Ok, I'm in to modding and know my way around a dremel but still kinda new to the solder iron and basic electronics. I was wondering if there is a formula or a calculator out there to tell you the approximate life of a battery under certain loads. I searched but didn't find much I understand . Still learning volts and amps (thanks to the Electronics Sticky). Sorry if a bit too wordy. Thanks in advance for input.
The primary rating for battery capacity is the mAh rating, or Milliamp-Hours. For example, a battery with a 700mAh rating can have 1mA drawn from it and provide the same level of current for 700 hours, or have 700mA drawn from it and be able to last for 1 hour. Obviously, higher is better. But that's just theoretical. For example, there's the matter of internal resistance. Since batteries are a voltage source, this means the the battery is drawing it's own current, so that adds to the total amount of current being drawn. This probably isn't that big of a deal, but it's something you should be aware of. And there's also the voltage drop. All batteries, no matter what their composition is, will have their output voltage decrease as more current is drawn from them. Ideally, you'd have a battery with a very steep voltage drop; the same output voltage is maintained until the battery is almost dead, then it drops like a rock. This screws with the amount of battery "life" left, as if we're talking about a consumer device, they'll need the battery to have a voltage above X amount, even if it still has some life left in it. Finally, we come to the memory effect. This only applies to Nickel-Cadmium (NiCad) batteries IIRC. What happens is if they are recharged and are not almost depleted, then the batteries apparent capacity is equal to the amount consumed before being recharged (i.e. I charge it when it's half-empty, and when it's done, it only has half of the capacity). This can only be reversed if the battery is almost completely drained (that "almost" is important, as completely draining a NiCad will apparently reverse its polarity), then recharged. The missing capacity will then return. For the most part though, you can stick with the mAh rating.
Great! Exactly what I needed. So let's say you are running a LED (General is about 3v and about 20 mA, right?) off of 2 AA batteries; How long would the battery last according to these specs? Average specs for a AA batt about 2,850 mAh (Rated capacity at 25 mA continuous drain) Battery Specs Linky Would someone tell me how to figure that? Thanks and sorry for my ignorance.
That's a good example as the current is in the LED operating range. At 25mA led current the batteries will last in theory 2850/25 = 114 hours, or about 4¾ days, but if you drop the led current to half as much they'll last twice as long. However, the figure quoted is for the 1.5V battery going right down to 0.8V, and the led will stop lighting up at somewhere around 0.9V per battery, so actual life will be slightly different.
