How to probe and log a small DC current without cutting the wires? - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: December 07, 2017, 10:19:10 pm
content: Sometimes, but not always, I find my car battery completely flat, with zero volts after letting the car parked for 2-4 weeks.The battery seems totally OK after recharging, but since this happened more than a few times now (randomly) I would like to log the current and try to find out why and when it is happening. The battery is 80Ah/12V, and I suspect it to go totally flat in 2 weeks, maybe faster, so a resolution of 10-100mA will be needed. Sometimes the battery is just fine and can start the car after 1-3 months, sometimes is zero volts flat after 2 weeks. I can provide another Pb battery (from a UPS) for the logging device.Any ideas how to probe and log such a small current (10-100mA) without disconnecting/cutting any wires, please?

author: CopperCone
author_rank: Super Contributor
timestamp: December 07, 2017, 10:23:12 pm
content: thats large enough to use a hall effect sensor. Smaller would require a fluxgate circuit.

author: trevwhite
author_rank: Frequent Contributor
timestamp: December 07, 2017, 10:25:55 pm
content: Quote from: RoGeorge on December 07, 2017, 10:19:10 pmHall effect current probe will measure dc current. Some can be plugged into a scope or multimeter for logging I think.Sent from my SGP311 using Tapatalk

author: RoGeorge
author_rank: Super Contributor
timestamp: December 07, 2017, 10:39:21 pm
content: Quote from: CopperCone on December 07, 2017, 10:23:12 pmI didn't know about the fluxgate circuite, thanks!Quote from: trevwhite on December 07, 2017, 10:25:55 pmThe analog hall sensors I have salvaged from a PC fan doesn't seem sensitive enough, but since both of you are telling a hall sensor should work, I will look for more sensitive ones, thank you!

author: Benta
author_rank: Super Contributor
timestamp: December 07, 2017, 10:41:39 pm
content: I'd simply buy a clamp meter with USB interface for logging. DC types are available.

author: cdev
author_rank: Super Contributor
timestamp: December 07, 2017, 10:44:54 pm
content: Uni-T UT-210e but doesn't have logging out of the box but so many people are modifying - there is a wealth of info here on the internals, which could be used I'm certain, to let you do that.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 07, 2017, 11:03:38 pm
content: Outch!Never used a clamp meter, and always assumed they are AC only. Embarrassing, I will go hide under a rock nowThank you, will look for clamp-meters, too.

author: cdev
author_rank: Super Contributor
timestamp: December 08, 2017, 12:25:41 am
content: I have the UT-210e and its quite useful. People here are investigating turning them into oscilloscope current probes.Mine can measure down to 1 ma without tapping into a circuit. Also it has a NCV sensor to tell you where the dangerous voltages are.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 08, 2017, 07:37:48 am
content: 100mA is small for clamp meters. You could try a fluke i30s, maybe looping a few times.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 08, 2017, 09:52:23 am
content: Ordered a UNI-T210E from Aliexpress, seems like a nice tool to have around. Can hardly wait to hack it, thank you all!

author: xani
author_rank: Frequent Contributor
timestamp: December 08, 2017, 12:35:57 pm
content: Just remember that on DC mode clamp will react on any magnetic field which means you should zero it before each measurement

author: glarsson
author_rank: Frequent Contributor
timestamp: December 08, 2017, 01:07:38 pm
content: It is also important not to move the clamp after zeroing. Angle of any magnetic field matters.

author: cdev
author_rank: Super Contributor
timestamp: December 23, 2017, 11:56:14 pm
content: I want to get something like the old "silly putty" to hold it steady. If you do that its accurate to 1 ma. Which is very very good.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 24, 2017, 12:51:21 am
content: Quote from: RoGeorge on December 08, 2017, 09:52:23 amI bought a UNI-T210E for this very purpose. In my case the "vampire" drain was there all the time so I didn't need logging.What you could do to avoid the trouble of hacking and logging is to just check the battery drain several times a day for a few days. When you catch it happening it should last for a few minutes and that's all you need to track it down with the nifty little clamp-on you now have. Good luck.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: December 24, 2017, 02:26:28 am
content: Quote from: RoGeorge on December 07, 2017, 10:19:10 pmHmm, what kind of car starts at 10-100mA? Or am I misunderstanding what you are saying? The cold cranking amps of most motors are in the 100s of amps, depending on the engine size. Also, if you are logging the current while it sits idle over weeks or months, how do you know the leakage is 10-100 milli-amps?If this is a lead/acid battery, you may have a cell (or all cells) starting to build up sulfates on the plates, which shorts out the cell and drains the charge, especially true if the battery sits unused for long spans. Lead/acid batteries can be very unpredictable because when you charge at 30 amps, which is reasonable for an alternator to do, this action can break the built up sulfates and when you stop, the battery doesn't discharge itself until the sulfates build up again. After some time, however the battery dies because of the level of sulfates deposited build up to a level that the charging will not break them.Or, like maybe you are probably thinking, you have some type of short in your system that drains the battery over time... who knows? In the US, most automotive parts store, like O'Reilly's, or even Walmart will check your battery with a sophisticated testing system and be able to tell if you have bad cells, for free.If you are thinking something else other than what I mentioned, never mind... Hope you find your gremlin.

author: tboy32
author_rank: Unknown
timestamp: December 24, 2017, 08:16:17 am
content: Quote from: tpowell1830 on December 24, 2017, 02:26:28 amI think he's not talking about cranking the engine. Since he knows the capacity is 80Ah and it goes from fully charged to fully flat in 2 weeks he can approximate the average leakage current without continuously monitoring it.If he's got another 12v battery he could swap it out then come back after a week and test its voltage. If it's at like 6v then he should be able to deduce the problem is indeed some electrical system slowly draining the battery (and proceed with the original logging idea). If, however, the battery is still charged then it's possible the original battery itself is the problem (as tpowell1830 stated).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 24, 2017, 09:16:18 am
content: It's going to be hard to accurately log current. Probably easier to record voltage with as much resolution as you can and look at the slope to estimate current.

author: jeroen79
author_rank: Frequent Contributor
timestamp: December 24, 2017, 10:24:40 am
content: How about using one of the cables leading to the battery as a shunt?Attach one sense wire to the negative terminal and another to the point where it is connected to the chassis.

author: xani
author_rank: Frequent Contributor
timestamp: December 24, 2017, 03:04:49 pm
content: If car is just sitting there in garage you can:Disconnect one leg of the batteryConnect car to lab power supplyConnect logger shunt (either multimeter or something hacked together with arduino) in series with it.No cutting cables and getting car back to work is just re-connecting one battery clamp so nice and quick.