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Re: [IP] RE: batteries, when to change

At 08:09 PM 9/7/99 -0800, you wrote:
>Batteries usually last several days after the alarm if they have gone 
>several weeks to begin with. The child can change the batteries when 
>they get home from school. The low battery alarm is most likely to 
>happen during a bolus since that is when the most demand is placed on 
>them. The bolus needs to be finished and the pump should continue to 
>function normally.
>email @ redacted

I'd like to add a thought to Michael's excellent points.  Voltage in
complex IC circuits is very important.  Unfortunately the silver-oxide
battery has very little droop in voltage prior to becoming unusable.  A
"battery low" warning must sense the small droop in voltage (prior to
battery exhaustion) and turn on the message that the pump wearer sees.  The
brain of the pump with all the accompanying self checks must then operate
on marginal voltage that might vary significantly during a bolus or an
accidental illumination of the night light.

I have had more than one incident where the pump alarmed "No Delivery" and
I could find no cause.  Shortly thereafter I would have a "Battery Low"
warning.  I am convinced that low batteries can cause a "No Delivery" alarm.

The MiniMed pump uses three batteries.  Any one of them may be defective
and provide slightly decreased voltage under load.  The surfaces that act
as electrical contacts for the batteries are sometimes contaminated by oil
from the skin or even dirt or sweat.  The interface is trying to pass
electrons at a total voltage of 4.65 volts -- total.  Not much.  

My point is that battery usage in insulin pumps is pretty tricky and can
cause unexpected difficulties.  No pump is exactly like another, not
really.  Every pump will behave differently to low voltage but, suffice it
to say, the quirks are usually undesirable.

I would suggest that fresh batteries be available where ever the pump
wearer goes.  Particularly at school.  They must remain wrapped and
separated.  Putting batteries in a pocket or in a handbag will result in
dead spare batteries when they are needed.

In review:
Take the "Low Battery" warning seriously and change the batteries with
fresh ones at the first opportunity.

Be suspicious of pump anomalies when battery voltage is low.  It could be
the pump but it could be the batteries.

Try to have fresh batteries available where ever the pump wearer spends
time.  Of course, it is imperative to be sure that the pump wearer is
trained to change the batteries.  It does require familiarity and a sharp
eye!  It is not intuitively obvious (not to me, at least).

Bob Blakely
email @ redacted
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