How to measure current used by a device that consume power in short bursts ? - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: July 29, 2024, 07:39:57 am
content: I have an aneng 8008 and would like to measure the current used by a device that is hooked up to my car (for pest control)The device does not use current in a constant way but rather in peaks (eg: it's waken up by a timer, emit a short beep, then goes to sleep and so).How to measure current usage in that case ?I would like to find out how much current it use in average, in order to have an idea how much it drain the battery.Ideally I would measure voltage of battery at some point, leave the car for a few days without driving, then measure again and then do the same but without the device but that will be rather difficult to do since I use that car almost everyday.

author: MrAl
author_rank: Super Contributor
timestamp: July 29, 2024, 07:53:42 am
content: Quote from: tigrou on July 29, 2024, 07:39:57 amHi,Depending how short the bursts are you may have to use an oscilloscope. You then integrate the current over time.Since the bursts are probably the same, you may only have to measure one burst.If you do not know how to integrate the current myself or someone else here can show you how.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2024, 08:00:31 am
content: A great tool for this is the JouleScopehttps://www.joulescope.com/But it costs too much for hobbyists use.The cheap way is to just measure (using an oscilloscope) the voltage across a resistor in series with the device.But you have to pick the right value resistor, because it will drop voltage and affect the current.It really depends how noisy your oscilloscope is and how low you can set V/div before the noise is to much to get a good signal.Another way is to use a supercapacitor to power the device and watch the voltage on the cap drop over time to figure out how much energy was removed from the cap. Then you can convert that back to an average current.

author: BennoG
author_rank: Frequent Contributor
timestamp: July 29, 2024, 08:57:33 am
content: Quote from: Psi on July 29, 2024, 08:00:31 amI like that out of the box thinkingBenno

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 29, 2024, 09:11:10 am
content: The supercap idea is quite charming, but always make a reference measurement without any load before and after the real measurement. Check that cap leakiness is stable and subtract it from the measured discharge.Lot´s of supercaps are super leaky when they were idling for a long time. The electrodes reform after a few hours of use and so the leakiness get´s better again.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2024, 09:14:37 am
content: Quote from: Phil1977 on July 29, 2024, 09:11:10 amYes, you have to get a baseline to confirm what capacity the cap has between the two voltages.Normally do that after taking the real measurement since you then know what starting and ending voltage to do the capacity check between.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 29, 2024, 09:27:03 am
content: You could also switch a discharged supercap in series with your lab PSU and the load. Then its leakage is lower and maybe the capacity change over voltage is not so significant.

author: MrAl
author_rank: Super Contributor
timestamp: July 30, 2024, 06:14:18 am
content: Quote from: Psi on July 29, 2024, 08:00:31 amThe super cap is a good idea. The capacitor acts as the integrator (in reverse) and I would think you could do the same just using a capacitor as a regular integrator. But you don't have to use a super cap if the bursts are short and repetitive.We actually had to do this a long time ago to 'measure' the BH curve of a magnetic device that was to be used in a product. In that setup the cycles were all the same because there was a constant frequency, so just a regular cap could be used.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2024, 08:41:36 am
content: An oscilloscope current probecanbe the most valuable tool in your box of tools.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2024, 11:53:54 am
content: Quote from: Phil1977 on July 29, 2024, 09:11:10 amIt's actually a bit simpler than that, in my opinion; or, arguably I guess, but it seems simpler to me.The terminal current of a supercap can be described as the sum of two currents: "normal" capacitor current, and leakage.Leakage increases exponentially around rated voltage. The mechanism is simply solvent electrolysis, much as the self-discharge or failure mechanism(s) are for battery over/under dis/charge. The self-discharge time constant can be some days near ratings (maybe even hours when hot), weeks or months at a modest fraction.There is no "reforming" mechanism -- there's no oxide barrier at all, unlike aluminum electrolytics for example. The barrier is the solvent itself (or the salt dissolved in it: forming an ionic double layer), so leakage doesn't go down the longer you hold it there, it's purely a voltage thing.I've had one sit on my shelf at ~2.1V for whole months; I'd have to write down what the actual changes are to see for real, but it was quite slow at room temp and this modestly derated voltage."Normal" capacitor current, for a supercap, is highly diffusion mediated. That is, instead of X = 1/(2*pi*F*C), you get R = X ~ sqrt(Ro / (2*pi*F*C)). This F^(-0.5) asymptote extends over a wide frequency range: perhaps 10s µHz to 10Hz+. (It may not be consistently 0.5 exponent, but varying; or alternately, it might be well described by a modest-size lumped equivalent model, give or take the diffusion element still; I haven't seen data on specific parts, unfortunately -- this is mostly a hand-wave.) Whatever the exact function, the important part is to think about capacitance and ESR as variables, even moreso than for regular capacitor types -- and exactly as much as battery impedance models do, and for similar reasons (ionic diffusion).The biggest take-aways are: capacitance varies with frequency, so it depends on how long you let it dis/charge, what efficiency (both charge and energy) you measure, what value, and say if setting a constant voltage to measure leakage, how long you need to let it settle for before you're finally measuring the steady-state leakage per se.And, because the tail on this response isjust so long(days, weeks), you might think the current has stabilized to leakage, and measure it then, but actually it's still creeping down, little by little. Datasheets themselves fall victim to this -- I would say more as a matter of convenience, as, who can wait full daysor weeksto test otherwise-normal components before shipping them? So, you see somewhat pessimistic leakage values in datasheets, and you can expect better leakage if you're patient.This long tail can look like reforming, but it's actually a linear effect, and everything that applies for charge, applies for discharge as well! It's just a very slow to reach steady state, depending on how close you need it to approach "steady".Another way to put it: it might look like dielectric absorption, but, it's kind of... the whole damn thing is absorption?! The "droop rate" for a step charge can be quite massive, like 20, 30% (i.e., charge the capacitor from 0V to 2.5V in some seconds or minutes, then disconnect and wait).So, if you're testing something like an IoT gadget that sits around for weeks or months at a time (or years?), and just wakes up and beeps once in a while or whatever it's doing, expect to "soak" the capacitor(s) for a good week or two before starting the test, and take "mid stream" sample data -- measure it periodically, note the initial droop rate and discard if it's still settling, then calculate from there.Of course, if energy consumption varies with data, or configuration, or activity or whatever, make sure to set that to a reasonably-worst-case condition so you're getting a representative upper bound here.For shorter durations like single wake events, coulometry can be done with a much smaller capacitor, and, say, measuring the delta V on the oscilloscope. Then extrapolate average power based on average wake rate.Dielectric absorption of other capacitor types is much lower -- AFAIK, a similar diffusion effect underlies those, too (perhaps not ionic diffusion, but relaxation of polarization sites in the dielectric, and electrolyte where applicable, stuff like that), it's just a smaller fraction of the total (electrolytics might be a few percent, etc.).Tim

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 30, 2024, 12:40:33 pm
content: I do not know what electrochemically happens in supercaps. I can just tell that I have a few balanced supercap packs that quite strongly heat up when they are brought to their nominal voltage after long storage time.They are 14.4V 500F modules with 6 serial 2.4V 3000F capacitors. If I charge them with constant current, then they quite linearly charge up to 10V, and then the voltage curve flattens and they start getting warm.After half a day of operation at 12V their leakage current is <50mA and they´re not warm anymore.I assumed it´s some electrode reforming that´s happening inside the cap, but maybe it´s something about the balancing, maybe the voltage limiting transistors produce the heat. Maybe I find time to wire them up to a voltage, current and temperature logger to learn more.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 30, 2024, 01:50:17 pm
content: Instead of a super cap you could ust use some AAA batteries you probably already have and see how long they last. Compare that to the capacity rating or if you have it the discharge curve.

author: Peabody
author_rank: Super Contributor
timestamp: July 30, 2024, 04:35:22 pm
content: Can you tell us what the timing actually is? How long is the short spike, and how often does that happen?

author: MrAl
author_rank: Super Contributor
timestamp: July 31, 2024, 09:37:20 am
content: Quote from: ejeffrey on July 30, 2024, 01:50:17 pmThat would not be very accurate, but when compared to other run tests I think it would paint a clear enough picture.If a run test with a resistor shows 20 hours run time, then the new test shows 20 hours run time, I think it would be a reasonable comparison. Batteries have some strange characteristics though especially the difference between pulsed loads and constant loads. This could complicate things quite a bit and could render the comparison as error prone. If the entire experiment was to determine run time on AAA batteries though, it might be just right.

author: Psi
author_rank: Super Contributor
timestamp: July 31, 2024, 10:16:32 am
content: I have some 2.5V 2600F maxwell supercaps that have been sitting around here for like 5 years.They're the ones I had in my car instead of a car battery that some of you may remember me posting photos off on here.After I bought a new car I removed them from the old one before selling. I fully charged each cap and put them in a box.I checked in on them from time to time. They dropped from 2.5V down to 2.4V pretty fast, like under a month.Then down to ~2.3V after 4 more months or so. They were all still above 2V 2 years later.I last check them around 3 years ago, so I just went to have a look now.2 had started to leaked their guts out. The other 4 seem fine and are around 1.7V.So the leakage is stuff-all once you get down around 2V