Battery Output and Amp-hours
Date: Summer 2012
I have a 12V 250 amp-hour lead acid (lawn tractor) battery. I need to run a small 80mA 12V muffin fan to ventilate a small area for about 8 weeks every summer. (at a shady camp with no electricity)
The battery voltage will degrade, but the fan works with as little as 3 volts...
My question: Should an 0.08Amp 12V fan run for at least 1350 hours before the battery gives up? 1350 hours is roughly 8 weeks.
and 250 amp/hours tells ME the battery will put out 1 amp for 250 hours. Of course it not linear, but a 0.5 amp draw should go for about 500 hours, a 0.1 amp draw should last 2500 hours... Am I way off base here?
No, you are correct. If you have a 250 amp-hour battery, it means (ideally) that you could draw 1 amp for 250 hours, and 0.1 amp (100mA) for 2500 hours. For your case, it should be well within the capacity of this battery when it is fully charged. Just a caveat: a 250 amp-hour rating does not mean you should try to draw 250 amps for a full hour from it. This current is quite high, and it could cause the battery to overheat. In your case, you are not planning to draw so much current, and your expected need is much less than the battery capacity, so you should be fine. Be certain to take precautions with a lead-acid battery, though, especially if you leave it unattended. They contain acid as their name implies, and as such can cause injury if they leak. They should not be charged or discharged at too high a rate. They can vent hydrogen and oxygen when they are being charged causing an explosive hazard. Do not seal the battery in a box or anything else that does not have proper ventilation.
Kyle Bunch, PhD, PE
Your calculation is correct. The fan can run as long as 3125 hours based on the current draw of 0.08 A.
Loker Hydrocarbon Research Institute,
Department of Chemistry
University of Southern California,
The power source rating of 250 amp-hours means it can supply 250 amps for one hour or 1 amp for 250 hours.
Ideally (that is disregarding physical limits of the actual equipment that may limit current or voltage delivery capacity):
12 Volts (V) Times 250 Amp-hours (A-h) = 3,000 Watt-hours (W-h)
12 V Times 0.08 Amps (A) = 0.96 Watts (W)
3000 W-h Divided by .96 W = 3,125 hours (h)
3,125 h Divided by 24 hours per day (h/d) = 130.21 days (d)
130.21 d Divided by 7 days per week (d/w) = 18.60 weeks.
A half Amp load = 12 * .5 = 6 Watts; 3,000/6 = 500 hours; 500/24 = 20.83 days; 20.83/7 = 2.98 weeks
A 0.1 Amp load = 12 * 0.1 = 1.2 Watts; 3,000/1.2 = 2,500 hours; 2,500/24 = 104.17 days; 104.17/7 = 14.88 weeks.
3,000 W-h drawn at
.08 Amps lasts 130.21 days or 18.60 weeks
0.1 Amps lasts 104.17 days or 14.88 weeks
0.5 Amps lasts 20.83 days or 2.98 weeks.
The expectations set is almost correct. A 250 Ah battery is expected to give 10 hours at 25 amps. This is called 10 hour discharge rate. If the current drawn is lesser say 2.5 amp, then hours will be even more than 100 hours.. perhaps 120 hours, and at 80mA, one might expect the number of hours to be perhaps 1.2 to 1.5 times 250/0.08
Dr. V. S. V. Mani.
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Update: November 2011