Cap size vs battery drain

[swe-dude]

How can the size of the caps affect the battery life of a sensor?

Here is the setup i have 2 sensors with dht22 attiny13 nr24l01 3aaa batteries and a XC6206P332MR 3.3v reg  (3.3v to nrf).

attiny13 and dht are powered directly from battery and nrf from 3v3.

On one i have 0.1uf ceramic cap and 220Uf electrolyte on battery side and on 3v3 side from regulator.
On the other i have 0.1uf ceramic cap and 10uf ceramic cap on battery side and on 3v3 side from regulator.

The reason for using 3aaa is that the dht22 need 3v+ for the humidity readings to be accurate.

The one with 220uf electrolyte have lasted well over a year so far. but the 10uf one only lasted half a year, all other components are identical as far as i can tell. Any ideas why the diff are so big?

[allanhurst]

Unless they're faulty the capacitors will have (almost? - depends if large peak currents are required..)

 no effect on battery life.

regards

Allan.

[Grumpy_Mike]

The difference is probably due to the leakage current of the capacitor. Every capacitor will leak a very small amount of current. I would have expected the large capacitor to leak more than the small one but maybe they are of different dielectrics.

Google
capacitor leakage
or see
http://electronicdesign.com/analog/whats-all-capacitor-leakage-stuff-anyhow

[allanhurst]

A quick thought - leakage of capacitors (and other devices) goes up as the temperature increases ....
and for that matter humidity.

was one of your units in a higher temperature or very humid environment ?

regards

Allan.

[jremington]

(3.3v to nrf)

If this means that you have an NRF24L01 radio module transmitting the data AND you are using ACK/NACK protocols to transmit data, then it is possible that the module with the (possibly too small) 10 uF capacitor is error prone, making many retries and wasting power.

Why not take a moment to fully describe the system that you have?

[DrAzzy]

The power consumption of the NRF comes in spikes, which will cause voltage drop.

As the batteries are used, you see lower output voltage under load - so those spikes result in the voltage drooping. The two bigger caps would be better able to counter this, since it transmits very quickly, and then the load is mostly removed, and the battery voltage recovers and the caps recharge - so the batteries can get more-dead before the device stops working. That'd be my guess, at least.

[swe-dude]

OK here is a schematic of the 220uf setup the 10uf is the same but with 10 instead of 220


Not the best one i have done but better then what i can do with pen and paper.
And no I'm not using Auto acc i have that disabled and only transmits 1 message per wake up.

[swe-dude]

I forgot to mention the voltage divider used to measure battery voltage sorry..
When the voltage drops below 3v the humidity error starts to increase more and more +10% when the voltage drops to 2.9 and then it just gets worse.
So i change it at 3v. (the nrf/attiny part are OK down to 1.7v thou) i read abut current drain spikes killing cr2032 batteries faster but i did not think AAA batteries had the same problem at least not with a few ms long spikes once per 8 second.

[swe-dude]

A quick thought - leakage of capacitors (and other devices) goes up as the temperature increases ....
and for that matter humidity.

was one of your units in a higher temperature or very humid environment ?

regards

Allan.

In this case no, but that is good to know I'm planning to replace the temperature sensor in the greenhouse with a humidity/temp one.

[DrAzzy]

All batteries will show that kind of behavior - it's fundamental to electrochemical devices. It comes up more with coin cells, since their capacity is fairly small compared to typical currents involved in hobby electronics.

Whether it's relevant under a given set of circumstances depends on the battery chemistry, the size of the cell, and the load. If you have a 'scope, put it on the power rail with a dying battery and see how much the voltage is bouncing around.

[jremington]

A simple and rather obvious experiment is to replace the 10 uF cap with a 220 uF cap.

[swe-dude]

If i replace the 2 220uf electrolyte caps with 8 47uf SMD ceramic caps is there any other problem that i might run into?

[Grumpy_Mike]

You get eight iptimes the leakage.

[Chagrin]

If i replace the 2 220uf electrolyte caps with 8 47uf SMD ceramic caps is there any other problem that i might run into?

Your regulator is designed for low ESR capacitors but putting eight ceramic caps in parallel will lower that ESR by a factor of eight. The datasheet for your regulator does not specify the ESR allowed but I would predict you'd be pushing yourself into problems.

TI has an application note "ESR, Stability, and the LDO Regulator" that explains the phenomenon.

[swe-dude]

I had not thought abut the ESR at all i will have to read up on that, BIG thank you for the warning!