A(MAX)
    DC 5V/2.1A(MAX)

    The only thing between the Anker Pro2 and my Laptop which is Aspire V5-552G-X414 is the cable connecting them, the Anker Pro2 came with the cable and it had a tip that looks exactly the same as the one that come with my laptop.

    So basically my understand was that since my laptops battery is 3560mAh and this external battery(Astro Pro2) is 20,000mAh, I should be able to charge that little battery in my laptop atleast twice, actually I thought five times since its five times the capacity, or does it not work that way?

    Again I left the laptop off with a completely drained battery, after Three and a Half hours the laptop was fully charged and the anker pro2 lights indicated it have between 1-20% battery remaining.

    I don't know much about electronics so I'm trying to understand, thanks!
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would measure the current during the charge and see how much power is being supplied by your external battery.

    I suspect that you are simply mis-reading the specs (or they are intentionally misleading)

    If you look at 74Wh, and then the mAh rating, you can see that the battery voltage is nominally 3.7V

    Your laptop has a 3650 mAh battery, but I bet it's nominally 14.8V (or possibly higher). That would make it a 54Wh battery.

    After taking 54Wh from your 74Wh battery, only 20Wh will remain. That's 27%, but charging is less than 100% efficient, so the amount remaining will be less.

    Now, I'd be VERY surprised if that Ankler battery really was 3.7V with a boost DC-DC converter to produce all the rails.

    We spoke about this earlier didn't we? Were you the person comparing 2 possible solutions? (edit: No he wasn't, but that was a link about the same type of battery)
    Last edited: Nov 17, 2013
  5. pally


    Nov 17, 2013
    I do not believe we spoke yesterday,

    You know a lot more than I do about electronics, I'm not sure I completely follow you

    I'm 99.9% sure I am reading the specs correct, I am starting to think as you mentioned the information is misleading.

    So in laymens terms basically the Anker Pro2 external battery is really only 3.7V 20,000mAh capacity, and somehow its getting up to 19V which reduces the amps it can supply in total?

    Thus my math skills were assuming I really have a 19V battery with 20,000mAh capacity?

    Now I guess the real question is

    If you were me, and wanted to find a way to power a laptop whose adapter supplies 19V 4.57A, what is the best external battery I can get around $200 for most battery life? Or what specs should I be looking for to not get fooled again.

    Thanks :D

    PS: Not sure how to measure the current during the charge since the wire to and from external battery to laptop is fully coated, I do have a good multimeter on hand though
    Last edited: Nov 18, 2013
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    No, I had a conversation some months ago with someone regarding a battery like this. I went back and checked and they had not specified a Wh rating for the battery, so the normal assumption is that if you are told 19V and 20,000 mAh that the capacity is related to the output voltage.

    Yes, you sound like you were doing rational math based on a sensible assumption.

    As to what battery is the best... That's the $64 question (or in this case the $200 question).

    Essentially you want something that has the highest capacity measured in Wh.

    mAh is also a measure of capacity, but it is not an absolute measure (Wh is absolute).

    No real need to measure the current any more. I think we've discovered who dun it.

    In most cases, assuming the same battery chemistry, and assuming that the batteries make up most of the weight of the device, the weight will be roughly proportional to the capacity. You should expect a 100Wh battery to weigh about twice what a 50Wh battery does.

    As an aside, batteries can be compared using the measure of Wh/lb (Watt-hours per pound) which remains relatively constant for a given chemistry. This link gives you some interesting comparisons. Note that they cover 2 different lead-acid battery types, and 2 different Lithium types.
  7. pally


    Nov 17, 2013
    Thanks so much for your help and clarity, I know now to look at Wh,

    based on your link I know now Li-ion batteries are more expensive when you add up their lifetime value. Maybe a NI-MH battery is better for my case considering those are pretty popular.

    Thanks again!
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That is up to you to decide.

    If I had to carry it in my backpack whilst climbing a mountain I might make a decision biased more toward weight then cost. However it's possible that for you, the "best" choice is something else.
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