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Based on empirical measurement, the voltage characteristic of the lithium polymer battery on the Nexus 7 is mostly linear between 25% and 85% of total capacity when draining charge constantly. For measurements to be reliable, capacity needs to be kept within those parameters.

Based on this measurement, for tests to be accurate, they shouldn’t draw more than 20% of battery, or they won’t be as accurate.

Sometimes, odd values come up when sampling, mostly 99% capacity even though the battery is far from that value already. The sampling on this test was done twice per second. This has been fixed by https://bugs.launchpad.net/ubuntu-nexus7/+bug/1093543.

Draining the battery at short intervals, increases the error of the calculation slightly at some charges, this is a graph showing the draining during a dd running for 20 seconds, resting for 30 seconds, then repeating until the battery is empty. It is still better to run test cases between 20% and 85% capacity.

After the new kernel was available, with Bug 1093543 fixed, this is how the picture looks like:

The outliers that could be interpreted like the readings not being accurate or reliable in the first picture are not there any more.