# Thread: Calculating the battery life..

1. Hi,

I've forgotten the calculation for working out battery life.

How do I work out the battery life of this:

DC 12V, 7AH with a 90W bulb

Cheers,

Neil

2.

3. Looks like a homework question - so you'll have to google it -there's plenty on the net...

4. Originally Posted by Megabrain
Looks like a homework question - so you'll have to google it -there's plenty on the net...
It's not actually, don't jump to conclusions. Considering that I'm on my third year of university studying criminal psychology, I'm not really up to date with what I learnt back at secondary school, I only asked because when I Googled it I couldn't find a straight formula.

Regards

5. There is simply no straight formula. It depends upon several things like internal resistance. From memory the AH is the maximum charge it will hold. Just because you might get 40 amps for 1 hour does not mean you automatically get 1 amp fo 40 hours, at this lower rate you will get more than 40 hours worth , what you need to do is look at the data sheet for the specific battery you have in mind and look at the discharge curves.

6. Originally Posted by Megabrain
There is simply no straight formula. It depends upon several things like internal resistance. From memory the AH is the maximum charge it will hold. Just because you might get 40 amps for 1 hour does not mean you automatically get 1 amp fo 40 hours, at this lower rate you will get more than 40 hours worth , what you need to do is look at the data sheet for the specific battery you have in mind and look at the discharge curves.
OK thank you. I will search for the datasheet for the battery. I have managed to find a formula however for this - well more of a theoretical formula, and I have calculated it to around 3 hours, purely off the digits I previously said. But this calculation is obviously not taking into consideration the battery discharge curve you mentioned.

Regards

7. 7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

Whatever formula you found is either wrong or you did not work it out properly.

8. Originally Posted by Megabrain
7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

Whatever formula you found is either wrong or you did not work it out properly.
thats what i worked it out as :wink: 90w from a 12v battery gave me 7.5A

9. Well done CC - Shouted out the answer after it was written on the board, go and stand in the corner....

10. Originally Posted by Megabrain
Well done CC - Shouted out the answer after it was written on the board, go and stand in the corner....
i used this thing

11. Originally Posted by Megabrain
7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

Whatever formula you found is either wrong or you did not work it out properly.
Well done. I was just testing your knowledge before, I knew that this was correct.

But thanks again

12. CC at last! a post of yours that can be read!

13. Originally Posted by Megabrain
CC at last! a post of yours that can be read!
not sure how to take that

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement