Do I understand this correctly... battery power and life

Clint

I have a circuit powered by a 5v regulator. When I·hook 9v to the voltage regulator, I·measure about·200mA of current going into the regulator. How much current is my curcuit using at 5v? Still 200mA, right?

I am trying to figure out battery life if I use 4-AA batteries in series·to power my curcuit. Acording to http://data.energizer.com/·an alkaline AA battery will provide 2850mAh. When I have four batteries in series drawing 200mA @ 6v, what life could I expect? Would it be 2850mAh/200mA=14.25h?

Post Edited (Clint) : 7/3/2008 2:22:50 AM GMT

Mike Green

The regulator uses a little bit of current for itself, maybe a few hundred uA or a mA or two depending on the type of regulator. All the rest of the current is used by the 5V load.

You have the battery lifetime calculation mostly correct. The battery capacity depends partly on the amount of current drawn. At very high currents, the battery capacity will be lower due to internal battery losses. Most batteries also have some "self-discharge" rate where there's some internal leakage of current. At very low currents, this affects the shelf life of the battery.

Beau Schwabe

Clint,

In theory yes, 14.25 hours.... but as your battery weakens so does the voltage and the battery's (4 xAA) ability to perform. Another factor that you need to consider is what the minimum voltage and current is that will allow your circuit to still properly function. If those requirements are under the discharge curve pertaining specifically to your battery, then the amount of available usage will be affected.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Clint

Thank you for the feedback. You've been very helpful. I will look a little deeper into how the battery life is affected by current draw and also what the minimum voltage is for my circuit to function correctly.