# Calculating capacity and how long to charge a battery

Discussion in 'General Electronics Chat' started by ptownbro, Oct 14, 2013.

1. ### ptownbro Thread Starter New Member

Sep 28, 2013
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Trying to learn charge times and how capacity is measure. Best way for me to understand it is to look at the math equations. Any help would be greatly appreciated!

Assume you drew 100 amps from a 12V car battery for 30 minutes and the amp-hour rating on the battery was 55 Ah. Two questions:

1) What would be the capacity of the battery at 20 minutes expressed in a percentage (like, for example, in comparison to the percentage expressed on your cell phone after you used it for a given time).

2) How long would it take to charge the battery back to full after 30 minutes using a 12V alternator providing it 100 amps?

The way I'm envisioning this to make it make sense to me is what happens when using the cell phone as a (maybe imperfect) example. If I talk on it for 20 minutes it will go down to X%, then after you plug it in it will take X minutes to charge back to 100%.

Please show me the math to get to your results and feel free to correct any of my terminology if I've worded something incorrectly. Like I said I'm trying to learn.

Thanks!

Last edited: Oct 14, 2013
2. ### ronv AAC Fanatic!

Nov 12, 2008
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Alas, it is not so simple. When discharging the harder you make the battery work the lower it's capacity is. If you look at the curve below for example you can see that this battery will run for about 4 hours at 6.6 amps discharge rate so you would expect it to last 0.4 hours at 66 amps, but it will only run 0.2 hours.

Charging is a bit different in that it is only about 75% efficient (varies) and is usually limited in magnitude by heating in the battery. Most lead acid batteries for example recommend about 10% of the capacity for the maximum charge rate.

Aug 27, 2009
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