Current draw and battery life

Aaron Wall

I'm testing the battery life of a device I made with a BS2, GPS Module and running off 4 nicad cell phone batteries. Each is 3.7v. two are wired in parallel and then put in series with the other two put in parallel. This gives me just under 8v to work with.

When I hooked my $35 craftsman multimeter up in series with various fully active devices, I got these current draws:
BS2 + GPS = .15mV
BS2sx + GPS = .21mv
GPS (tracking and searching) = .12mV

Now when I looked up the current draw in the data table for the various stamps (catalog p. 12), it listed the stamps as drawing 3mA for the BS2 and 60mA for the BS2sx.

Why are my numbers so much lower than the table?

Is there a way to rate batteries on their current/time capacity? Measuring how long they will last at a given amperage? Is this a valid capacity test?

allanlane5

Well, first of all, the "currents" you recorded are "millivolts" in unit. So, either you're not measuring current, or you HAVE hooked it up properly and you're measuring AMPS.

so, 0.15 Amps == 150 milli-Amps. Bs2sx etc == 210 milli-Amps, or 60 mA more, which is what you'd expect. This implies the GPS modules is pulling 150 mA or so, which I would believe.

Batteries ARE rated at a certain "milli-amp-hours" -- like some of my AA NiMh are rated 3200 mAH. This means, you can run at 100 mA for 32 hours, or 200 mA for 16 hours, or 320 mA for 10 hours, or...

In fact, the numbers are NOT as linear as the above appears. The battery DOES have "internal resistance", which means you can't pull 3.2 amps for 1 hour -- you'd get less time than that. But for 'small' current draws (like 200 mA or so) it's "approximately" linear.

Chris Savage

As a note, most multi-meters require you to move the probe into a different jack for current measurements. Both my Beckman and my Fluke meters will not measure current if the leads are in the ‘standard’ jacks.

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Chris Savage
Parallax Tech Support