Battery Life: One Month with the 640g

We had to change our fifth AA battery on the 640g last night. Based on our experiences to date, I thought it might worth a quick post for two key reasons that might be of interest:

• We get an average of around 9.5 days out of each Lithium AA battery [range: 8.5 - 10 days] 
• Battery status changes quickly from amber to red  (in a few hours)

We've used the recommended Energizer Ultimate Lithium AA batteries and have changed the battery at the first "Change Battery" Alarm i.e. in theory we would have got some more use out of it if we'd ignored that and gone through to the "Empty battery" symbol (5% estimated I think).

The battery status moves from amber (~ half full) to red (battery change warning notice at - I think around 20-25%) in just a few hours (less than 4 hours last night), which surprises me, given the week plus lifetime of the battery. I guess this may be due to the more constant power output of a Lithium (Li-Iron Disulphide) cell over the battery lifetime, with a (relatively) rapid drop in capacity only towards the end of its service life [geeky stuff here]. Maybe that's why the 640g feels it has to wake you up in the small hours to warn you about the battery, at the nominal 20-25% level warning, and not be smart / confident enough to wait until your morning basal rate change i.e. the change signifying that you - or your little pumper at any rate - is probably wide awake? I don't know if any of the other pumps have this feature? I imagine its something left on the 'would be nice' list when it comes to keeping Medical Devices as simple as possible to get them through approval and because of a genuine desire to keep risks of failure to a minimum.

Our first two batteries were supplied by Medtronic with the pump, the other batteries came from us (ordered directly with Amazon to ensure we got the real thing and not a factory reject or - even worse - a fake). We did try to order from Medtronic but they've been out of stock (I guess either someone didn't get the memo about the 640g launch and the change in battery size, or they ran out supplying all the new pumpers :-) )

Our settings will impact on battery life (as will the number of alarms Janki triggers). We've had low alarms on, high alarms off, we use audible and vibrate alarm settings, auto brightness and a 60 second timeout on the display.

Ok, so the battery life is not as good as Medtronic originally quoted: we were told the same as others in the lead up to its launch, namely that expected battery life would be 2-4 weeks. I noticed that Medtronic have updated their battery life on their Customer Support page to quote 7-14 days. According to this page, some features that will decrease battery life include (on one line):-

- high basal rates –  “excessive” button pressing, bolusing and rewinding  

I have this lovely picture in my head of pumpers trying to play Tetris on their 640g or setting a challenge of bolusing for each mouthful at mealtime ;-) 

Whilst this reminds me of buying a mobile phone and never, ever reaching the quoted battery or talk time figure, I'm pleased to see Medtronic UK have now put realistic battery times up on their site. I'm not sure why the projected figures were so much higher before - presumably these came from the clinical trial data and their internal 'lab tests'. The 50% 'tweak' in the battery figures isn't a total disaster (for us) and a week plus of battery life is ok (for us). We haven't put it to the test, but there is still the inbuilt rechargeable backup battery, which is a fantastic idea and one of the plus points for us weighing up Janki's pump options. The backup battery should give us up to 8 hours of operation (or is that 4 hours? ;-) )

Some positive news is that if you are caught out and about without a Lithium AA battery, a standard Alkaline AA appears to put in a remarkably good performance (compared to its heavy-weight Lithium colleagues). The one Alkaline battery we've used (a Philips PowerLife AA) lasted 5.5 days, with plenty of button pushes, bolusing and rewinding going on :-)

Overall, battery life of around nine days is ok for us, but, as with the dual bolus cancellation, a small tweak to the software in terms of battery life estimation (for Lithium AAs) and an intelligent warning schedule would make a difference - I think - to living with the pump 24/7 over the years ahead.