Checking a battery with a multimeter

[SDE]

I have two very cheap meters. I put a charger on a battery in the Saturn Vue. I tried to check the voltage at the battery to confirm that the charger was charging the battery. I was getting very low readings that varied quickly, going up and down. I tried both meters, and they both got the same irregular readings, with the battery in the vehicle. I removed the charger and still got inconsistent reading. I thought that the meters were faulty. When I checked a battery that is in the garage, I got a steady reading. So now I am thinking that must be a problem in the Vue. I know the ignition switch has a problem and a dealer is going to check it out on the 28th. So can someone explain to me why the readings on my cheap meters were jumping from 2.5 to 5 to 3 to 7 and 10 to 6 and eventually 12.4?
TY
SDE

 

[Charles in Aus.]

It does sound as if something is causing an intermittent and inconsistent drain . The battery is most likely compromised by this as well . The ignition switch could well be the culprit , a loose or partial short draws current then falls away as the wire or contact heats then contacts again as it cools . I would isolate the battery to be safe , that is if the car's security system will let you .

 

[Fawteen]

Probably because the output of some battery chargers isn't pure DC. Some have a half-wave rectifier that produces a pulsing DC and a digital meter will reflect that.

 

[MarkB_MI]

I'm inclined to believe the battery in your Vue is shot. No matter how noisy the output of the charger is, when hooked up to a good battery you should get a good, solid DC reading. Also, when you hook a charger up to a totally dead (but good) battery, the charger is going to start limiting itself so it will be turning on and off, making measurements difficult.

It would be interesting to know what sort of reading you would get with a high-end DMM. Pretty much all good multimeters perform analog-to-digital conversion using a technique called dual-slope integration. A side benefit to dual-slope integration is high noise immunity. It's possible your meters use a different conversion technique (such as successive approximation) that is more sensitive to noise.

 

[MarkB_MI]

> Probably because the output of some battery chargers isn't pure DC. Some have a half-wave rectifier that produces a pulsing DC and a digital meter will reflect that.

Actually, all battery chargers produce pulsed DC. The diodes only conduct when they're forward biased, which only happens during the portion of time the rectified AC output is greater than the battery's voltage. So if you looked at the actual current going into the battery on an oscilloscope, you would see it pulsing between zero amps and whatever current the charger can produce. The pulsing has a shorter duty cycle if the output is half-wave rectified (a common occurrence if one diode is blown), but it will pulse even with a full-wave rectified output.

 

[guido]

SDE,

Check the Meyer's battery voltages.
Anything under 8.4v bolts will make the readings irregular,

Guido.

 

[Bret4207]

(quoted from post at 07:20:15 06/21/21) > Probably because the output of some battery chargers isn't pure DC. Some have a half-wave rectifier that produces a pulsing DC and a digital meter will reflect that.

Actually, all battery chargers produce pulsed DC. The diodes only conduct when they're forward biased, which only happens during the portion of time the rectified AC output is greater than the battery's voltage. So if you looked at the actual current going into the battery on an oscilloscope, you would see it pulsing between zero amps and whatever current the charger can produce. The pulsing has a shorter duty cycle if the output is half-wave rectified (a common occurrence if one diode is blown), but it will pulse even with a full-wave rectified output.

Explanations like that are why some of us are okay with the idea of anything electrical being magic...and leaving it at that! :lol:

 

[John T]

I think you may well be right, the problem is in the Vue !!!!!!

NOTE a cheap digital meter is NOT known to produce perfect smooth steady voltage readings when a battery is connected to a charger or an inconsistent load..... A good battery at rest and stabilized not connected to loads or a charger, however, can yield a steady voltage even with a digital meter.....

SDE, since you get steady readings on a known good battery sitting with no charger connected, yet in the Vue with or without the charger connected the readings are erratic, I suspect the Vue battery is bad and/or some sort of an intermittent switching/loading due to a faulty switch may be the problem........ Would it make a difference if you disconnected the Vue battery from the vehicle but left ONLY the charger connected ??? If it is smooth and steady unhooked from the vehicle there may be some sort of a switch or other Vue problem.......

While battery chargers produce pulsing DC (they are NOT filtered like a quality DC power supply) the battery sits there like a huge electron bank buffer which smooths out the voltage.

Even if your meters are NOT the problem I still like my old reliable analog Simpson 260 with its damping action

Many auto or battery stores will test a battery and charging system FREE. If the battery and charging system are all good, a fully charged lead acid battery at rest and stabilized reads about 12.6 volts while under a good charging system it may raise from at least 13 to 14+ volts

Have the battery and your charging system tested

Best wishes yall God Bless America keep her safe strong and great

John T

 

[Russ from MN]

Take off one battery cable and then check it!

 

[Fawteen]

Ooops...missed the part about still getting funky readings with the charger disconnected.

 

[Janicholson]

Digital volt meters (on the DC scale) vary from giveaway store promotions, to expensive Fluke brand certified units that cost hundreds of dollars. All of them sample the source of voltage using a timed sample sequence. This sampling is done rapidly taking a measure lasting only tenths of a millisecond. When a source of voltage is full of static, it reads the static not the average voltage. The display then changes, showing the voltage it found in that tiny (wrong) sample. High dollar digitals take maybe a thousand samples, compute the statistical average, and display that result. They result in a representation of the net voltage that is pretty close.
A filing car electrical system (or just a running car/tractor or truck) can generate the static causing this garbage reading in lesser meters.
An analog meter (with a moving needle on a number scale) is doing this averaging because of the inertia of the needle mechanism. In some cases the needle will vibrate, but rarely. This reading is also an approximation, but usable. Even cheap analog meters work OK in a static laden measurement.
Jim

 

[SDE]

Thank you everyone for your responses. I regret not doing as John T. suggested and checking the battery with one cable removed. I have returned the VUE to her, so that she could get to work today. She texted that, Car ok getting to work. I hope that that means that the battery light stayed off. Of course tomorrow is another day. I am hoping that she can use this vehicle until the dealership can replace the ignition switch. If a problem persist after that, she might need to hire a better mechanic.
TY
SDE

 

[SDE]

I agree

 

[tech4]

I think your meter leads are not making good contact. I can't imagine an auto battery changing voltage that erratic. Turn the lights on and see if the lights are fluctuating. The cheap meter leads and some expensive leads have a coating that is hard to make contact on some materials.

 

[bc]

Sounds like we got the whole low down on chargers and multimeters.

Next two days are Amazing Prime Days. Just ordered a new multimeter with all the functions for diodes, capacitors, etc. Kaiweets model HT118A for 28 bux. It auto ranges and does 6000 counts. Saw one that does 10,000 counts (sample counts I assume) but more than I need. Looked at the flukes and kleins and this one seems comparable except not american or have the fluke quality.

When you get the car back you can disconnect one battery cable, forget which one but I think the positive, and connect a voltage meter between the battery post and the cable. If the car is discharging while setting then you know you have a problem somewhere. Then it is a matter of pulling one fuse at a time inside the car and under the hood and see if the discharge stops and it helps to have a helper watch the meter. Once you find the circuit then you can trouble shoot it. Lots of time it could be a door light switch or some relay such as a wiper relay that is sticking or motor or about all circuits have a relay and it could be a short or burned out wire and countless other possibilities.

 

[Geo-TH,In]

I don't leave home without

this meter in my truck.

Last week the 10 year old in Jubilee wouldn't start tractor.
I left the charger on all night. Voltage after charging 12.8 volts. Cranking amps 75. Battery was junk. This meter also measures cranking voltage and charging voltage.

 

[MARK ROBKE]

Ditto. Leads making poor connection. Mark.

 

[KayBur]

I use Cartman Booster Cable. Shows battery values very accurately. I bought this device quite recently, but I have already managed to evaluate its benefits.

 

[Ted in NE-OH]

If it is the give away Harbor Freight red meter, when the meter battery is low the display will give erroneous reading such as 120 VAC can read 140 VAC. Change the meters battery to see if that fixes the problem. BTDT