When designing a low power product, it’s important to know the voltage profile of the batteries you’re using. I decided to set up a test on Friday (it’s Sunday now) to make sure the batteries we’re shipping with the Mooshimeter live up to their specs.
I pulled a single AA from our battery stock and set it up so that it will discharge through a 5.6Ω load while logging both the current through the load and the voltage across the battery. The battery being tested is a Duracell Coppertop with expiration date marked March 2021 and a manufacturing date stamp saying 2014/09.
I came in to the office today and pulled the data logs from the SD card. This was also a test of the latest firmware version, which logs directly to CSV instead of the intermediate binary format that production meters use right now. I pulled the data in to excel and did some math to generate the profile below. You can download the data log here.
For readers unfamiliar with battery metrics: battery capacity is typically measured in Amp Hours (AH), which is the amount of current supplied times the amount of time it is supplied. Unfortunately there are many factors that influence the number of AH a battery can supply, the biggest one being the rate of discharge. Generally speaking, the lower the rate of discharge, the more AH you can pull from a battery before it is depleted. Another issue is the voltage requirement of your device. If your device needs a certain voltage to run, battery capacity below that threshold voltage is useless to you.
So how’d the battery do? Extremely well! In this test we pulled almost exactly 2AH from the AA battery before passing the 1V threshold (1V per battery is necessary to run the Mooshimeter). We can expect the 2AH number to be extremely conservative, because this test was in to a 5.6Ω load, which means we were drawing an average of about 200mA for the usable portion of the profile. This discharge rate is about 3 orders of magnitude higher than the nominal discharge rate for the Mooshimeter, so we can expect to extract substantially more than 2AH before the batteries become unusable.
If you’re unfamiliar with the format of the plot above, here’s a simple plot of voltage and current from the battery vs. time. Something interesting happened around 7 hours in to the test, it looks like the test setup was jostled in a way that the contact resistance in the current path increased for a few samples, at least that’s my best theory right now.
If you’re interested in the raw data that came off the Mooshimeter, you can download it here.
Thanks for reading
Edit March 2016: The excel file I used to make the graphs, you can download it here.