Can someone post a formula / help me figure out how to figure out the battery run time for the following scenario?
I have a single 12v led light bulb with a draw of 510mA @ 12v and 1400mA @ 8v. If I start with a AGM 12v 80Ah battery and let it run for say 10 hours how do I figure out the run time given that the amp draw will rise as the battery is discharged? I know battery discharge is non-linear so I am not sure how to calculate.
Also it is worth noting that I plan on using 4 of the these light bulbs so ultimately I need to also calculate that in as well.
Battery Specs:
Product ID: 24M-XHD
Cranking Amps: 1000
Cold Cranking Amps: 800
Voltage: 12
Termination: Common Code M
Weight (lbs): 44.6
Width (in): 6.88
Length (in): 11.00
Height (in): 9.50
ReserveCapacity-25: 135.00
WET/DRY: W
Best Answer
What you need is the battery capacity, usually expressed in Amp-hours. If you want to run something that takes 510 mA for 10 hours, then it will drain 5.1 Ah from the battery.
Any 12 V car battery will handily exceed that minimum rating. Car batteries don't like to be deep discharged, so plan to use only half the rated capacity. At that level, the voltage will be fairly close to 12 V the whole time, so you shouldn't have to worry about the 10 V draw.
In any case, are you really sure about this light taking 1.4 A at 10 V, but only 510 mA at 12 V? That sounds really odd and implausible, like maybe you read something wrong. This implies it uses 6.1 W at 12 V but 14 W at 10 V. Where do the extra 7.9 W go? If it has a switching power supply I could believe a little less efficiency at 10 V compared to 12 V, but not such a whopping change with just a little less input voltage. Something isn't right in what you're telling us.