Calculating MacBook Pro charging requirements?

Discussion in 'Power - Site Power/Batteries/Generators/Solar' started by Stamourowl, Jul 10, 2019.

  1. Stamourowl

    Stamourowl If it involves dogs and hiking count me in.

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    Hi All,
    I am trying to figure out how power charging my MaBook will be on my battery. It is a Marine 24 battery. The MacBook is a MacBook Pro 2015 model. My best info is it has uses the 85 watt charger. If I am calculating correctly:
    85 watts/12 volts = 7 amps. Is this per hour? I have a dc connector for the MacBook. It looks like this.
    It currently works great in the car but I would like to use it in the popup.
    Thanks for any help!
     

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  2. Eric Webber

    Eric Webber Active Member

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    Technically, it’s momentary amps. Running that load for an hour will consume 7ah (amp hours) at 12v. This assumes 100% efficiency.

    Using a dc charger, assuming it isn’t really just a small inverter (which would be less efficient) you will probably be consuming “real world” about 8ah per hour.

    But, good news here, it won’t actually use that much power for the full charge. The last few percentage points are much lower wattage to float/top it off

    Maybe a better way of looking at it is to
    A) make sure the dc charger works (sounds like it does)
    B) look at the actual battery capacity (8.230Ah at 12v or 100wh)

    That’s what it receives, not what the battery sends.

    But if your battery is (as an example) a 40ah 12v with about half usable (recommended not to drop under 50% on most type of deep cycle) you have enough to charge it up fully twice from zero with a bit to spare, even factoring in inefficiency
     
  3. Stamourowl

    Stamourowl If it involves dogs and hiking count me in.

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    Thanks!
     
  4. SteveP

    SteveP Well-Known Member

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    Assuming this is the actual charger you'll be using: Dual USB Ports Charger for New MacBook 12 (5V 3A USB to Type C only)

    If you're charging one item through the USB C port you'll be using 15 watts, or 1.25 amps/hr, + conversion overhead, which I'd calc at 20% to be safe. Using both usb ports and/or the DC port will increase your draw.
     
  5. David Blackwell

    David Blackwell Member

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    The best way to measure AC devices is to use a Kill-A-Watt. See Amazon ($25). Below are my measurements. To have a complete energy picture, you will need to estimate the amount of time (time X minin order to come up with watt-hours which can be converted into amp-hours for sizing loads and battery capacity. If using an inverter, don't forget to include inverter losses of at least 15% (general rule-of-thumb). I have never measured my own inverter. It's also possible to find 12v car chargers for the DC charging of laptops as well as 12v USB chargers for phones etc.

    Computer/Activity (AC Voltage, Amps, Watts)
    2011 AirBook
    Charging: 117.5, 0.07, 8
    Running with Screen On: 117.5, 0.28, 47

    2008 MacBook Pro
    Charging Only: 117.5, 0.07, 8
    Running with Screen On: 117.5, 0.28, 33
    Running with Screen On & Hard Drive: 117.5, 0.28, 45
     
    Last edited: Jul 14, 2019
  6. Fil_Kay

    Fil_Kay New Member

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    I don't know about a MacBook, but years ago I found a 12 VDC laptop charger for use in a car with a number of replaceable ends for various laptops. It wouldn't work for a MacBook, but you may be able to find something similar.

    If you really want to find something to charge your laptop during extended trips without being hooked into shore power, I would recommend finding something that'll work directly off of 12 VDC due to the inefficiencies of converting from 12 VDC to 120 VAC and then back to whatever the MacBook takes (looks to be 20.3 VDC at 3 amps for my charger). Even if each conversion is 80% efficient, that means you're only at 64% efficiency if you need to go to 120 VAC first.

    Another option would be to pick up a charger/battery pack all-in-one unit from Apple; I've seen them available online for about $100. Then again, at that price, you can probably consider adding a second deep cycle battery in parallel and doubling your capacity for the entire trailer.

    If you're not able to find a 12 VDC charger, or don't want to get an external battery pack or add a second battery to your trailer, then you can still probably get by on your existing deep cycle battery even with charging the laptop. At 120 VDC and 1.5 amps, my charger would draw 180 W, and it can charge the battery from dead (about 5%) to full over the span of a couple hours (I think it hits about 80% in the first hour or so). Also keep in mind what your power consumption is going to be when you're actually out far enough to not have shore power; chances are, you likely won't have internet access, so you can probably keep your WiFi turned off to save power. You can also keep your screen dimmed (if you're not in direct sunlight), and keep the key backlights off as well. Additionally, if you keep the power-hungry processes to a minimum (e.g., don't have videos running in the background for music, don't stream things via bluetooth, don't run intensive calculations through Excel, don't have a dozen desktops open with various programs going), then you can prolong your charge. With that, you may only have to charge the laptop a couple times, even on a multi-day trip.

    Plus, don't forget that a properly-wired 7-blade trailer connector will provide 12 VDC power from your tow vehicle, so you can always start your vehicle up and let it run for a bit to re-charge the battery. If you've only got a 4-pin connector, you could invest in a pair of jumper cables and use those to charge the trailer battery as well (although I recommend investing in good jumper cables regardless).
     

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