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Coin cell pulsed drain

Added by ltj almost 6 years ago

I know this topic has been covered several times on the blog and the forum in the past. But I’m still wondering what the pulsed drain limit might be for coin cell used with a JeeNode micro for instance.
A few days ago some fellow students asked me for help, because they had trouble running a 5V Ardu pro micro with several leds, sensors and an xbee with 2032 litihium cells. The answer was quite obvious that these cells, by far, cannot sustain such a load (think it was about 70mA idle!). But it got me thinking. I personally run some micros (v3 direct power) with a single 2032 cell, and it works fine even with a couple of sensors attached. They are of course sleeping most of the time and only becomes power hungry when waking up, reading sensors and sending the data over the air.
According to the Panasonic spec sheet these batteries are rated for 0.03mA sustained drain. Definitely achievable when “sleeping”, and perhaps also on average. Pulse drain however, is not recommended to exceed 10mA, and I suspect ATTINY84 + RFM12B transmitting goes well beyond the 10mA’s :) I’m no EE or anything like that, so I’m a bit unsure about how far you can push these cells in terms of pulse magnitude and duration, and how it will affect performance when battery charge decreases. Any thoughts on this?

/ltj


Replies (4)

Coin cell pulsed drain - Added by martynj almost 6 years ago

ltj, a short pulse load is fed mainly from the capacitor in parallel with the battery. Provided this is a large enough value, the charge extracted is enough for a radio “blip” without the voltage sagging too much and then the battery refills the charge over a much longer time constant (defined by its effective internal resistance, Re and the C value of the ‘reservoir’ capacitor).

RE: Coin cell pulsed drain - Added by ltj almost 6 years ago

martynj, thanks, I wanted to ask about using a capacitor in parallel with battery as well. But it works without one, and as far as I can see no cap is on JCW’s radio-blip micro with coin cell. So when do you need one, and when can you leave it out?

RE: Coin cell pulsed drain - Added by martynj almost 6 years ago

ltj,

Yes, it’s there  - just hiding ;-) 
If you refer to the JMv3 Schematic (found on the Jeenode Micro Wiki), capacitor C1 is present, effectively acting as the reservoir since it is directly across the battery. 

RE: Coin cell pulsed drain - Added by ltj almost 6 years ago

Sigh. I will bother to check schematics and the board itself the next time :) So it seems 10uF is a fine value for stuff like radioblip and a little beyond. I guess I can always play around with the C1 value if I run into trouble. Thanks again for helping me out with the obvious @martynj.

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