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Voltage fluctuation

So im not sure what to think when the car is running it is 14v but will move all around between 6v and 14v at the battery and when its off it will go between .01v and 12v at the battery.

You probably have a bad connection at the gauge.

Have you checked all the cable connections? Poor or resistive connections will, give you a false reading, and a bad connection could easily cause erratic voltage.

You may also want to check the battery terminals to be sure they’re tight and secure. I’ve seen a post broken free inside the battery casing cause erratic voltage.

Post back.

Well this is with a volt meter and i took tgthee cables off the battery and cleaned them with coke and a wire brush they look pretty clean… Also after about 3 days of sitting the battery will be dead

When you don’t post the year/make/model of a vehicle, it’s impossible to see if there’s any TSB’s that may be related to said problem.


Remember that the cables each have another end. In the case of the + terminal, it probably also has a “fusible link”. That looks like a wire, but it’s actually a fuse. It’ll be the thinner red wire going to the relay box under the hood. The thicker wire will connect directly to the starter assembly. When you turn the key to START, a relay in the relay box closes its contacts, which causes the solenoid on the Bendix assembly to close internal contacts and allow the thick wire to carry the necessary current directly to the starter motor. When you release the key to ON, that circuit is disconnected and the smaller wire becomes the circuitry wire.

That smaller wire, the fusible link, protects your car’s operating circuitry and can blow. And I’ve seen one become intermittent, the insulation holding the blown stuff is proximity to its brethren sufficient to manifest itself as an intermittent circuit. Normally they open completely and cause the insulation to bubble up from the heat of melting.

As Tester said, it’d be a help to know what kind of vehicle we were discussing. Year, make, model, and engine (if there are options) can make a difference. My description above is very general.

You said you cleaned it with coke and a wire brush. Did you remove the cables and clean the inside holes and battery posts? If not, then you have to do that. I use one of these.

You made the two battery voltage measurements under the hood with your own volt meter, (aka DVM), not with one on the dashboard right? So you measure the two battery posts with your own volt meter and get readings which vary from 0.1 v to 12 volt with the engine off? If so, I think you may have a problem with your volt meter. One of the input cables to the volt meter might have a broken wire in it. I’ve had this happen to my own volt meter, and it displays erratic values like that. A car battery won’t usually change it voltage like that in a matter of seconds.

Another guess, maybe you aren’t making a good connection to the battery posts with the meter probes. Clean the top of the posts well of any oxidation and corrosion and measure there.

A third idea, try your volt meter on another car battery you know is working ok.

BTW, 14 volts with the engine running and a fairly well charged battery is normal. But 12 volts at the battery – with the engine off – is on the low side for a fully charged battery. I measure around 12.6 volts on both my Corolla and my truck in that situation. That could be indicative of a faulty battery and/or charging system.

Three days of sitting with the battery cables connected or disconnected?

You didn’t state where the voltage measurement was taken at. If it was directly across the battery then either something is wrong with your meter (a bad test lead possibly, as GSJ mentioned) or the battery has an internal connection problem and you need to replace the battery. The charging voltage seems to be okay. You seem to have another issue with a parasitic current drain on the battery while the vehicle is parked. Normal current drain should be less than 50 milliamps when things have gone into the sleep mode.

If measured at the batt terminals with a voltmeter and not relying on the interior gauge…if this were true you would be wise to look at all connections at alternator…or start suspecting the internal voltage regulator inside the Alternator as well as batt connections. All depends where you took the measurement from. At batt posts…then the connections on the batt can be suspect. On the terminals themselves…and it goes toward the Alternator


Its a grand am gt in not sure the year but its one of the batt styles that dont have post and i checked it with the cables on but i was testing at the battery but if i check right on top of where the cables screw into the battery i cant get a good read because the voltage is going all over the place but if i check where the wires are crimpt to the eye hole peice that goes in the battery i get a steady read on the voltage i was thinking about replacing the connections that go on the battery if that dont work get a new battery and if that dont work cut out the ground and hot wires all the way to the fuse box and put new ones in

Bradley, this not twitter and you are allowed to use proper sentences and punctuation to make reading your posts easier.

Voltage on a good battery cannot fluctuate between 0.1 and 12 volts. Or even a bad battery.

To move instantaneously from 12 to close to zero would mean an instantaneous short on the battery or an open connection. As an instantaneous short would cause the battery to explode, then you must have a bad connection.

Ordinary chrome plated meter probes do not make a good connection unless you use a lot of pressure. Or are you probing at the connectors, not the actual battery posts. There could be a bad connection there.

I have seen a battery with a cracked internal buss bar that would intermittently go from OK to open circuit.
However OP seems to have a battery with side connections that screw in.
Those can be tricky to properly clean.

Bill is correct…methinks you are viewing a wonky voltage regulator within the Alternator.


Your problem is not in the car, it is either the meter probes or lack of knowledge of how to use a meter.

Fluctuation can happen…usually right before total failure of output. Cold solder joints in the VR inside the distributor…loose connect somewhere internally …sure it can happen. I have to trust what Im being told is the symptom.


I don’t care what is happening in the alternator or regulator, there is no way the battery voltage can jump between 0.1 and 12 volts without the battery exploding.

Wait…Bill and Keith may be correct here… While its connected to the batt we really shouldn’t see wild fluctuations… This is true… the batts voltage would always be in the voltage measurement. Oops…my bad