# How long will 2 aaa batteries last when connected to a 5mm 25ma 3.3v led?

The batteries are connected to a simple switch, then to the LED. Just wondering - hours? Days? Is there a better option for power supply? I need them to last around 150 hours.

seandogue8 years ago
Since the LED is running under its specified forward voltage, the forward current will drop as well. Without knowing the specific LED, it's hard to say will a high degree of assurance.

Let's assume it operates at 18mA at the lowered forward voltage.

An average alkaline AAA battery has a capacity of ~1000mAhr

1000/18 =55.5

Now let's drop that value for losses...say to 45 hours.

Is there a better option? sure 2 D cells. (>10KmAhr) ;-)

you do the math.

Here's a nice link for battery capacity

lentze (author)  seandogue8 years ago
So if one aaa is 1000mAhr, does that mean the equation changes when there are two aaa batteries used? to 2000/18 = 111 hours?
8 years ago
no. That will be the capacity of the 3V system. Only by paralleling will you achieve increased capacity of the system. So for instance, two paralleled dual AAA 3V
8 years ago
Also, please note that 18mA is a total guess on my part. You need to connect it up and measure the current to see what it draws at 3V to make a more accurate calculation of what the operating value will be. And you also need to pad that required mAhr values to compensate for the fact that you're running the LED at lower than expected Vf (when the AAAs become depleted they will not output 3V and you may find that the diode stops conducting prematurely if you run at a marginal value of capacity...in other words, instead of 150Hours as the target calculation, shoot for 200 hours and you'll be safe. This is what's called an "engineering margin" or conservative estimation.). That's why I suggested the D cells.