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I am going to be road tripping and living out of my SUV for a while. I need a way to power a tiny portable cooler/fridge 24/7 and would like to be able to power my cell phone, laptop, and maybe a few other things when the vehicle is off.

I have been looking into the possibility of mounting a solar panel on the roof and using that to charge a deep cycle battery, but it would expensive and complicated. Using the alternator to charge the auxiliary battery would be so much simpler.

I know I would need to use a battery isolator. Here's a video example of how it works

My question is whether this could damage the alternator in any way? Will it wear it out faster, or is it just using energy that would otherwise be wasted?

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So I thought I'd come here to explain some physics.

There are two important equations:

  1. p=iv - power (watts) = amps * volts
  2. v=ir - volts = amps * resistance

Note - an amp is a measure of how many electrons are flowing through the wire per second. 1 amp is A LOT of them - I forget the exact number (I'm a mathematician, not a physicist)
A volt is how much energy each electron has. A generator is only able to provide so much energy to each electron (12 volts = 12 joules per coloumb (a coloumb is a large quantity of electrons)) but the amount of them it moves along can vary, that's your amps.

watts is a rate of energy, it's in joules per second. So 5 watts for 10 seconds is 50 joules of energy. Your fuel tank could be measured in joules of energy it holds and your engine is the magical device that turns fuel into mechanical energy.

As you know cars run pretty much exclusively on 12 volts - or some other fixed number. This means if you want, say 600 watts, your alternator needs to be able to provide "i" in 600=12i, which is 300=6i, 100=3i, i=33.3 amps, which is A LOT of current. This is why wires melt more often in cars then they do at home. Where to provide 600 watts at 220 volts would be about 2.73 amps. Over 10 times less electrons carrying charge per second through devices!

So the more power you draw the more electrons the alternator has to drag through your wiring, the more energy it takes from the engine. A good "back of the envelope" value for how much energy an engine produces (mechanical of course, not heat) is 2,400 watts. If you draw 600 you've only got 1800 watts left, so you put your foot down harder (thus using more fuel) to get back up to 2,400 watts for kinetic energy.

Okay so that's the first half.

Batteries

Car batteries are NOT BUILT FOR CONSTANT DRAINS they are good at one thing, and one thing only: short bursts of high drain (like a starter motor)

  • Also they can retain charge for a long time. But I wont contrast with Lithium based batteries here

If I recall correctly, car batteries are good for 70amp hours (they can provide 1 amp for 70 hours, or 70 amps for one hour) if you draw about 3 amps (which would last about 23 hours)

remember p=iv, so p=3*12 = 36 watts for for 23 hours. That's not much is it!

Or we can do p=iv=70*12 = 840 watts, but only for an hour. (INACCURATE as car batteries don't behave this way, expect nearer 840 watts for 40 minutes)

  • This implies a linear relationship. As I said car batteries are not good at small loads for long periods of time, they are good for short bursts of very high current. You get the idea though.

You want a laptop ~ 40 watt (depending on what it's doing, if it's charging its battery it'll draw more power, up to the charger's max rating).

Fridge ~ 90 watts

Mobile phone ~ 3 watts - lets ignore this.

You're looking at drawing about 130 watts. This'll require about 11 amps.

Your battery can do 70 amps, for an hour. Using this information that'd give 70/11 = 6.36 hours. Keep in mind it's not good at doing this for a long time

So I'd bank on it being approximately 5 hours until a full battery completely ran out.

IF however you get 3 batteries. You're nearer their comfort zone (3.67 amps per battery) which should give you about 19 hours. This is a more accurate number - You'd have to wire these up in parallel.

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    Note that deep-cycle marine batteries are designed for constant drains, so it might make more sense to use something like that for your secondary battery.
    – TMN
    Jul 25, 2016 at 18:47
  • Note that common lead-acid batteries take ages to recharge (compared with modern Lithium-based batteries). In this case, you may need to have the engine running for 6 hours a day or more to prepare the battery for the next night!
    – JimmyB
    Nov 9, 2016 at 14:55
  • This has been here now for 2.5 years and no one has gone "well actually..." in some sort of pissing contest for my simplified explanation. This is truly a magical place.
    – Alec Teal
    Apr 28, 2018 at 19:34
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You should not incur any issues using a secondary battery with a battery isolator. It will not cause any damage to your alternator. For your edification, it won't be using wasted energy, but your engine will probably using a tad more gas to operate the alternator, which will have to do more work.

On a side note, you might want to rethink exactly what you are going to be powering from your spare battery. From the sounds of it, you'll be using a lot more energy than a single battery can provide. A battery will only have so many amp hours (Ah) available to it. You need to find the power draw which will be against it for each piece of equipment you plan to power and size your battery accordingly.

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  • So is the alternator "disengaged" or something when the car battery is fully charged and there's not much power draw from electronics in the vehicle? I was thinking the alternator was "always on" and always generates a fixed amount of power, hence my comment about that power being "wasted" if unused.
    – Nate
    Sep 11, 2015 at 14:12
  • @Nate writing an answer
    – Alec Teal
    Sep 11, 2015 at 15:07
  • @Nate you'd get about 5 hours.
    – Alec Teal
    Sep 11, 2015 at 15:30
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    @Nate the physical work that it takes to turn the alternator is related to the current flow through the alternator. If the alternator is charging a "full" battery, very little current flows and the alternator spins easily. If the alternator is charging a low battery or there's a large load on the circuit, more current flows, which means the alternator is harder to turn, which means the engine has to burn slightly more fuel to maintain the same speed. So all the electrical power you use comes out of the gas tank in the end :)
    – hobbs
    Sep 11, 2015 at 17:52
  • @Paulster2, assuming the "tiny portable cooler/fridge" is a typical cheap TEC cooler, a 200 AH marine deep-cycle battery can go for a day or so between charges, assuming the "maybe a few other things" have cell-phone-level trivial power draws.
    – Mark
    Sep 11, 2015 at 21:54
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I lived out of my van for six months, I had a relay operating two hundred amp hour deep cycle batteries. The fridge isn't drawing max load when it's cool, if you're in a hot climate it will draw more. Charging my computer phone, fridge and doing 40 minutes driving a day my batteries will last about 6 days, and I would have to charge up somewhere or go without a fridge for a while. I'm going to be installing solar panels in conjunction with my relay. One is not enough you need a dual charging system, if you're going to be living in it for more than a week at a time, and doing less than 3 hours driving a day.

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Important: I recommend you check with your mechanic or trusted automotive advisor before proceeding with following recommendation:

Hook the batteries ( Deep cycle) in parallel using same battery gauge wires ( pos to pos, neg to neg) as long as their amps/ and volts are identical, ( using my factory supplied alternator ) Been doing it for 11 years in my RV'.

I also installed and off/on blade type switch directly to Positive battery post to disconnect auxiliary if needed, use an amp/voltmeter attached to your aux. battery to check the status. This arrangement will store double the amps capacity, at 12 DCV,

If you are considering upgrading your alternator ..check with manufacturer specs first to make sure it is compatible with your electrical system and onboard computers.

Best of luck

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