Holly, I wonder if even with LiIon one might get slightly better performance with a supercap IF one is doing really "peaky" stuff. Maybe not WiFi, or ypur ebooks, but cellphone digital or the like. I'm probably going to see if there's any supercap payoff with my WiFi app, even though it's likely to be a wash -- it's a prototype and I'm just messing around. I'm going to be using LiFeS primary cells during part of my testing.

It's always a pleasure to read your posts.

On 9/7/05, Holly Gates <hgates@eink.com> wrote:

> Holly Gates writes:
>  > The supercap extended the AAA lifetime from
>  > something like 20 minutes to a few hours.
>
> This is a very interesting observation.  Have you checked whether
> the pulsed current kills the batteries faster due to some weird
> electrochemical effect, or whether the current peaks simply cause
> the voltage to reach below the critical level earlier ?
>
Both. If you draw above a certain current from an alkaline battery, it
will actually reduce the amount of power you can get out of the battery.
I.e. if at 100mA you could last for 4 hours, at 400ma you will last for
less than 1 hour. The other effect, as you guessed, was that on a
current spike the battery voltage would sag significantly which would
eventually be enough to hang the gumstix. The two things combined to
give an extremely short battery life; what one would expect to be
something like 5 or 6 hours based on steady state draw and battery rated
capacity ended up being about 20 minutes.

Adding the supercap helped by smoothing the voltage dips which caused
the gumstix to hang earlier than it otherwise might. It also helped the
ultimate battery capacity by knocking down the current spikes.

>  > http://www.positron.org/projects/h2/cases.shtml
>
> Wow, how many gumstixes are there on your desk ?  Eight ?  :-)
>
Probably something like that. When I took these pictures I was building
10 of these ebooks for a customer, which practically killed me since
every one is meticulously handbuilt over ~20 hours!

So far I think we must have used 50-70 gumstix. Hopefully climbing in to
the 100s before long...

-Holly

> -- Pascal
>
>
>
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