Accurate Energy metering / couloumb counting for low power applications - Page 1
Forum section: electronics / projects

author: spacejunkie
author_rank: Newbie
timestamp: September 04, 2014, 06:35:40 am
content: Can someone recommend an IC or technique to accurately (relative term, I know) measure low power devices.Details:I am building an connected device which is powered by battery and an external power supply (which is not always available).In order to maximize battery life, I wanted to do some analytics on energy consumption so as to find which activity consumes how much energy. Since the product is already connected to the cloud, it would be great to have statistics around energy consumption collected from actual devices running on field. This way, the firmware can be improved based on the statistics and pushed over the air to the device resulting in immediate improvement in battery life.The problem is the current consumption is pretty low most of the time ~1.5 ma in srtandby mode, <100uA in sleep mode and up-to 650ma in Transmit mode.As I understand, accurately measuring small values of current/coulombs is difficult.Besides most wireless components like BLE, WiFi, GSM consume energy in short pulses and this might make it more difficult to measure actual energy consumption as compared to something that consumes energy at an almost constant rate.I have looked at some coulomb counting ICs from TI, Maxim and Linear but not sure how they perform under low power conditions. Also, I would ideally like the component to be cheap so that it can be integrated in every product we ship and not just in the development versions. Some trade off of accuracy for lower cost is acceptable.

author: hlokk
author_rank: Contributor
timestamp: September 04, 2014, 06:45:18 am
content: Dave has the uCurrent which is designed for measuring exactly those kind of currentsHowever, obviously you wont be able to attach it to every produce (but could be useful in development). However, the principle (as covered in one of the videos I believe) may work for your application. However, not being particularly familiar with all the types of solutions, I don't know what would work best for you, so treat this message as a "hey, heres something to look into to that maybe might work".

author: coppice
author_rank: Super Contributor
timestamp: September 04, 2014, 07:05:55 am
content: Most energy monitoring techniques are really bad at measuring the energy consumed by bursty applications, especially if the bursts are really short. TI has a new approach in their latest MSP430 tools. The supply feeds controlled shots of energy into the Vcc rail. A simple count of the number of shots tells you the total consumed energy, without needing to follow the size and length of the actual bursts of consumption. That works great for a development tool, but its not so good for embedding in a product. Measuring the time for which the high load activities are running, using a timer in an MCU, probably gets you closer to a real measure of burstily consumed energy than most coulomb counters.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 01:27:41 pm
content: The approach taken by Texas Instruments is not new. Charge balancing ADCs and DACs work that way and discontinuous constant on-time or peak current mode pulse modulation switching regulators may be used to do this by counting their repetition frequency. I suspect TI's patent is either for a very specific implementation or the patent office missed significant published prior art which would not surprise me since it could show up in someone's obscure service manual or a trade magazine going back decades.For a low current design, I would use a virtual ground with a charge pump.

author: coppice
author_rank: Super Contributor
timestamp: September 05, 2014, 01:49:40 pm
content: Quote from: David Hess on September 05, 2014, 01:27:41 pmWho said anything about patents? What TI have done in the new MSP430 tools has nothing to do with low power. Its about handling huge mark to space ratios well. It is a kind of switching regulator approach, but the pulses are of equal energy, and pulse density modulation is used to achieve a steady voltage. Other low cost approaches previously tried in various development tools have done a poor job of tracking the true energy consumption, and people have struggled to get a good fix on the battery life they can expect from their products. This approach seems to be pretty accurate.

author: max_torque
author_rank: Super Contributor
timestamp: September 05, 2014, 02:19:05 pm
content: Surely though, your battery doesn't care about the short sharp power draws either? If you use an upstream low pass filter in the power supply line, with a large capacitor to filter those pulses back to close to dc, then a simple current shunt and voltage measurement should provide an accurate measurement without having to actually work out how much each pulse takes?During development, you can just make some software "Load" tests that say carry out a certain process a certain number of times per sec, and that will allow you to work out how much power each event is using won't it?

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 03:32:06 pm
content: Quote from: mojo-chan on September 05, 2014, 02:20:56 pmThis brings up a very accurate alternative method. If you know which power state the microcontroller is in, then you can integrate samples over time to increase signal to noise ratio in the measurement. In the extreme case where sufficient time is spent in a power state, an integrating converter can be used or an average over a specific time duration can be taken.The above will not return real time status of the power consumption though.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 03:48:07 pm
content: Quote from: mojo-chan on September 05, 2014, 02:20:56 pmThis brings up a very accurate alternative method. If you know which power state the microcontroller is in, then you can integrate samples over time to increase signal to noise ratio in the measurement. In the extreme case where sufficient time is spent in a power state, an integrating converter can be used or an average over a specific time duration can be taken.The above will not return real time status of the power consumption though.Quote from: coppice on September 05, 2014, 01:49:40 pmTI has been making lots of noise about their new super-duper patented technique. They even sent me an email announcing their game changing innovation.QuoteI tend to think of it as a dynamic range and fast response issue.QuoteI agree that it is a great approach and development tool makers have done a poor job. Those with enough analog and sampling experience who needed this information solved it themselves.