Power Consumption

We are doing a project and trying to figure out the power consumption for a given amount of time. We are using an ATMEGA 2560 and can see from the graphs that it will use about 25mA and we are going to say 5.5V but it does not give us any time frames. We are trying to figure out how long it would last. We need to calculate the power usage per hour to try and find a battery that will allow the Arduino to last X amount of days. I know we have to add in all of the external parts and sensors etc, we are just trying to figure out a time from the information that is present in the datasheets. We do not have the parts to actually physically test them, we have to just use the datasheets for our projects.

I think you are really trying to energy** capacity as opposed to power.

Energy for this sort of thing is in Amp x Hours, commonly called Ahr or AmpHr and sometimes milliamp hour (mahr)

In addition, when looking for a battery you must consider the conditions the battery and Arduino are specified for.

Most batteries have a set of discharge curves that specify the deliverable energy capacity with a specific end voltage. Also temperature is a factor, if you are using this outside.

** Actually Energy also includes voltage but for this discussion we use the term slightly incorrectly to help in understanding.

John

Battery capacity is given in Ampere Hours or mAh

So in theory a battery of 1000mAh will last for 1 hour if you draw 1000mA or 1amp from it.
Likewise it will last 2 hours if you draw half as much ie 500mA.

Hope it helps

And then when the battery says 1000mah, calculate you only use about 50% of the full capacity of the battery. If you would use the ful capacity your battery will be dead.

JohnRob:
Energy for this sort of thing is in Amp x Hours, commonly called Ahr or AmpHr and sometimes milliamp hour (mahr)

I've only ever seen "Ah" and "mAh", as the abbreviations, and I've seen a lot of batteries and datasheets.

Another thing to be aware of is that the nominal capacity given by the manufacturer is a
best case figure for a fresh battery, fully charged, usually at the optimal temperature, and for
a slow discharge (often over 10 hours).

Expect a new cell to realistically have about 80% of that capacity in practice, dropping to 50% or
even less over the life of the cell as its discharged and recharged. (no battery chemistry is
anything remotely like perfect).

Let a cell/battery over-discharge even briefly and the capacity can be completely trashed.

So in reality if you want 2.5Ah capacity, buy a 5Ah battery or so, then its likely to actually
last (if treated well).

We need to calculate the power usage per hour

No, you need to determine the current, and multiply by the time to get the capacity. Capacity
for batteries is just electric charge, but is usually measured in ampere-hours rather than coulombs,
since the numbers are more handy. 1Ah = 3600 coulombs.

If you don't know already, an ampere is the flow of one coulomb per second, so one amp for one
second is a charge or capacity of 1C (1 coulomb), or equivalently about 0.28mAh

I think this is what you want