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2002 Mitsubishi Galant Battery Run Time Estimates?

I recently got a new job as a field service technician and need to run a mobile office out of my car. Of course leaving the engine running will waste alot of fuel so I want to know about how long can I safely run a 100W inverter off the battery, and how long I need to leave the car running for each charge cycle? I want conservative estimates with a large margin of error to account for the draw of other accessories such as radio and lights and ensuring enough charge left to start the engine afterwards

You just asked a question no one can answer. We have no idea how good your battery or charging system is. I doubt if any one has timed how long it takes to deplete a battery using the equipment you have. Idling a vehicle will not use a large amount of fuel and will certainly be less costly than a service call for a dead battery in the middle of nowhere.


I guess I could always just carry a spare battery in my trunk and wing it XD

Forget the extra battery, just keep a rechargeable battery pack with you for emergency jumping.

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Why not a second battery like they use in rv’s that is isolated from the starting battery when the ignition is off;. that way you still have a charged battery to start the car.

Get something like this to run the equipment and when you drive use the inverter to re charge it. Don’t let your car battery run down to prevent getting stuck somewhere.


They have lithium-ion jump packs now that are small enough to fit in your glovebox.

I think idling uses something like 0.25 gallons per hour, so that’s less than $1 per hour. That seems worthwhile to me for you to be comfortable and for your battery to stay in good shape.

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Doesn’t sound like a good idea to me at all. Even if you could calculate when the battery would be run down, they don’t like being discharged at all, least of all all the time and recharged. So what you need is the second battery arrangement like the police use for emergency lights with more of a marine battery for discharging all the time. Otherwise yeah a battery pack, just leave the car running, or get a small portable generator. I’m not crazy about running 110V stuff off the car anyway at all and think you are headed for problems if you do this routinely.

repeatedly running an invertor and the stuff it powers with the engine off will put a lot of extra strain on your battery and alternator. The more power you need to run, the bigger inverter you need, the more damage will be done.

keep the engine running- keep the a/c or heater on, and save the battery and alternator.

or better yet, find a starbucks or mcdonalds to camp in while you do your work (if possible.)

on another note:
invertor? inverter? spell check doesn’t like either…

So by mobile office I assume laptop and printer. Now I would have no hesitation about just powering up the printer when needed, we have some smaller ones that run off a power point, then I have to assume you are using a laptop and do not have to power an external monitor, run off the laptop battery my thought. The more info the better.
Both laptops I have think an estimated 9 hour battery life, why do you need an inverter? that will definitely draw down a battery in say an hour?

Barky is correct, a laptop PC will run for 4-8 hours without recharging it’s battery. And printers go into a sleep mode when not used. When it’s recharging it uses something like 30 watts (at a guess, check your documentation)

Just some arithmetic: If your car battery is 700 amp-hour capacity and you don’t want to use more than half the charge, and you are running the 100 watt inverter at full power level…

The inverter has losses, so assume you need 130 watts to power it. At 12 volts that is 11 amps (130 watts / 12v = 11 amps)
350 amp-hour / 11 amps = 32 hours.


I think the battery in my car is more like 50 AH

you are correct, I’m off by a factor of 10, old age memory.

If your car battery is 50 amp-hour capacity and you don’t want to use more than half the charge, and you are running the 100 watt inverter at full power level…

The inverter has losses, so assume you need 130 watts to power it. At 12 volts that is 11 amps (130 watts / 12v = 11 amps)
25 amp-hour / 11 amps = 2 hours.

Having the headlights on consumes about the same amount of current as your inverter, 15 minutes will not be a problem but you won’t be able to do this for 8 hours. How long will you be using the inverter between driving from one location to another?

I wouldn’t go over 10% discharge from a starting battery.
Even that repeatedly will shorten its life considerably.

50 amp hours is a reasonable guess for a full charge. That means it could theoretically source 600 watts for an entire hour.

50 amps * 12 volts = 600 watts

But that would run the battery down too much. So figure on 1/4 of that to be on the safe side, 125 watts for an hour.

Then it depends on what 100 W means for the inverter. I presume it can source 100 W output from the inverter. So with inefficiencies it’s going to take as much as 250 W input to the inverter for it to source 100 W output. So you are looking at maybe 1/2 hour to be on the safe side.

How long to recharge the battery after running the inverter for 30 minutes?

1/2 hour at 250 W is 125 W hours, or about 10 amp hours for the battery. The alternator should be able to source 20 amps continuously, it would take about 1/2 hour of running the engine to re-charge the battery. That would depend on what other electrical accessories the car was using of course, and whether you were idling or driving.

My 4cyl accord only uses 0.3 gallons of fuel per hour at idle, in an 8 hour day thats only 2.4 gallons, about $5 of fuel. You could also put in a deep cycle battery or a starting/deep cycle battery. I personally would install an auxiliary battery in the trunk in addition to the one in the engine bay.

It would be nice to know what sort of tools you plan on using. There are many options depending on what you are planning on doing. Some are as simple as a long extension cord that you plug into your customers outlet, or a small portable generator, or using a complete 12VDC system that gives you tons of options, including spare battery charged by your engine and/or solar.

One thing is for sure, whatever battery you are using, it will be ruined quickly by frequent use of an inverter.