How do I calculate battery life for my devices?

Question

I want to power coupe devices via car battery, but before I do that I want to figure out how long battery will last. Most of the devices work from 5V so I am using 12Vto5V converter (link to item on ebay) The convert powers RaspberryPi, Arduino Pro Mini, and usb hub&devices for raspberry pi. Total current draw is around 2A @5V. Second converter is 12Vto9V and current drain is 1A at most. Lastly I am using one of the cigarette ligher socket 12V to usb devices (link on amazon). I have yet to measure idle current draw but at most this will draw 2A@5V.

Now, how do I go about calculating battery life with those devices? I can measure current between 12V and all the devices&converters, but what then?

Answer 1

The current rating for the converters is only the maximum current it can deliver, thus is not useful to calculate the power drainage. To get the time you have to know the real input current for every DC converter(\$I_i\$) then, since they are in parallel,

$$I_{tot}=\sum_i{I_i}$$

To get the time, suppose \$Q_p\$ is the capacity in Amp/h of your battery, \$T\$ is approximately the time it will last (in hours)

$$T=Q_p/I_{tot}$$

So, if you can measure the \$ I_{tot} \$, you can directly use the aforementioned formula.
Keep in mind you should not completely discharge a lead battery or it will decrease sensibly the capacity.