While it’s only possible to provide an estimate the app is an easy way to plan a portable Pi project without doing the required maths by hand. By using the features available it is also quick to re-calculate to test various changes to your system.
The calculation presents a time range to give a reasonable estimation of the run-time assuming the battery is healthy and fully charged.
The app allows you to :
- Select a Raspberry Pi Model (A,B,A+,B+,2,3,Zero)
- Select different capacities of USB power banks
- Select different AA cell capacities and voltages
- Select different AAA cell capacities and voltages
- Choose from a set of features that affect energy use such as Ethernet, HDMI and WiFi
- Choose number of processor cores in use for the Pi 2 & 3
- Choose from a set of “wildcard” current values
Wildcards are useful for :
- Adding extra current if you know your keyboard or WiFi dongle uses a bit more than my estimates.
- Allowing for additional add-on boards such as LED boards
The maths is fairly simple. It takes your selections and creates a current draw total. This varies for each Pi model as their idle power usage varies. It then calculates a battery capacity at 5V given the capacity selected. For AA/AAA cells this depends on the individual cell capacity and cell voltage.
For USB power banks their advertised capacity is usually measured at 3.7V.
The runtime is then just the capacity at 5V divided by the average current drawn by the Pi. The lower estimate is 80% of the higher estimate.
When available you may want to power the HDMIPi Screen from the same power bank so that is listed in the “Features” so you can calculate an estimated time with that hooked up as well. In order to keep the interface as simple as possible this is the only Pi accessory I have specifically included but the wildcard options should cover anything else you may want to use at the same time.
The current consumption of the Pi and the options selected are based on real measurements and not merely plucked from a manufacturers datasheet. This gives a more sensible real-world average rather than being a peak current draw which gives unhelpful estimates.
It’s hard to compensate for the age of batteries which may be past their best but the lower estimate should help balance the higher estimate which older batteries may never reach.
Robots & Motors
You might be wondering why I didn’t include an option for motors. The reason I didn’t is they vary a lot and it would be difficult to calculate a sensible result based a pre-defined average. Using the wildcard values you could add an additional 1200mA to provide a starting point.
This app does not gather or transmit any information from your device. It simply uses the option you select to calculate a runtime value.
It does not contain any adverts but does contain some clearly described links to RaspberryPi-spy.co.uk and my social media channels. These are directly related to the Raspberry Pi.
I created this app because it is something I wanted to use myself and I hope it is useful to you to. If you like it please G+1 and rate it in the Google Play Store. If you don’t like let me know and I’ll see what I can do to improve it in future updates.