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I am planning to charge a lot of electronic devices and photo-equipment in my car (using 12V DC only). I've got a lot of chargers connected, including 100W for Macbook Pro and multiple 36W smaller chargers. I'd like to charge as many devices as possible (while driving) – overall could be up to 300W.

My questions are:

  1. What is the reasonable maximum power consumption of all power-drawing so that it doesn't harm car's alternator?
  2. If all these devices are drawing 300W of power while car engine is off, might it hinder cranking an engine?
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  • What's the total amount of power you are expecting to draw? Are these devices charged by 12 volt DC, household current, or both? Apr 24, 2023 at 13:10
  • Also, what's the car? I can give you some more specifics with that information. Apr 24, 2023 at 14:11
  • @MoveMoreCommentsLinkToTop it's an old Renault Master 2003; I was planning to draw around 300W max using only 12V DC from car's crank-battery (updated the original post). thanks for the useful questions!
    – soshial
    Apr 24, 2023 at 15:40
  • I have update my answer with more info Apr 24, 2023 at 16:47

2 Answers 2

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Don't care about the alternator; it won't be overloaded.

Instead, care about the fuse. Most cigarette lighter sockets can't supply more than 10 amperes which at 13.5 volts is 135 watts. If you run the chargers through an inverter, the efficiency may be bit below 90% which gives about 120 watts.

If you draw more power, the fuse will burn, there's no way around that.

I suggest you should probably run your Macbook Pro with a USB-C charger that's rated far below 100 watts, maybe 65 watts could be ok. Then you have room for several other chargers too. I don't believe the MBP draws the full 100 watts, most of that goes to charging battery while at the same time powering the computer. If you use a 65W USB-C charger, then the computer will run, the battery will charge, but at a lower rate.

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  • You can absolutely overload an alternator, not likely in this use case, but it's definitely something that needs to be considered. Apr 24, 2023 at 13:15
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    Well yes ... but not with a 10 ampere fuse!
    – juhist
    Apr 25, 2023 at 18:14
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It depends on how much total power (all devices added together that will be charging at the same time) you are planning to draw (use) and how the devices are charged 12 volt or household current.

If it plugs into the cigarette lighter you'll be limited to the rated capacity of that circuit which can be determined by looking in the owners manual or at the fuse size suppling it and multiplying by 0.8, this will give you %80 of the fused size. Take that number and multiply it by 12 to get the watts for that circuit.

If you need more than that you can add additional power points, either by going straight to the battery with a properly sized and circuit protected wire, or pulling from an accessory power point that's provided by the manufacturer on the vehicle for just this purpose.

It doesn't seem likely for your use case, but ultimately you will be limited by the size of the alternator (rated output) but that can be upgraded by replacing with a higher output one, or adding a second one. Some vehicles come equipped with 2 and/or have an option (OEM or aftermarket) to add a second one.

For your specific use:

100 watt MacBook charger
36 watt other
36 watt other
36 watt other
208 watts total

You're going to need multiple outlets, that are separately fused as most cigarette lighter outlets and built in AC outlets (household current type plug) max out at 150 watts on the high end, many are less.

Here's an example from an owners manual (Renault Master year unknown, but likely the third generation)

page 3.19

Since it's a commercial vehicle it may have accessory circuits already installed that are designed to be used for addons. Try to find the Owners Manual, Service Manual, or Upfitters manual (AKA Body Builder manual) for your year model.

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  • That's a great idea and I will definitely use multiple cigarette outlets! Although I noticed below the steering wheel a scheme table for fuses: and the cigarette lighter outlet fuse is rated 25A. I guess, it implies all cigarette lighter outlets combined? Which doesn't mean that if there's only 1 outlet, that it'll be rated 25A, right?
    – soshial
    Apr 24, 2023 at 15:51
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    @soshial, yes that's likely for all the outlets in the car, and there may be a maximum for individual outlets, but the ones I've seen like that are written on the outlet from the OEM Apr 24, 2023 at 16:18
  • If all chargers are outputting for a total of 300 watts, divided by charging voltage at approximately 14.5vdc (above idle rpm), this is 20.7 amps of draw from the alternator. Engine off, battery voltage of 12.5v increases current draw to 24 amps. Not a good idea of running everything off the car battery. Do some calculations whether on battery power or while driving (idling does not increase alternator output) to determine optimum use of electronics/charging systems. At night more current is used by the vehicle for lighting. One spec states your alternator is rated up to 150 amps.
    – F Dryer
    May 1, 2023 at 0:12

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