testing a car batteries terminals with a DMM with continuity setting- beeping!

Submitted: Thursday, Sep 13, 2012 at 10:39
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Hi

I have been having a few issues with my vehicles aux battery staying charged- need a new one ive decided!. I have a 140A isolator under the bonnet and a second battery in the tub of the rodeo. I also have a solar regulator and a hard wired 10A 240v charger installed in the canopy.

My second battery wasnt holding charge so i started to look over the system, got the multimeter out and checked for voltage, checked the isolator for continuity and switching etc. I came accross that there seemed to be continuity between the + and - points but only when the + leads were touching the - points on the car (earth etc) but the beep sounded different, not as pronounced. no matter where i checked. I thought- ah ha..... i have a short. I completely isolated the aux battery, regulator and charger and tested it again at the isolator- still continuity????

I disconnected the isolator and checked directly at the main battery (still connected to the vehicles wiring etc) and there was still continuity??? i started the engine and it stopped. turned the engine off and it started again????

Is it normal to have some continuity between + and - ? or at least some low resistance or similar? enough to triggered the multimeter to indicate a circuit?

Why would it only happen when the + lead of the DMM is on the - points (i didnt think it would mater as it was only testing for a circuit)? and not the other way around


The chime was a lower tone and faint (nothing in DMM instructions to explain the difference)

I was thinking it may be detecting the circuit of the ECU, clock or even the red flashing led on the dash.

Is someone able to clarify this? Do i have a big problem?

Thanks for any help.

doey
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Reply By: Allan B (Member, SunCoast) - Thursday, Sep 13, 2012 at 11:49

Thursday, Sep 13, 2012 at 11:49
Hi Doey,

Do you have a big problem? Well yes, I think you do.
I am not trying to be offensive but with great sincerity I would say that the big problem is your lack of understanding and experience of electrical matters and the use of DMM's and testing techniques.

Although it may appear simple and there are people who will tell you that is is simple, testing and properly understanding the results can be fairly complex. It is easy to arrive at false conclusions.

For a start, it is inappropriate to be trying to do "continuity" tests whilst the battery, or any other voltage source is connected to the circuit. Secondly, a DMM continuity test may not correctly indicate that some current may flow when there is a 12v source applied.

If you wish to determine if current is "leaking" from your battery to the cars circuits then the appropriate method is to set the DMM to "Amps or milliamps", disconnect the wiring from the battery terminal post and connect the DMM between the battery terminal and the wiring. Any current "leakage" will then be indicated on the DMM. Finding just which circuit or device that is consuming that current is another matter but generally can be determined by progressively disconnecting sections until the current indication ceases. Note that some few milliamps current drain is normal due to such things as ECU, radio, security etc.

Cheers
Allan

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Follow Up By: Ross M - Thursday, Sep 13, 2012 at 12:19

Thursday, Sep 13, 2012 at 12:19
G'day Doey
I totally agree with all of Allan B's comments. He explained it "more better" than I probably would have done.

As he said, the testing is relatively simple and is mostly yes/no answers via a multimeter but the logic and understanding behind it is the important bit.

I found the explanation of what you said about the testing with the multimeter, to be confusing and have no idea of what you have done or what was actually tested. Was it about the vehicle battery or the aux? or both?


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Reply By: Member - John and Val - Thursday, Sep 13, 2012 at 12:26

Thursday, Sep 13, 2012 at 12:26
Doey,

Very risky using the meter's continuity tester when there is a battery connected - too easy to let all the smoke out of it! This also extends to gear that may have a charged capacitor, as the capacitor may store suficient charge to do damage.

First, provided there is no battery or other source of charge in the circuit, it shouldn't matter which way round you connect a continuity tester. If there is a diode involved, measuring continuity can be misleading. (A diode conducts in only one direction, so will give diferent answers depending on which way round you connect to it. They also have a significant voltage drop, so, depending on the meter, may appear open circuit when in fact they will conduct at voltages higher than the meter is using - can be very confusing.)

I suspect that your meter has died, but if it's still working, I'd proceed as follows: You have quite a bit of stuff connected to the battery and my approach would be to fully charge the battery, then disconnect individual bits to decide where your discharge path is.

You could insert your multimeter in series with one aux battery terminal and measure the current flowing from the battery, BUT, there will probably be smoothing capacitors in both your chargers, and the inrush current when you make the connection will very likely blow the fuse in the meter. My preference would be to insert a low value resistor (less than an ohm) between a battery terminal and all connections, then measure the voltage across that resistor. Using ohms law (volts/resistance= amps) will allow you to safely measure current flow. (Ideally, a 1/1000 or 1/100 ohm resistor so you get a direct readout in amps when measuring millivolts.) Now connect and disconnect every suspect item until you find the offender. My bet is on the 240V charger that probably has an internal blead resistor to discharge it's output capacitor.

Why does the contnuity checker's beep change? Dunno, but probably covered in paragraph1!

"My second battery wasnt holding charge so i started to look over the system, got the multimeter out and checked for voltage, checked the isolator for continuity and switching etc. I came accross that there seemed to be continuity between the + and - points but only when the + leads were touching the - points on the car (earth etc) but the beep sounded different, not as pronounced. no matter where i checked. I thought- ah ha..... i have a short. I completely isolated the aux battery, regulator and charger and tested it again at the isolator- still continuity????" Not sure just what you were measuring and just where, but by now your meter has probably died - I think you checked for contuity directly across a battery which would be instant death to some at least of its functions.

"I disconnected the isolator and checked directly at the main battery (still connected to the vehicles wiring etc) and there was still continuity??? i started the engine and it stopped. turned the engine off and it started again????" As above.

"Is it normal to have some continuity between + and - ? or at least some low resistance or similar? enough to triggered the multimeter to indicate a circuit?" As above.

John


J and V
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Follow Up By: Member - John and Val - Thursday, Sep 13, 2012 at 12:31

Thursday, Sep 13, 2012 at 12:31
While i was writing the above, Allan has responded more succinctly and I'd have to agree with his comments

Cheers

John
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Reply By: DOEY- Thursday, Sep 13, 2012 at 13:11

Thursday, Sep 13, 2012 at 13:11
hi
thanks for your replies.

after reading back through my OP i realise it sounded confusing. my apologies. You guys did well to decipher it.

I understand all that you have said.

cheers
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Follow Up By: Allan B (Member, SunCoast) - Thursday, Sep 13, 2012 at 13:38

Thursday, Sep 13, 2012 at 13:38
Thanks Ross & John for your endorsement. So far this is one "Electrics" thread where there is no disputation.......so far! LOL

John, I'm sympathetic to your disquiet at having composed an extensive reply to then find someone else has already responded in like manner. It has happened to me often such that now I sometimes open the thread again in a new window to check before hitting the Submit button to my response.

Cheers
Allan

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Follow Up By: Ross M - Thursday, Sep 13, 2012 at 16:07

Thursday, Sep 13, 2012 at 16:07
Allan B
When you said you check, by opening another window, I thought you must have Irish blood ie. to be sure to be sure.
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Reply By: Allan B (Member, SunCoast) - Thursday, Sep 13, 2012 at 14:10

Thursday, Sep 13, 2012 at 14:10
In truth, for some time I have found my cranking battery falling-off in charge when not being used. So I am now about to do just what I advised above..... connect a milliammeter and see if there is a draining load. If not then I have a self-discharging battery..... oh woe!

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Allan

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Follow Up By: Allan B (Member, SunCoast) - Thursday, Sep 13, 2012 at 15:33

Thursday, Sep 13, 2012 at 15:33
Well that was interesting but inconclusive. There is 25mA drain on my cranking battery. I have three outgoing circuits, one to the vehicle electrics and one each to two Redarc BCDC1220 chargers thence to two auxiliary batteries.

Of these outgoings, the vehicle drain is essentially zero, one aux circuit is 5mA and the other is 20mA. Redarc specs for the BCDC1220 is <5mA current in standby, so one is drawing 20mA. I will investigate this further but on the evidence, 25mA constant drain is only 4.2AH per week battery discharge. Not enough to cause the performance loss I am experiencing so there may be some battery internal discharge also. Bugger, now where is that warranty invoice?

For the moment, I have left the battery disconnected from all wiring and will see if it still discharges. Mind you, it is only a problem when garaged. When touring the daily usage eliminates the problem.

Doey, sorry to be hijacking your thread but hopefully there may be some lesson in it for you.

Cheers
Allan

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Follow Up By: DOEY- Thursday, Sep 13, 2012 at 16:21

Thursday, Sep 13, 2012 at 16:21
no probs Allan B

Very interested in your input. I will try what you have done tomorrow. Im expecting some current drain too! i have a couple of leds om a few circuits that indicate power at certain points in the curcuit ie- after the fuse as some of the fuses arent easy to get too!

I realise now that trying to check continuity of the battery was a silly thing to do in hindsight. I came accross it when testing the isolator between the 2 + posts, starting the engine and making sure the isolator opened to allow the flow of power to the aux battery.

I accidentally touched the negative post with one of the test leads and heard the beep- this is when an alarm bell went off in my head and at first i thought it was a short. when re-tested again out of interest it only happened when + test lead was on and - post or eath. and a different toned buzz!
After some more research and input from the above posts i have concluded that the internal resistance of the battery, the fact that the circuit would be live (ecu, leds etc). all factors i cant rule it out.

I also concluded that the power outlets on my 240v charger allow for reverse polarity and correct themselves once it detects a load.

i may have just jumped the gun with this post but i do appreciate the help of people who take the time to reply.


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Follow Up By: Bob Y. - Qld - Thursday, Sep 13, 2012 at 16:59

Thursday, Sep 13, 2012 at 16:59
Over the years I've found it difficult to accurately test whether a battery is "cactus" unless you've got an offsider, to turn the engine over, while you take some readings. I have another method, which involves gloves, No.8 fencing wire and a multimeter, but don't think I'll pas that one on today. LOL

We had one at the station, a load tester, so I bought my own. About $80, at Bearoing Service, in the Isa.



From my experience on stations, "flat" batteries are often misdiagnosed, when the terminals are dirty, or other simple problems.

As far as battery drain, and current draw, haven't got a clue.

Bob.

Seen it all, Done it all.
Can't remember most of it.

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Follow Up By: Allan B (Member, SunCoast) - Thursday, Sep 13, 2012 at 18:44

Thursday, Sep 13, 2012 at 18:44
Think I know the method Bob. And gloves would be a good idea.

In the Good Old Days, battery individual cells were accessible and the Load Testers looked like this. Gloves were an option.



This placed a high-amp load on an individual cell whist indicating its voltage. It gave some idea of the ability of the cell to deliver current and a bit like engine compression testing, showed up irregularity between cells. Nowadays the only way is a tester like you have which does the same but only the whole battery, not individual cells.

Neither type are the "be-all and end-all" but together with other observations do help with diagnosis. Mind you, if you are sure the battery has been fully charged, has clean terminals, yet still can't handle the starter load, then it is cactus.
Cheers
Allan

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Follow Up By: Bob Y. - Qld - Thursday, Sep 13, 2012 at 20:43

Thursday, Sep 13, 2012 at 20:43
Really tested the brains cells then, Allan. I'd forgotten about those old wet cells.

Have seen a tester like in your photo, but never used one. Probably didn't know what it was when I saw it anyway.

Those old batteries, had like a hard rubber case(???), with bitumen or something around the cells didn't they?? Remember they were a trap for young players, especially careless ones, whenever they dropped a ring spanner across the exposed terminals.

I could probably write a blog about dirty battery terminals, buggered terminals and
burnt out fusible links, over the years.

Thanks Allan,

Bob.

Seen it all, Done it all.
Can't remember most of it.

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