Measuring Car Battery DC Current with UNI-T UT139C multimeter - Page 1
Forum section: electronics / beginners

author: symbianas
author_rank: Regular Contributor
timestamp: February 23, 2021, 11:49:09 am
content: Hi, I want to measure car battery DC current with UNI-T UT139C multimeter. Instructions image got me confused:How should I measure the current of car battery? 1 or 2?Also is max 10A reading enough for measuring car battery current (engine off)?

author: symbianas
author_rank: Regular Contributor
timestamp: February 23, 2021, 05:10:11 pm
content: It seems it worked with multimeter inline with the circuit, but the drawing is really weird in instruction...I have measured current while car sleeps:with "after marked" alarm armed - 0.095Awithout alarm armed - 0.06AIs that too much? Car sits without driving for 2weeks and battery discharges to the point that I cant start the engine... The battery is new.Should I consider removing "after market" alarm system?

author: tunk
author_rank: Super Contributor
timestamp: February 23, 2021, 05:39:27 pm
content: 0.095A over two weeks is 32Ah (0.06A is 20Ah).Depending on the battery capacity and state ofcharge, then it could well be flat after two weeks.You could remove one and one fuse to see whatis drawing current.

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2021, 06:58:05 pm
content: First, I'd NEVER measure car battery draw in this way. If you have a short or a mishap, your multimeter is dead.I'd do the following:1: disconnect the battery grund lead.2: insert a 10 ohm resistor between ground lead and battery pole.3: measure the voltage over the resistor.This is safe, the worst thing that can happen is that the resistor burns.That being said, 95 mA of dark current is far too high. Rule-of-thumb for cars is 50 mA max., 20...30 mA typical. Give the aftermarket alarm system to someone you don't like, and check your radio´s consumption.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2021, 07:15:38 pm
content: It's not necessarily quite that easy.You need to unlock the car to open the bonnet (hood). Once you lock the car again, it will take time to go through its shutdown and housekeeping stuff before it reaches its final standby current. A 10R series resistor will drop too much voltage to allow this to happen. Other current consuming stuff will also happen after you disconnect the battery to insert the resistor and reconnect.I would suggest:1. Disable the bonnet detection switch - this will interfere with the alarm function if the car is locked with the bonnet open.2. Fit the shunt resistor, but clip a short across it to allow the housekeeping functions to complete.3. Wait for 10 minutes or more before removing the short and measuring the standby current.

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2021, 08:31:49 pm
content: Quote from: Gyro on February 23, 2021, 07:15:38 pmAll good points.But as we don't know which car it is, whether the battery is in the engine compartment or the trunk, whether the door switches are in play, and, and, it's moot.My point was, that putting a multimeter in series with a car battery is potential "multimeter suicide".

author: Monkeh
author_rank: Super Contributor
timestamp: February 23, 2021, 08:35:36 pm
content: Quote from: Benta on February 23, 2021, 06:58:05 pmNo, the fuse is blown.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 25, 2021, 12:26:43 am
content: Opinion: Aftermarket alarm systems are usually poorly installed, on crtical wiring, causing all manner of problems in pursuit of one that it rarely solves. I advocate ripping them out on sight and avoid buying a used car with one fitted. Chase every wire to it's termination and fix all crappy splices and scotchlok (only thing 3M makes that I hate) junk.It's better to pull fuses of suspect circuits and measure across the fuse socket. Bad practice to disconnect the battery of a modern car if it can be avoided, and as mentioned it takes a while for everything to calm down and give a good idle current measurement when you do. If you do use a meter in line with the battery I'd put a cheap 5A automotive fuse in line to protect the more expensive fuse in the meter.

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2021, 09:31:01 am
content: Quote from: Monkeh on February 23, 2021, 08:35:36 pmNot all multimeters have one. And if it does, it's always some special type with two weeks delivery.

author: symbianas
author_rank: Regular Contributor
timestamp: February 27, 2021, 05:54:24 pm
content: I managed not to blow the fuse. I measured up to 5.9A when car is not in sleep.The car is 2010 BMW with 80AH battery in the trunk and yes it has lots of MCUs and it takes 30mins for it to go to sleep.So I waited for it to get to sleep and measured 0.06A in sleep as I said. Then started removing fuses. When I removed Fuse 36 car aftermarket alarm started beeping current dropped to 0.009A. So It might be aftermarket alarm fault although that fuse is for other functions too I guess, after alarm was only hooked to that fuse additionally... I will need to try to remove alarm system, but first I will need to find where it is as it was installed by previous car owner.I might need to try to retrofit car manufacturers alarm system hopefully it wont draw that much power.

author: cliffyk
author_rank: Frequent Contributor
timestamp: February 27, 2021, 06:36:50 pm
content: Quote from: Benta on February 23, 2021, 08:31:49 pmI agree 110% however I believe semantically it would be murder...

author: rstofer
author_rank: Super Contributor
timestamp: February 27, 2021, 06:50:13 pm
content: We have found that we need to physically drive the car and truck at least once a week to keep the batteries charged. I bought an industrial scale charger and very long jumper cables but those are just crutches in case we don't drive the vehicles.

author: cliffyk
author_rank: Frequent Contributor
timestamp: February 27, 2021, 07:07:48 pm
content: Quote from: rstofer on February 27, 2021, 06:50:13 pmGet a good battery microprocceser-controlled battery maintainer (not a cheap wall-wart based "trickle charger"). I use a Schumacher SEM1562 I've had for over 10 years as I'm retired and my car often sits for a month or more--I use my motorcycle and UTV for 99.44% of my daily needs. I believe the latest model is the SEM1562A

author: radiolistener
author_rank: Super Contributor
timestamp: February 27, 2021, 07:14:53 pm
content: Car battery has 600-800 Amps current. This is very dangerous for your DMM

author: cliffyk
author_rank: Frequent Contributor
timestamp: February 27, 2021, 10:54:41 pm
content: Quote from: radiolistener on February 27, 2021, 07:14:53 pmCan be much more than that, we just put a new AGM battery in my wife's 2017 Subaru Outback. My first conductance test showed 12.6 V with an internal resistance of 3.82 mA --that's a 3350 A potential dead short discharge rate--good thing it's only 12 V...

author: Stray Electron
author_rank: Super Contributor
timestamp: February 27, 2021, 11:55:36 pm
content: Quote from: Benta on February 25, 2021, 09:31:01 amI've never seen a MM without one but even so, any meter that doesn't have a fuse should be fried! What? Aren't there any Boy Scouts in your country? Always "Be Prepared" and buy extra fuses ahead of time!I also check the type fuse (and battery type) before buying the meter since I don't want to deal with weird fuses. I buy extra fuses and stuff as will fit into the battery compartment of the meter so I always have a couple on hand. BTW I also never use an expensive meter for battery work! The cheapo Harbor Freight meters work fine and I'm not worried if I damage one. I got one of the free ones that they were giving away with any purchase and it lives in my tool box and it's been thrown around for over ten years and it's still working fine!If there's a possibility of the meter being exposed to too much current then use a current shunt! I have a 500 Amp one permanently wired into the battery circuit in my truck. Everything except the starter is routed through it.

author: cliffyk
author_rank: Frequent Contributor
timestamp: February 28, 2021, 12:23:41 am
content: Quote from: Stray Electron on February 27, 2021, 11:55:36 pm!!!^^^All of THIS^^^!!!I have 3 or 4 of the HF freebies and IIRC, AFAIK they have fuses¹. Never seen a MM (digital or otherwise) that didn't--many are right in the battery compartment. Usually just plain ol' dime-a-dozen glass fuses.-----------------------------------------------------¹ - I just dragged my butt out to the barn and checked--they do have fuses, they are tiny little "grain o' rice" soldered-in-place thingys--but they could be easily replaced with a 3/4" glass fuse in a pinch).

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 28, 2021, 12:57:56 am
content: The cheap HF freebie type meters are not fused on 10 amps. I even pulled one out of the junk box to check.

author: cliffyk
author_rank: Frequent Contributor
timestamp: February 28, 2021, 04:58:53 am
content: Well there's headline news:ALERT: $0.00 TO $2.99 MULTI-METERS MAY NOT BE SAFE!A truth I have found in my 70+ years living amongst the humans is that:“After a time, you may find that your buying the cheapest there is, may end up with your having the cheapest there is. It is quite logical, and very often true.”My apologies to Mr. SpockI spent years customizing cars and bikes, it was always said of doing so that there are always three goals in such projects "Cheap, Fast and Safe", but here in the real world you only get to have two.

author: symbianas
author_rank: Regular Contributor
timestamp: February 28, 2021, 12:11:19 pm
content: Well I am not concerned much about the multimeter as I bough it for this reason and much more concerned about the car. But good to know that I should not overload the meter with current, tried to turn everything off if possible. Anyway max current was 5.9A I hope multimeter can survive that.As for car I have made progress! Started removing the adtermarket alarm and now current is 0.008A! So it is much less.The question is what do I do now with this relay and plug? And will car even start?Found pins of alarm system:What is that black wire loop?

author: symbianas
author_rank: Regular Contributor
timestamp: February 28, 2021, 05:25:57 pm
content: I got overwhelmed by wires, but it seems that its not that hard. Most of them just tap into other wires. Only the relay disconnected one thick black wire, I have cut and reconnected it, hope its ok not to solder it?Have not tried to start the car though.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 28, 2021, 08:41:28 pm
content: Opinions vary, but you want either a quality crimp joint or quality solder joint. Always give splices a tug test for mechanical strength. The splice should be protected from moisture and abrasion, marine type (hot melt adhesive lined) heat shrink tubing is ideal, though not really critical for interior wiring.

author: Benta
author_rank: Super Contributor
timestamp: February 28, 2021, 08:51:11 pm
content: Quote from: BrokenYugo on February 28, 2021, 08:41:28 pmNever, ever solder joints in car wiring. Quality crimp is the way to go.