I’ve previously written a fairly detailed analysis of the performance of various AAA batteries in Medtronic insulin pumps being used in a closed-loop setup (the frequent radio traffic shortens the battery life significantly from when the pump is operating in standalone mode). In the 8 weeks since that post I have continued to collect data from more batteries, and can now review the initial conclusions of that post. I did put out a partial update a month ago, but this post supercedes that.
As before, this data is gathered by my OpenAPS rig recording the battery voltage measured by my Medtronic 554 pump. When the voltage drops low enough to trigger the pump’s LOW BATTERY warning (usually around 1.16V) the pump turns off the radio and it drops out of the loop. The pump usually keeps going with its programmed basal rates, but we replace the battery ASAP at that point to restore looping.
It can seem extreme if you’re having to replace the battery at about the same frequency as the insulin reservoir, so we want to be sure we’ve chosen the best compromise between cost and convenience when selecting batteries for use in our pumps. To this end I’ve selected seven models of AAA battery, and have been adding more samples to each. All of the batteries were sourced at mainstream retail locations in Australia, and were used fresh out of their packaging.
The high-level view
Here everything’s on one graph:
Click on the image for a larger view.
For observations about the shape of these graphs (including the voltage spikes that coincide with reservoir priming, and the effect of temperature on lithium batteries) please go back and read Part 1 of this study.
The major new information here is from having had time to test more than one battery of each type. For example, the Energizer Ultimate Lithium (green on this graph) has had two batteries lasting about 10.5 days, whereas on only lasted 9.5 days. There will usually be the occasional “outlier” in real-world testing, and I feel comfortable sticking with 10 days as being an expected lifetime for one of these batteries in this equipment.
At the other extreme the Duracell Coppertop (red on this graph) has had only two results so far, both clustered just past 3.5 days. The odds seem fair that if I test another Coppertop that it will achieve about the same, and thus my estimate of 3 days seems safe.
The Energizer MAX (light blue) only managed just past 4 days on my first test, so I had to round that down to 3.5 days for safety. However it seems that was another outlier, and the next two MAX batteries have been closer to 5 days, so now 4-4.5 days seems fair.
The Energizer MAX is one of the cheaper models I surveyed, and this change to predicted life does make a large difference to any ongoing cost calculations (although the IKEA Alkalisk is still winning on that front).
Simplified battery life results!
The MAX is now clustered with the rest of the alkaline batteries: the IKEA Alkalisk (purple), Duracell Ultra (dark blue), Energizer Advanced (yellow), and Energizer Eco Advanced (orange). Because most people will want to change the battery (or at least have a spare on hand) before the old one expires, it now seems reasonable to estimate 4.5 days for all those models. That just leaves the Duracell Coppertop (3.5 days) and Energizer Ultimate Lithium (10 days) as the exceptions.
Where to from here?
I don’t have any particular outstanding queries left about this subject, so don’t at this point expect a further update post. But my rigs will continue to keep gathering the data as I use up the batteries in my store.
When I move to a different insulin pump (am hoping for this in early 2018!) we’ll have to see what voltage data is available from my new setup, but my measurement of Medtronic/OpenAPS battery use will presumably cease.
Addendum – price check
Since my October post the prices have changed slightly for a few of the batteries at the suppliers I surveyed. So with the updated lifespans and with today’s prices, here are the cheapest options I could find:
|Energizer Ultimate Lithium||Bunnings||AU$0.37||10||4|
|Energizer Eco Advanced||Woolworths||AU$0.37||4.5||10|