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After being hit by Hurricane Irene last week, I happened to loose power for literally one week so rather than rely completely on batteries and being a nomad, I used my Cobra 400 watt power inverter (wired directly to my car battery such that I can keep it powered even without the key on accessory) on and off to power a small emergency lamp, charge my laptop battery, power my modem, and other small items like a mini-fan.

For the most part this was limited to maybe five hours tops per day and I usually would idle my engine every hour for a few minutes to make sure the battery got a boost again, and while I never had anything fail, I wanted to ask if the drain/charge cycle from the power inverter could've caused any wear on the battery, and also if it's okay to idle your engine to power an inverter.

The reason for the latter question is because I heard that in the winter it's best to not let the engine idle to warm up as it allows carbon to build up, however wouldn't Seafoam or a similar cleaner in the intake fix that? I also heard that the idling affects the oxygen content in the fuel, but don't most modern cars have sensors to compensate?

Also, my final question is how long you need to idle your car for the battery to get a good charge. I heard from my father who is an Electrical Engineer that car batteries are designed for a burst of power rather than a steady stream (like in Marine batteries) so it's never ideal to have an inverter running without the engine -- but if the battery is designed to burst output, does that mean it would have a rapid/instant recharge once you crank the engine?

Thanks very much in advance for any insights,

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    I know this question is old but for some reason no one mentioned things like a deep cycle battery or a Optima Yellow Top, which can take the abuse of being discharged and recharged over and over again. They are what are used in marine applications but there's no reason you can't use them for your truck/car. I use a yellow top to compensate for my sound system's load and for when I run heavy accessories like an air compressor or a winch.
    – DJSpud
    Commented Mar 3, 2015 at 16:30

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As your father says, car batteries are designed for a burst of power (the high-current starter motor), not a steady stream. If you're going to use a battery a a general power supply, you need to get a "deep-discharge" battery, as designed for caravans/marine etc usage.

Constantly discharging/charging a normal car battery will reduce it's life significantly, so it is best avoided where possible.

The time taken to recharge the battery depends on the size of the battery and the size of your alternator, but it will take a lot longer at idle than in normal driving - car alternators are designed to give peak output at normal cruising revs (usually around 3,000rpm).

A proper generator will have an engine designed to run at a certain speed, and a generator matched to that speed, so that it is constantly running at peak efficiency.

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    will reduce it's life significantly, so it is best avoided where possible. . This is true, but ruining a $100 car battery is cheap compared to a week-long power outage. Thanks for your informative answer.
    – Stefan Lasiewski
    Commented Apr 14, 2012 at 16:07
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    And I am curious if this would be different for Hybrids: green.blogs.nytimes.com/2008/12/23/…
    – Stefan Lasiewski
    Commented Apr 14, 2012 at 16:09
  • Car alternators have a pulley ratio 3:1 or so, so 1000 RPM at the engine should give 3000 RPM at the alternator. It may not give peak output at that RPM, but the point is that modern alternators can provide quite a lot of amps even at engine idle.
    – juhist
    Commented Mar 11, 2018 at 16:42
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Using a 400 watt inverter for a week will not likely substantially harm your car battery. Car batteries, while designed for short, higher current "starts" can handle longer term loads. It is good to avoid draining them down so that they will not start the car, but you apparently did not drain the battery down substantially.

The charge time is situational. I have a couple of VW Diesel cars, with larger alternators which put out 70+ amps at idle. The diesel cars have electric heaters to bring the coolant up to temperature, which come on as soon as the car starts cold, hence the different need for more power at idle. On the other hand a Dodge truck with a similar sized alternator, and a gas engine, puts out about 15A at idle, and would take longer to top off the battery.

The best way to charge your car battery is to drive it, perhaps running errands, or whatever.

It is true that carbon build up, plug fouling, catalytic converter loading and all kinds of nasty things can happen with 30 minutes of idling each day. But for just a few days, you will probably not have a problem, unless your car's engine has problems like high oil consumption already.

Most modern cars do perform mixture adjustments so that the mixture is managed during idle and warm up. Remember that in some parts of the country, cars spend quite a bit of time idling in traffic.

When faced with an emergency, sometimes you have to do what makes sense. Depleting 3% of your car battery total life, to provide some of the other "essentials" during that week seems like a good trade off. In the end, what you did was reasonable, and most likely did not impact the life of either the battery or the car engine.

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  • 400 watts for a week is 67200 watt hours or 67.2 kilowatt hours. The tiny little battery can provide perhaps half a kilowatt hour. Even large electric car batteries are typically not 67.2 kilowatt hours. Ok, perhaps a 400 watt inverter uses only 4 watts at idle (if you just keep it on without using any electricity at all...). Even then, you'd be using 672 watt hours, more than what the battery can provide.
    – juhist
    Commented Dec 10, 2018 at 16:25
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    The OP said he ran low draw items on and off, for 5 hours a day, plus hourly start and charge cycles. Much different from the scenario you mention.
    – mongo
    Commented Dec 10, 2018 at 16:48

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