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Beginner Amp Hour Question.

Discussion in 'Power Electronics' started by Hsnopi, May 22, 2019.

  1. Hsnopi

    Hsnopi

    43
    6
    Dec 9, 2015
    Hello all,
    I'm trying to understand amp hours in order to get a battery for my CPAP for off grip camping. I want to make sure I'm doing this properly.
    I have a CPAP that draws, from a chart, 3.49 amps at 12v. (The chart says "Current draw @12V DC (amps)).
    So I found a 250W solar rechargeable power supply. It states: 12V 20Ah.

    If I understand, this means it has 20Ah of cycle(?) over 10 hours. If I run my CPAP off it then i divide the 20Ah/3.49 this means I get 5.73 hours of run-time. IS that correct?
    Thanks!
    Jeff


    Sources:
    https://www.resmed.com/us/dam/documents/articles/198103_battery-guide_glo_eng.pdf
    p7 (Airsense 10 Autoset Slimline @8)

    https://www.amazon.com/Rockpals-250...d=1558491353&s=lawn-garden&sr=1-1-spons&psc=1
     
  2. Ylli

    Ylli

    230
    46
    Jun 19, 2018
    Close, but since you are drawing more current than the 10 hr rate, the actual usable battery capacity will be a bit less. Probably 4.5 - 5 hours.
     
  3. dave9

    dave9

    821
    194
    Mar 5, 2017
    The power supply specs could be an issue.

    1) If it uses a lead acid battery, you won't want to drain that deeply or else it will have quite a bit fewer recharge cycles. It helps a lot to use a deep cycle battery as suggested in your linked document but lead acid tech still results in a far longer lifespan if you don't try to get every last Ah out of them.

    2) If it uses a Li-Ion battery, as your linked amazon product does, the 12V/20Ah spec may be very questionable. It could have fraudulently overrated generic cells in it or they could be doing the creative math that some (I will not mention the country, lol) manufacturers use to deceive customers.

    For example see the product page:
    https://www.amazon.com/Rockpals-250-Watt-Generator-Rechargeable-Emergency/dp/B075SSMR6K

    It claims "240WH(3.7V 64.8Ah/12V 20Ah)". Now it may be true that it uses a series of cells to achieve 64.8Ah at 3.7V "IF" the cells are honestly rated (see above) but there is no series of Li-Ion cells that can achieve 12.0V. It would be far more likely if not *necessary* to have a buck or boost regulated circuit to achieve this and doing so will incur loss so it is unlikely for them to arrive at a correct 12V capacity by dividing the 240WH by 20Ah.

    Lower down on the same product page it states "240WH(3.7V 64.8Ah/12V 16.2Ah)". HOW CONVENIENT that the 20Ah figure has transformed into 16.2Ah.

    My main point is depending on exactly what your power supply is, the real capacity may vary from the specs and the expected runtime may be lower than a simple calculation would suggest. You may have to find someone who has submitted believable test data or test it yourself to determine the true usable capacity at a similar current draw.
     
  4. dave9

    dave9

    821
    194
    Mar 5, 2017
    This is incorrect/poorly worded. It couldn't use a series of cells to achieve that 64.8Ah at 3.7V, but they might have used the # of cells in the battery multiplied by the Ah rating of each to come up with the equation to produce this (probably incorrect) spec.
     
  5. Hsnopi

    Hsnopi

    43
    6
    Dec 9, 2015
    Thank you all. As I was looking into this I saw a lot of what I can only guess was marketing. Listing things in different units and so forth to confuse people I guess.
    I'll have to keep digging.
    thanks!
     
  6. Audioguru

    Audioguru

    2,618
    593
    Sep 24, 2016
    A CPAP is a medical device that keeps you breathing. I looked at Amazon's Details to see if the Rockpals battery powered generator has a medical certification and warranty and Amazon said, "see the Seller". I looked for Rockpals in Google and found a Chinese trademark. I would not trust my life to that cheap junk.
     
    hevans1944 and Jim Deaven like this.
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