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Complete Noob Question about the capacity of an external battery for a Dell laptop...

  1. Apr 4, 2017 #1
    I'm a complete noob when it comes to electrical stuff, but I have a question about something, so please bear with me.

    I have a Dell laptop (Vostro 3550), and recently bought an external battery for it ("Dell Power Companion"). The external battery is rated at 12,000 mAh. Since my laptop voltage is 11.1 V, I assumed I'd be getting 12 * 11.1 = 132 WHr. Turns out, though, I'm only getting the equivalent of about a 43 WHr battery with it.

    To further confuse things, my laptop voltage is 11.1 VDC. But the DC power converter's output voltage is 19.5 V.

    Anyway, below are the specs for the laptop and for the external battery. If someone can explain to me why I'm only getting 43 Whr from a battery that's rated at 12,000 mAh with 11.1 volts, I'd appreciate it.

    Thanks!

    Laptop specs:
    90W power supply: 19.5V 4.62A
    Original 43 Wh laptop battery: 2.2 Ah
    Laptop Voltage: 11.1VDC

    External battery specs (copied from back of battery):
    12000 mAh
    14.8V 43 Wh
    19.5V 2.3A
     
    Last edited by a moderator: Apr 4, 2017
  2. jcsd
  3. Apr 4, 2017 #2

    jedishrfu

    Staff: Mentor

    What region of the world are you in? What are your AC specs? 120v, 240v, 50 or 60 cycles / sec or something else...
     
  4. Apr 4, 2017 #3

    Grinkle

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    Gold Member

    Power is Voltage times Current. The battery capacity is 19.5V * 2.3A = ~45Wh. It looks like the advertised capacity was de-rated to 43Wh from the specs you are showing.

    To know your actual capacity, you would need to know the average current draw of the computer over the life of a single charge. You could approximate the battery voltage at a constant 14.8V, it would be lower towards the end of charge in reality but you can ignore that for first order calculations. I don't think its easy to measure the current draw of your laptop, though.

    edit - 2.2Ah means 2.2A for one hour, so the more accurate calculation is -

    19.5V * 2.2A * 1h = 42.9

    It wasn't de-rated, I just made a mistake and missed the 2.2Ah part.
     
  5. Apr 4, 2017 #4

    Drakkith

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    Staff: Mentor

    Battery capacities are given at a certain rate of discharge and going above or below can get you more or less total energy than the stated amount. If I remember correctly, drawing more amperage from the battery than what was used to rate it will give you less watt-hours/amp-hours. In other words, the higher the current you draw from the battery, the less total energy you can get from it. I'm not quite sure why.

    Also, looking at the external battery, it appears that it has its own built in power/voltage converter (not sure of the technical name for this), otherwise it can't output 19.5 volts with only 4 lithium-ion cells (each cell has a maximum of 4.2 volts, and 4x4.2 = 16.8v). I'd bet you're losing energy in this conversion process.

    I've read that 19 volts is high enough to charge 4 lithium-ion cells in series and still leaves a little "headroom" for other things like the converter and controlling electronics. The laptop itself steps down the 19 volts to whatever it needs to run off of or just uses the voltage from the battery.There's a long discussion here on stack exchange about it.
     
  6. Apr 4, 2017 #5

    russ_watters

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    So is this just misleading advertising? They rate the pack as if the batteries were in parallel when they are really in series?
     
  7. Apr 4, 2017 #6

    Drakkith

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    No idea. I don't know if they rate each cell and then add the ratings together, rate them in parallel, or rate them in series.
    Now that you mention it, I'm not even sure the cells are in series. I assumed they were, but I could be wrong. None of the technical specs I looked at gave that information. They all just said "4-cell lithium-ion".
     
  8. Apr 4, 2017 #7

    russ_watters

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    So....I own a 10ah USB charger battery. Powering USB ports is all it does, so I'm pretty sure that's 10ah at 5V. Seems like it should be just that clear-cut here.
     
  9. Apr 4, 2017 #8

    Drakkith

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    I doubt it's even that clear cut with your own battery. Everything I've read says that the capacity of a battery is reduced as you increase the current drawn from it (perhaps a lot of power is eaten up by internal resistance?). Also, since lithium-ion batteries don't provide 5 volts, there's a DC-DC converter and some other circuitry that eats up power as well. How much I don't know.
     
  10. Apr 4, 2017 #9
    I'm in the U.S. 120V. Don't know about the Hz.
     
  11. Apr 4, 2017 #10
    But that's the thing. WHY is the battery capacity * 2.3A? If the battery is rated at 12,000 mAh, wouldn't that be 12 Ah, not 2.3A? (I realize I'm mixing A and Ah; but it seems that the A should be the same whether it's A or Ah.)

    Anyway, that's my question -- where does the 12,000 mAh come into play? Seems that that's not there at all, and it's going by 2300 mA instead of 12000 mA.

    Again, I'm a total noob, so forgive me if the answer is obvious. But I'm just not seeing why 2.3 instead of 12 is being used in the calculation.
     
  12. Apr 4, 2017 #11

    Drakkith

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    Okay, some further reading has revealed that lithium ion battery cells have an average voltage of 3.7 volts, and a max of near 4.2V. The 14.8 volts here is likely from four 3.7 V cells in series, and your laptop's battery voltage of 11.1V is just 3.7V x 3. Assuming the battery can actually provide 43 watts for an entire hour (for 43 watt-hours), dividing 43 W by 14.8V gives 2.9 A, while dividing by 19.5V gives about 2.2A, which is close to the advertised 2.3A. Since watt-hours is actually a measure of energy, while amp-hours aren't, the 43 Wh is the number you should be looking at. As you've already discovered, this is likely correct. Also, if we divide 43 Wh by 12 Ah we get about 3.6 volts, close to the 3.7V for each cell. Perhaps the advertised mAh rating is simply the rating of a single cell that was then multiplied by four? I can't see any other explanation at this time as to why the mAh rating and the Wh rating are so different when you do the math. Very confusing...
     
  13. Apr 5, 2017 #12

    jedishrfu

    Staff: Mentor

    This article describes the amps vs amp-hours for batteries and why they aren't the same:

    https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

     
  14. Apr 5, 2017 #13

    russ_watters

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    This really annoys me. I have trouble interpreting it as anything but false advertising. It's like when fan manufacturers give the airflow and pressure of a fan at different operating points. I guess it's ok to screw with consumers (?), but if I fell for it, I'd get sued for installing the wrong fan.

    ....And we have an electrical department at my office. Can you imagine sizing an electrical system for an amperage capacity without specifying the voltage? Oops, bought the wrong size transformer!
    That's fine(difference between theory and reality), but it isn't the OP's problem: the problem (as i see it) is the theoretical numbers don't even compute because the manufacturer is intentionally misleading about what voltage goes with what amperage to mislead about the battery capacity.

    I don't think they play that game with the internal batteries, because it is less plausible when the pack only outputs one voltage.

    ...though I'm not even sure about that, since one I just Googled says 11.1V *and* 7200 mAh, instead of 7200 mAh *at* 11.1V.
     
  15. Apr 5, 2017 #14

    Grinkle

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    The answer is not obvious, you are asking valid questions. Can you post a picture of the 2 labels you reference in the original post? I wonder if there is any other context that might be on the label to explain the 12Ah number - its very odd, I agree. I can't think of any better explanation than others have already posted - hard to think of a pair of use-contexts where 12 and 2.3 can each be correct.
     
  16. Apr 5, 2017 #15

    jedishrfu

    Staff: Mentor

    BAsed on the previous article I posted then your battery having a capacity of 12 amp hours and maximum output of 2.3 amps would mean the battery would last for about 5.2 hours before its exhausted.
     
  17. Apr 5, 2017 #16

    Grinkle

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    I read the article this way as well, and imo it is a reasonable interpretation of 12Ah / 2.3A as provided specs. I can't make that seem consistent with the other specs printed on the battery; 2.3A @ 19.5V * 5.2H = 234Wh, and the label also says the battery capacity is 43Wh at 14.8V.
     
  18. Apr 5, 2017 #17

    russ_watters

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    ...assuming the capacity and output ratings are provided at the same voltage. From the numbers provided by the OP, it looks to me like 12 Ah / 2.2 A = 1 hr

    From the OP, it looks to me like Dell is selling two different versions of the same battery, one accurately rated at 2.2ah and one inaccurately rated at 12ah.
     
  19. Apr 5, 2017 #18
    I don't think I missed anything, but I uploaded a picture from the back of the external battery with the specs.

    Also, here is the spec document for the external battery (discussing only the power DC jack here, not the USB port): http://www.sets.com.lb/wp-content/uploads/2015/05/Dell-Power-Companion.pdf

    And here is the Owners Manual for the laptop, which has the specs for 4 different laptop models. The electrical specs for the 3550 are only pp. 108-110. To save you the trouble, here's what it says:

    Battery:
    Vostro 3450/3550/3555 Type • 6-cell "smart" lithium ion (2.2 AHr)
    Voltage 11.1 VDC
    AC Adapter:
    Input voltage 100 VAC to 240 VAC Input current (maximum) 1.5 A / 1.6 A / 1.7 A / 2.3 A / 2.5 A
    Input frequency 50 Hz to 60 Hz
    Output power 65 W, 90 W, or 130 W [My model uses the 90W power supply.]
    Output current: 65 W • 4.34 A (maximum at 4- second pulse) • 3.34 A (continuous) 90 W • 5.62 A (maximum at 4- second pulse) • 4.62 A (continuous) 130 W 6.70 A (continuous)
    Rated output voltage 19.5 +/– 1.0 VDC
     

    Attached Files:

  20. Apr 5, 2017 #19

    Grinkle

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    And on the battery itself, it says -

    ?? The datasheet does not say anything about 43Wh @ 14.8V. The datasheet very clearly calls this a 12Ah battery.
     
  21. Apr 5, 2017 #20

    Drakkith

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    Indeed. Hence the immense confusion in this thread. The battery is labeled as 43 Wh with no Ah listed, but the datasheet does the opposite.

    Edit: Wait. On the back of the battery, middle right, it clearly says EU 2200 mAh...
     
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