Measured Lipo discharge curve

formatting link

This is a 1Ah Lipo powering my Geiger counter GPS logger. It lasted about 434 minutes, or say 7.3 hours. It only takes 4 minutes or so for the battery to drop from 3.2V to below 3.0V.

8 minutes for it to drop from about 3.3V to 3.0 V.

Now SDcard runs a a supply derived from a 5 V upconverter, but it makes sense from this to run it from a low dropout regulator directly from the battery, ordered some TPS73133 for that. As now the efficiency loss should be gone from the upconverter, the thing should run even longer.

Battery was fully charged. Not bad at all already. This curve compares nicely to other lipo discharge curves I have seen on the internet. Lipos are cool, you just got to study them a bit to get the maximum out of it.

I got the data from the SDcad, it logges battery voltage too... # read_gmp_card -f 220 -l 654 | grep "battery voltage" | awk '// {print $3}' > dots

Looks like this:

3.944 3.936 ..

# gnuplot # gnuplot> plot "dots"

formatting link

Same way you can filter for radiation: # read_gmp_card -f 220 -l 654 | grep -i "cpm" | awk '// {print $2}' > ticks # gunplot #gnuplot "ticks"

formatting link

Now aint that cool, truely random ;-)

On a sunny day (Tue, 11 Mar 2014 15:55:00 GMT) it happened Jan Panteltje wrote in :

Anyways, had hardly written this and the mail brought low-drop-out TPS73133 regulator chips. So recharged the Lipo, and had it run all day:

formatting link

Note the difference with the previous one:

formatting link

7.5 Hours for the one with the low drop out regulator versus 7.33 hours for the previous one. It looks as though the lipo was charged a bit higher to begin with.... (Did exactly the same thing, maybe it gets better after some charges.) Although the difference is small, there are many other advantages of using this TPS73133 chip in this design. as when used without GPS but with SDcard, I can switch of the 3.7 to 5 V power converter completely, reducing the current consumption by at least 50 %. Fixing the audio amp (replacing it with a complemenatary emittor follower pair without any bias (bases tied together) ) should give even longer battery life [1]. 8 hours should be possible, a whole workday of uranium prospecting in the boonies. And that, was the target,

Still waiting for the MCP73831T lipo charger chip before trying the USB and solar charging thing, be summer by then ... :-) So.. it takes the lipo about 8 minutes to drop from 3.344 to 3.064, at that point the system switches off. Note the current is really high in those few minutes as the switcher will try to maintain +5V on the GPS module:

1 3.320 2 3.304 3 3.280 4 3.248 5 3.232 6 3.200 7 3.152 8 3.120 9 3.064 So maybe it is safer, or better for lipo battery life, to just switch of below 3.3V... That keeps SDcard at 3.x volt range too. [1] I ran out of BC547 NPN, thought I had hundreds, where did they all go? so many projects, ordered 100 BC847 and BC857 universal SOT NPN/PNP transistors....