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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. – Jhawins Mar 3 '15 at 16:30
up vote 4 down vote accepted

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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Thanks very much for the info. Very much appreciated. – theonlylos Sep 5 '11 at 21:48
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 Apr 14 '12 at 16:07
And I am curious if this would be different for Hybrids:… – Stefan Lasiewski Apr 14 '12 at 16:09

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