# Car battery amps?

by MountainDew
Tags: amps, battery
 P: 9 hello, i know car battery amps range from 40 amps to whatever, i just cant understand them, the amps aren't printed on the batteries and i don't know why? i have a pretty good ideal that my car battery maybe 45 amps, what does this mean? i know the alternator produces 55 amps, if i hooked a 100 watt light bulb up to my 45 amp car battery i know that the light bulb uses 100w/12v = 8.3 amps, so how do i figure how long the 45 amp battery will run a 8 amp bulb?
 P: 9 i was reading something on the web and was wondering if this could be right, knowing the battery is 45 amps, and i am using around an 8 amp bulb, i would assume 8 amps is how much the bulb uses per hour??? if so, 8/45 = around 5.6 hours? could this be right?
 P: 9 i think you can do it this way to? take the 45 amps * 12v to get 540 watts, so we know the bulb is 100 watts, so i guess the bulb uses 100 watts per hour, which means it would last, estimating in my head, looking at the battery which is 540 watts, id say it would run little over 5 hours? does it sound like i'm on the right track?
 PF Gold P: 8,964 Car battery amps? Welcome to PF, Mountain. There must be a misunderstanding somewhere. If you have a 45 amp battery, you sure as hell aren't driving anywhere. The minimum cold cranking amps for a car battery is probably around 350. My Roadrunner draws 1,000 amps in sub-freezing temperatures.
 P: n/a Yes 45 amps sounds way too low. Automotive batteries supply much more current - the one I keep around the house is 650CCA (550 otherwise) but is slightly larger than what you would see in a car. Anyway, some batteries will give a rating of the full charge in Amp-hours, usually in the form of "y amps for x minutes" written somewhere on the identification label. The product of the current supplied and the discharge time is relatively constant, so if you know how many amps your lightbulb will pull then you can guess roughly how long it will take to discharge the battery.
 P: 198 cold cranking amps are different from amp-hours which is what you want, cca is how many amps can be delivered short term during cold weather, amp-hours is how many amps can be continuously be delivered during one hour. so if you have 45 amps-hour then you get 540 watts that can be delivered continuous for an hour, the cca number will be higher, say 120 amps that can be delivered to the starter instantaneously during winter conditions.
 P: 9 my friend on msn is telling me that a car battery as way more than 45 amps, and ive seen it here, i apreciate all the help, but would you not agree that a 100 watt household bulb on a 12 volt battery would be using around 8.3 amps? and how long could i exspect this bulb to run? 5.4 hours? if so, would that prove the battery was 45 amps? im sorry, i am fairly new to this and do understand i am lacking some knowledge
 P: n/a You're confusing the relationship between max output current (rate of change of charge / time) with the amount of charge (Q) the battery has. You can't get the max current (dQ/dt) the battery could ever output based on how long it took for the battery to discharge from loading it with a lightbulb that consumes such little power. The lightbulb will draw only so much current - period, and as long as the battery can deliver 100W then the lightbulb will shine. What you *could* do is attach 5 100W lightbulbs in parallel and confirm that they all are bright and observe upon the addition of a 6th or 7th that there would be some diminishing brightness. That would confirm more or less you have a (5 to 6) * 8.3A ~=45A of supply current.
 P: 198 that wouldn't work, the battery would supply more current. the way you measure a car batteries output is by it's voltage level for rough estimates. hook up the 5 light bulbs and put a voltmeter in parallel, watch as the voltage drops from 13.5 volts at no load to 12.5~13 at rated amp-hour load to 11~11.5. that should tell you more or less how much power the battery can supply over a givin time, if you want more amps increase the number of light bulbs. car batteries charge to ~13.5 volts and are considered dead at 11.
P: 6
 Quote by MountainDew would you not agree that a 100 watt household bulb on a 12 volt battery would be using around 8.3 amps?
I'm no expert, but I think this is incorrect. The resistance of the bulb remains the same when it's hooked up to 12V battery, therefore the current it draws should be 0.083 amps (i.e. 1/10th of what it draws at 120V).

This turns the bulb into a 1watt (12V * 0.083amps) bulb; i.e. its glow will be barely detectable.
 P: 198 a 60watt 120v bulb draws 7.2watts and is 20 ohms cold off of 12v so a 100watt bulb will draw about 10.2 watts cold.
 P: 143 MountainDew, The 45Amp your talking about is a per hour rating, meaning that your battery could give you a 45Amp current for one hour before it dies out. About the second part of your question, when you take a 120Watt bulb and connect it to a 12V car battery, you can simply calculate the current drawn by the bulb from the relation P=VI, now since it's a 120Watt bulb and the car battery voltage is 12V the current drawn will be 10Amps, now since your battery can supply 45Amp for one hour then it can Supply 10Amps for 4.5 hours.
 P: 2,506 Wow! Lot's of misinformation here. An incandescent light bulb is not a linear device. The more current that flows through the bulb, the higher the resistance of the bulb gets. Also, why do you assume that no matter what voltage source you hook a bulb to it will automatically dissipate the rated wattage? The 120 volt 120 watt bulb will dissipate 120 watts AT THE VOLTAGE IT WAS DESIGNED TO OPERATE AT! It will NOT draw 10 amps, ever.
 P: 143 Averagesupernova, I think most of us assumed an ideal condition, we looked at it as a HW problem rather than the practical case.
 Mentor P: 22,280 The "ideal" condition wouldn't be holding the power constant, it would be holding the resistance constant.