How Long Can I Run A Laptop Off The Car Battery? - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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Old 11-07-2007, 08:27 PM   #1 (permalink)
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I have a lilliput screen and a laptop carputer in my car with the charger hooked up to the car battery. I know I shouldn't make it a habit to run off of the car battery when the engine is off, but I was wondering how long could I run the laptop without the engine on before I will have trouble starting my car? Or how would I calculate an estimate? I am curious because I actually have no idea of the timescale (several minutes, or several hours?). And would constant "engine off" use (i.e. draining the battery and recharging again when I start the engine) have affect on the car's battery life?
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Old 11-07-2007, 08:42 PM   #2 (permalink)
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it all depends on the battery that you have. All batteries are different and perform differently given the application(s).

Quote:
Reserve Capacity (RC) or Amp Hour (AH) Capacity

For car batteries, an equally important consideration to CCA is the Reserve Capacity (RC) or Amp Hour (AH) endurance ratings because of the effects of increased parasitic (ignition key off) loads while long term parking, power demands during short trips and emergencies. Endurance is defined by Eurobat as the actual combination of the energy content stored in a battery and the rate which the battery is discharged over the life time. RC is the number of minutes a fully charged battery at 80 F (26.7 C) can be discharged at a constant 25 amps until the voltage falls below 10.5 volts. European and Asian starting and deep cycle batteries are usually rated in Amp Hours (AH). To convert RC to AH (or AH to RC), check the battery manufacturer's capacity specifications. More RC (or AH) is better in every case. In a hot climate, if your car has a 360 OEM cold cranking amps requirement, then a 400 C CA rated battery with 120 minutes of RC and more electrolyte for cooling would be more desirable than one with 600 CCA with 90 minutes of RC. There is also a relationship between the weight of the battery and the amount of RC (or AH). The heavier the battery, the more lead is has and potentially a longer service life.

For deep cycle batteries, important considerations are will the Ampere-Hour (AH) rating meet or exceed the requirements based on your application? Peukert Effect? and how much weight you can carry? Most deep cycle batteries are normally rated in number of hours it take to discharge a fully charged battery to 10.5 volts in 20 hours at 80 F (26.7 C), denoted as "C/20". Discharge rates of 100 hours (C/100), 10 hours (C/10), 8 hours (C/8) or 6 hours (C/6) are also common ratings. When comparing amp hour capacities of deep cycle batteries, use the same discharge rating periods which can be obtained form the battery manufacturer. For example due to the Peukert Effect, the same wet deep cycle battery with the amp hour capacity of 240 discharged over 20 hours could have a capacity of 176 amp hours when discharged over six hours or 115 amp hours when discharged in one hour. Within a BCI Group Size, the battery with higher AH (or RC) will tend to larger in physical size, have longer service lives and weigh more because of thicker plates and more lead than car batteries.[/b]


Here's a good info site: (http://jgdarden.com/batteryfaq/carfaq7.htm#rc) .
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Old 11-07-2007, 09:17 PM   #3 (permalink)
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Quote:
it all depends on the battery that you have. All batteries are different and perform differently given the application(s).
Here's a good info site: (http://jgdarden.com/batteryfaq/carfaq7.htm#rc) .[/b]
What a great link! Thank you for posting it here.

To the OP (timwoo): chances are, you have the OEM battery, which is a BCI Group 40R size Motorcraft BXT-40R, made by Johnson Controls; that battery isidentical to several other brands of batteries made by Johnson Controls under the Interstate, AC Delco, DieHard, and other, brand names. You can get slightly better AH and RC ratings from a Deka battery or a Carquest house brand battery in the same size Group 40R, both of which are made -- in the United States of America -- by East Penn. You can get a battery with better yet AH and RC ratings by switching to an AGM (absorbed glass mat) battery in BCI Group 48 size (also made by East Penn), Deka model 9AGM48.
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