This little item paid off.

John B. · Oct 24, 2013

I've always liked electricity and had a battery connection problem with my 98 Chevy Blazer. So I decided to buy a voltmeter that plugs into my cigarette lighter.

I noticed the last two days the charging voltage in the Blazer has been creeping up to 14 volts and today it hit 14.5 volts.

Tonight I took the side post battery connection loose and cleaned them. They weren't that dirty and the battery is only 2 yrs old. After I cleaned them and hooked them back up it now charges 13.7 volts. So this little ebay item paid off.
Volt Meter for Cigaretter Lighter, Cute

omahagreg · Oct 24, 2013

A properly working alternator will put out 14.6 under FULL load! At 13.7 something in your system is not working!

John B. · Oct 24, 2013

I agree at full load an alternator is capable of putting out 14.6. But not with no load after 20 miles.

Then tell me why after cleaning the battery connections would it lower the charging voltage?

I know why it lowered the voltage because it lowered the resistance in the circuit.

I've put this volt meter in our other three vehicles and they all showed the same charge voltage of 13.8

omahagreg · Oct 24, 2013

Dad has been a mechanic all of his life-started on the farm as a kid! I have quizzed him out numerous times about this one. A rested battery-first thing in the morning, 8 hours or more of not running-must read 12.6 or something is wrong (undercharged from bad belt or bad alternator or battery is bad.) When running, at any load, battery voltage will read 14.6, or something is wrong. It will read 14.6 with nothing on, or, give it a load test, and turn the headlights on AND the AC-it should still read 14.6, or something is wrong! This has always proven right in any vehicle I have tested.

Anonymous-0 · Oct 24, 2013

Wonder if the meter is a bit off calibration ?

Switchflicker · Oct 24, 2013

Here's a question about volts then. I fixed up a voltmeter with a DPDT switch to measure the volts on two different batteries in the motorhome. One battery is the starting battery and the other is the house battery. I can flick the switch left or right to read the individual batteries. The meter is analog, but that shouldn't make a difference.

The question is this. Why does one battery read about one volt or so more than the other one? Wire length to the meter is about the same. Meter is the same so no calibration issue.

Sw

toolz · Oct 24, 2013

SW- If your system uses a battery separator, that"s usually what happens. The primary battery set will read normal, and the secondary will read lower due to a voltage drop across the separator. I"ve had nothing but problems with the separators, and now use high amp continuous duty solenoids to separate the battery banks. They can be triggered with a manual switch, or off the ignition switch.

Greg K · Oct 24, 2013

Metric volts vs sae volts.

todd hamilton · Oct 24, 2013

Ohm's law states E/I is oooohhhmm, let me think about it a minute

Owen Aaland · Oct 24, 2013

(quoted from post at 22:26:52 10/24/13) Metric volts vs sae volts.

I'm going to have to remember that for the next time I run into a problem!

js305 · Oct 24, 2013

This has created quite a discussion. My experience is more like the first post. When all things are working correctly battery voltage does change. After sitting overnight a battery will lose some voltage. If healthy it will be at or slightly above 12.5 volts or so. This is assuming there is no abnormal draw. After starting the system will jump up to the max allowed by the regulator, 14.5 or so, until the battery is charged back up. The voltage will gradually drop back to 13.5 or so. I think the assumption is correct that connections can and do make a difference. We have learned this on the old six volt tractors.

Anonymous-0 · Oct 25, 2013

There is an allowable variance. Not every charging
system is going to be exactly 14.6 volts.

Bob · Oct 25, 2013

Typically, vehicles new enough to have an alternator system from the factory have remote voltage sensing... a wire from the voltage regulator to near the (+) battery post... and keep the battery voltage in the 14 to 14.5 Volt range regardless of load (within the charging capabilities of the alternator). I"d be SURPRISED to see a charging voltage as LOW as you are reporting.

B2200 · Oct 25, 2013

Switchflicker, The best way to explain this is that your main starting battery was probably up to a fuller state of charge, so it would read a higher voltage. The auxiliary battery, if in a low state of charge requires more amperage from the alternator to bring it up to full charge. Until it gets to that point, the voltage on it will show slightly lower. As it charges up, they will equalize. Also, charging a battery for say and hour does not mean it is fully charged. The longer a battery stays at a lower state of charge, the longer it will take to bring it up to full state of charge. It will require more amperage from the charging system to bring it to full charge. Don't assume that the auxiliary battery is fully charged after say even a four hour drive after being drawn down. Especially if it is left in a low state of charge for a while, that battery should be put on a charger.

MarkB_MI · Oct 25, 2013

Will resistance in the charging circuit increase the alternator's output voltage? Sure, voltage regulators are imperfect devices and you'll get a higher voltage across a high-resistance load than one with low resistance.

However, there's a serious problem with your theory: If the resistance was high enough to cause a measurable rise in voltage, you would have had starting problems. Do the math: When the battery is fully charged, the charging current is only a few amps. Let's be generous and say 10 amps. A one volt rise translates to 0.1 ohms. But that same one-tenth of an ohm is present during starting. So let's say it takes 200 amps to crank over your engine. 200 times 0.1 = 20 volts. Oops! We only had about 12 volts to work with. I don't think that engine is going to turn over with a tenth of an ohm in the circuit.

Here's the deal: Voltage regulators are temperature compensated. The hotter it gets, the lower the voltage. That reduces overcharging while ensuring a rapid charge after starting. If you watch the voltage from a cold start until you've been driving for half an hour, you'll see the voltage slowly drop off. It'll probably start out close to 15 volts and drop down to less than 14. That's just normal behavior and you'll see it in any vehicle with a voltage regulator.

MarkB_MI · Oct 25, 2013

You probably have a battery isolator, which is nothing more than a big diode. The forward-bias voltage drop across a silicon diode is around 0.7 volts. So the battery behind the isolator (the starting battery, I assume) will always be slightly undercharged.

Texasmark1 · Oct 25, 2013

Or one may be a deep cycle storage battery. I haven't bothered to get into the tech part of it but I did have a battery charger with a switch for regular lead acid batteries and deep cycle/something else batteries. Agree that one may charge to a different voltage. Don't know and don't care. Just an observation.

But I certainly agree that the comments that were made on here that a diode in the circuit is .7 volt loss normally and at 200 amps cranking current 1/10 ohm of resistance in the starting circuit is too much.......but you shouldn't see that in a static voltage test since the current to to operate the voltmeter is micro amperes and microamperes through 1/10 ohm of resistance is microvolts (.000001 V is a microvolt).

Mark

Switchflicker · Oct 25, 2013

Thanks for your replies Gentlemen (and ladies if there are some lurking around)

There is an isolator. Big blue thing with cooling fins and a number of electrical connections. I suspected there was a voltage drop because of this. I'm not sure if it's worth the work to change it out for a high amp relay. This would be to connect the batteries together while driving, right?.

It works, so I'm not going to fix it right now. Just needed a logical explaination. That's all

Sw

pete 23 · Oct 25, 2013

Like said, voltage regulators are temperature compensated. Some are very active, some are not. Some read battery temperature through a sensor under the battery , most don't. Some regulators for continuous use like farm tractors running for many hours full bore have a lower set point to reduce overcharging. When IH started using the lower voltage regulators , most thought the alternator was defective. That being said, normally when checking for full rated output of a alternator, you load the battery until it drops to 13 volts and at that voltage the alternator should be capable of turning out it's rated amperage, like 100 amps for a l00 amp alternator or within 90 percent of that. So, you can see , that voltage will normally drop some when using a lot of current.

MarkB_MI · Oct 25, 2013

Switchflicker, the voltage drop across a solid-state isolator is a small price to pay to have a simple device that's almost 100 percent reliable. Relays are inherently unreliable and require a certain amount of control circuitry that's also unreliable.

Switchflicker · Oct 25, 2013

(quoted from post at 21:00:56 10/25/13) Switchflicker, the voltage drop across a solid-state isolator is a small price to pay to have a simple device that's almost 100 percent reliable. Relays are inherently unreliable and require a certain amount of control circuitry that's also unreliable.

Thanks Mark,

Sometimes the best thing to do is nothing. And who knows, maybe the manufacturer knew what he was doing!!

Sw

This little item paid off.

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