Calculating A-hrs: Understanding a Car Battery's Performance and Efficiency

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A car battery rated at 70 A-hrs can be calculated for its duration at a current of 15A. The correct calculation involves dividing the amp-hour rating by the current, resulting in 4.67 hours or approximately 280 minutes, not 17.5 minutes as initially suggested. The discussion highlights a misunderstanding of the conversion between amp-hours and minutes, clarifying that 70 A-hrs divided by 15 A gives a straightforward answer in hours. The conversion to coulombs is unnecessary for this calculation. The final conclusion is that the battery can last around 280 minutes at a 15A draw.
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My teacher posted a practice exam with no answers so I want to make sure I am doing my problems right.

A car battery is rated at 70 A-hrs. How many minutes can this battery last if it operates at a current of 15A?

I made an equation 70A/60min = Xmin/15A

So, X=17.5 min

Am I right?
 
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nckaytee said:
My teacher posted a practice exam with no answers so I want to make sure I am doing my problems right.

A car battery is rated at 70 A-hrs. How many minutes can this battery last if it operates at a current of 15A?

I made an equation 70A/60min = Xmin/15A

So, X=17.5 min

Am I right?

An amp is one coulombs per sec. That means that an Amp hour is 3600 coulombs.

70 amp hours is 252,000 coulombs.

15 amps is 15 coulombs per sec = 54,000 coulombs in an hour.
 


so 900 minutes?
 


nckaytee said:
so 900 minutes?

How did you get 900 minutes?

You have 252,000 coulombs to spend and you spend 54,000 an hour? How many hours is that again? Times how many minutes an hour?
 


Just divide 70 A-h by the 15 A, the A cross themselves out and your left with just the hours not necessary to switch to coulombs.
 

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