Is it possible to use a clamp meter and check car battery charging? - Page 1
Forum section: electronics / testgear

author: asilva10
author_rank: Contributor
timestamp: November 30, 2013, 03:03:58 am
content: I never tried and I do not have a clamp meter.Anyway, I know that this tool can measure AC current by means of evaluating the magnetic field strength around a wire in the AC circuitry.So, I have the intuition that we can at least get some reading spikes (or some other indication) at this tool for a "DC" circuitry with some sort of transient behavior.Had anyone at the forum make this test, that is, using a clamp meter to somewhat check the car battery charging process?Besides this question, what would be the proper instrumentation, if exists, related to measuring the positive/negative current of a car battery without interrupting the circuitry (no serial-like ammeter solution)?Thanks

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 30, 2013, 03:21:24 am
content: The only way to truly measure DC current is to break the connection, and insert the current meter (or shunt) in series with the circuit.I have seen some devices that claim to be able to measure DC current without being installed in series, but have never found the claims to be accurate.

author: asilva10
author_rank: Contributor
timestamp: November 30, 2013, 03:24:49 am
content: @Six_Shooter: thanks for the reply. But if we forget "accuracy", can these devices at least inform that an "event" (i.e., charging of a battery) is occurring?

author: moepower
author_rank: Contributor
timestamp: November 30, 2013, 03:41:28 am
content: Hi,Yes, just use an ac/dc clamp meter. Fluke is usually a good brand to get reasonable accuracy, even for low current measurements like that. You can monitor the charging/discharging just by observing the direction of the current on the meter, which will be indicated by a minus sign or not.

author: mtdoc
author_rank: Super Contributor
timestamp: November 30, 2013, 03:42:12 am
content: Some clamp meters are designed to measure to measure DC. I have a Fluke 336A that does this. I've been told thatTHISinexpensive one works well.So the answer is yes, you can measure DC current with the right clamp meter.

author: TMM
author_rank: Frequent Contributor
timestamp: November 30, 2013, 03:44:27 am
content: Quote from: Six_Shooter on November 30, 2013, 03:21:24 amWhat. You haven't seen the myriad of DC Clamp meters out there?@OP: What you need is a clamp meter capable of DC current measurement. They use a hall effect sensor to measure DC current. $40 will get you one with 2-5% accuracy (e.g. Uni-T UT203) which is more than good enough to determine if a battery is charging.

author: Psi
author_rank: Super Contributor
timestamp: November 30, 2013, 03:50:45 am
content: You can get passive current meters for measuring the car battery current during cranking. You place them on top of the starter cable and the large magnetic field causes the needle to deflect on an amps scale.

author: SLJ
author_rank: Frequent Contributor
timestamp: November 30, 2013, 04:14:51 am
content: Do you really need to know the current to see if the charging circuit is working?There should be an increase in voltage if the charging circuit is working when the car is running.14.4V to 14.7V on a running 12 volt system is typical. Above 14.8V will cook a battery.Never disconnect a battery or break the battery connections when the car is running.

author: Smokey
author_rank: Super Contributor
timestamp: November 30, 2013, 04:23:06 am
content: Quote from: Six_Shooter on November 30, 2013, 03:21:24 amWatch out here. You don't give any numbers to back up your statements. The question really comes down to what is "accurate" enough. Is 1% accurate enough? Then this clamp (which I use daily at work) does not need to break the circuit and measures DC.http://www.fluke.com/fluke/auen/accessories/Current-Clamps/i30.htm?PID=56292If you need more current than 30A, then how about 1% to 300Ahttp://www.fluke.com/fluke/auen/accessories/Current-Clamps/i310s.htm?PID=56298Yes those are expensive, but they are far from the only DC current clamps available. While these are actually BNC output probes and not self contained meters, all you need is a BNC-Banana adapter and it jacks into any multimeter.

author: BravoV
author_rank: Super Contributor
timestamp: November 30, 2013, 04:35:01 am
content: Quote from: Six_Shooter on November 30, 2013, 03:21:24 amJust because you never seen or heard of, it doesn't mean its not exist.Just an example out of many out there, clickHEREfor the specification, and if you're curious, then clickHEREfor the teardown, and it does AC ,"DC" and "DC+AC" current measurement"WITHOUT" breaking the line, and no, its not a snake oil.

author: Kohanbash
author_rank: Regular Contributor
timestamp: December 01, 2013, 01:04:03 am
content: Unfortunately I do not have the part number in front of me now. A few weeks ago I characterized a ~$50 hall effect current sensor (the wire runs through a hole in the center, and I had 5 of them). The error in current reading was +/- 0.2A. I am not sure how this generalizes to the entire range of ~$50 sensors but it is a data point.Note: I was using a 12bit ADC to read from the sensor. Current values were 0-10A.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: December 01, 2013, 01:13:33 am
content: I actually bought a ac/dc clamp meter just for this purpose, but there was so much variation in the readings I wasn't sure what to make of it.

author: asilva10
author_rank: Contributor
timestamp: December 01, 2013, 01:17:43 am
content: Quote from: sdk32285 on December 01, 2013, 01:04:03 amHi, if you figure out the part number, let me know, OK? Thanks.

author: moepower
author_rank: Contributor
timestamp: December 01, 2013, 02:09:37 am
content: Yup, you can get erratic readings if you are not careful.If the clamp is near other large magnetic fields (i.e., transformers, inductors, etc.) then the readings can be biased. Of course, if the clamp isn't fully enclosed around the conductor, it won't work. And you must remember to zero the readings before you put the clamp on the conductor to avoid drift.I've usually had very good measurements with Fluke or Hioki clamps, even for small currents less than 1 amp. So I still reckon you should get a good measurement for car battery charging.

author: nctnico
author_rank: Super Contributor
timestamp: December 01, 2013, 02:15:46 am
content: Quote from: sdk32285 on December 01, 2013, 01:04:03 amHall sensors do have offset which also changes with the position relative to the earth's magnetic field so that is probably what you are seeing. There are better sensors though. Allegro sells chips which are pretty accurate and way (over 10 times) cheaper than the ones with a hole. I recently used one of their 50A devices in a circuit.

author: asilva10
author_rank: Contributor
timestamp: December 01, 2013, 02:22:55 am
content: Quote from: nctnico on December 01, 2013, 02:15:46 amHI, any part number? I'm interested in investigating this possibility.

author: zapta
author_rank: Super Contributor
timestamp: December 01, 2013, 03:31:31 am
content: This thread discusses a DC clamp meter with 10ma resolution.https://www.eevblog.com/forum/testgear/yokogawa-cl220-clamp-meter/msg339484/#msg339484

author: Kohanbash
author_rank: Regular Contributor
timestamp: December 01, 2013, 03:54:48 am
content: QuoteI will try to check when I go into work on Monday.

author: Smokey
author_rank: Super Contributor
timestamp: December 01, 2013, 05:26:53 am
content: ACS710 from allegro are good chips. We use those all the time. They are PCB surface mount and not clamp on though, and require the circuit to be broken so they can put in series. There are some really good parts from LEM that you don't put in series, but instead stick the wire through a hole in the transducer (like those ones in the power meter dave tore down). LEM is great for through hole and panel mount stuff and Allegro is great for surface mount.Some things to look out for with hall sensors in general:1) They drift with temperature. This sucks. Try to null before using the data.2) They are sensitive to magnetic fields. This includes running traces under the chip, external magnets, and the earth's magnetic field.3) Read about the difference between open loop and closed loop hall effect current sensors. By putting a control loop around the sensor and actually measuring the control current to keep the hall zeroed, you get surprisingly good linearity. Check out this paper from LEM to learn more... (Skip to page 11)http://www.lyr-ing.com/DocumentosLyR/HallEffSensors/Applications_of_LEMTransducers.pdfIf I was going to rig something up to a data logger to measure car battery currents, here are some LEM sensors that I found after a quick search:http://www.lem.com/docs/products/hal%20standard_e.pdfOr if you wanted the higher accuracy:http://www.gmw.com/electric_current/LEM/docs/LEM-LF-205-S.pdf

author: nctnico
author_rank: Super Contributor
timestamp: December 01, 2013, 10:47:19 pm
content: Quote from: asilva10 on December 01, 2013, 02:22:55 amThis one: ACS756SCAhttp://www.allegromicro.com/en/Products/Part_Numbers/0756/0756.pdfThe datasheet isn't clear on this but these chips need a precise and low noise 5V power supply to meet their specs.