I am making my own Bill-tong box to dry meats. I have 4 fans which are 0.16A each Running at 5V from a battery pack that is 15000mAh
I have tried doing the math myself online but the answers I am getting back are crazy!
I would like to know how many hours this battery pack will run these fans for when fully charged
Total draw: 4 * 0.16 = 0.64A
Total capacity: 15Ah
Approximate time: 15/0.64 = 23-24hrs
That assumes the labels/ratings on the fans and power pack are accurate. Most cheap battery packs have much less then the rated capacity, and it falls off with time and cycles too.
You have four loads drawing 160 mA each, for a total of 640 mA. You have a battery pack rated for 15 Ah. Both the battery and load work at the same voltage.
(15 Ah)/(640 mA) = 23.4 hours
Yes, the math really is that simple.
However, batteries are a lot more complicated than just a single charge capacity rating. The battery capacity changes with temperature, previous history, rate of discharge, phase of the moon, and what your dog had for lunch yesterday.
Take the math as a rough guide. The answer is therefore "not quite a day".
If you have to give a minimum guaranteed spec to a customer, you should derate heavily. You should also do a few real tests then. Make sure the fans really do only draw 640 mA or less, and that the battery pack really can put out 15 Ah before the voltage goes too low to be useful. If everything is working correctly, you might specify 20 hours at room temperature. Derating by half in a more general case isn't out of line at all.
