# Battery Life for Changing Loads

Discussion in 'General Electronics Chat' started by craigfoo, Dec 15, 2013.

1. ### craigfoo Thread Starter New Member

May 16, 2013
22
0
I'm trying to figure out how long my battery would last in these conditions:

I have a 3Ah battery and I need to run a sequence. The sequence goes like this:

The maximum draw would be 2 amps.

0 secs: 1 amp draw
2 secs: 2 amp draw
4 secs: 2 amp draw
6 secs: 2 amp draw
8 secs: 2 amp draw
10 secs: 2 amp draw
12 secs: 1 amp draw
14 secs: 0 amp draw
20 secs: repeat sequence

If I were to take this sequence and graph it out, I would think I could take the area of the curve and get Amp-Seconds and do my calculation from there or am I way off base?

2. ### #12 Expert

Nov 30, 2010
16,705
7,354
It really is that simple to find the accumulation of amp-hours.
Then you consider whether you want to run your battery dead before recharging it. Most batteries wear out a lot quicker when run all the way empty between recharges.

3. ### craigfoo Thread Starter New Member

May 16, 2013
22
0
Ok, I thought I was losing it.

So that's 24 Amp-Secs for a cycle, 3 cycles per min which gives me 72 Amp-Secs for a 1 min run time.

It would be 72 Amp-Secs/3600 Secs to get 0.02 Amp-Hours. If I wanted to know how long it would take to only run my battery down to half, where do I go from here?

4. ### #12 Expert

Nov 30, 2010
16,705
7,354
1.5 amp hours divided by .02 amp hours = 75 minutes at 3 cycles per minute...if I didn't slip a digit.

5. ### craigfoo Thread Starter New Member

May 16, 2013
22
0
How does dividing amp-hours by amp-hours give me minutes?

6. ### #12 Expert

Nov 30, 2010
16,705
7,354
72 amp seconds per minute.
72 amp seconds x hour/3600 seconds = .02 amp hours per minute of use
Half the battery capacity is 1.5 amp hours.
1.5 amp hours x 1 minute of use/.02 amp hours = 75 minutes of use

7. ### craigfoo Thread Starter New Member

May 16, 2013
22
0
Ahha, touche. Thanks for your help!

8. ### #12 Expert

Nov 30, 2010
16,705
7,354
Forest for the trees today, huh?
We all do that occasionally.