Battery or alternator?

[Snagglefoot]

A bad battery puts a strain on an alternator. A bad alternator puts a strain on a battery. If in doubt change them both and be done with it.

 

[zrxkawboy]

You cant check a discharged battery and get any meaningful reading regarding its health. It should have been put on a charger before testing.

^^^This. You can't condemn the battery by testing it while discharged. Fully charge it first - using a proper charger overnight, not by driving it for 20 minutes - then re-test.

 

[burbguy82]

Disconnecting a battery with engine running will likely ruin the alternator...Just saying

Interested where that info comes from, never heard that. Of course, this would be momentary and not for an extended period. How would it ruin the alternator?

 

[mk378]

It causes a voltage spike which can be bad for anything electronic. Do not do the "disconnect the battery while running" test on a modern car-- or really any car. There are better ways to test an electrical system.

 

[willbur]

It causes a voltage spike which can be bad for anything electronic. Do not do the "disconnect the battery while running" test on a modern car-- or really any car. There are better ways to test an electrical system.

True- 200 volts with nowhere to go (like a battery).

 

[burbguy82]

makes sense, but I am not so sure it works quite like that.

True- 200 volts with nowhere to go (like a battery).

 

[SHOZ]

If the battery voltage is above the engine off voltage then the alternator is working. But how much? Newer cars turn them on and off. I don't like anything under 13.8 unless it's hot out. Gen Coupe OE alternator puts out a strong14.5v when charging and seldom is under 14v. 135k miles but the battery is in the trunk out of the heat.

 

[willbur]

makes sense, but I am not so sure it works quite like that.

Well, how does it work then? google "does disconnecting a battery on a running engine damage alternator or electronics" or something similar. Be informed- read what it tells you.

 

[bullwinkle]

The battery acts like a great big capacitor, dampening the pulses put out by the alternator (full wave rectifier means major ripple in output DC voltage). Disconnect the battery, you risk damaging EVERYTHING electronic in the vehicle! Back in the day, the radio was the only thing electronic (& could be turned off)-that’s why you could get away with disconnecting a battery on a running vehicle.

 

[burbguy82]

Well, how does it work then? google "does disconnecting a battery on a running engine damage alternator or electronics" or something similar. Be informed- read what it tells you.

good ole google

Well, how does it work then? google "does disconnecting a battery on a running engine damage alternator or electronics" or something similar. Be informed- read what it tells you.

True- 200 volts with nowhere to go (like a battery).

did you get this from google too?

 

[vpr5703]

makes sense, but I am not so sure it works quite like that.

The way I understand this to work is that the battery acts as a buffer for voltage and load changes because the load on a cars electrical system is never really constant. And as a result the ECU is constantly altering how much energy the alternator is putting out. The battery absorbs or lets out a little bit of energy when the system load varies, acting as a buffer for the short time that the alternator takes to respond to the requested output change.

Without the battery, the voltage coming out of the alternator could swing higher or lower than it should during these load changes and requested output changes. And if it swings too high, you burn stuff out.

 

[burbguy82]

The way I understand this to work is that the battery acts as a buffer for voltage and load changes because the load on a cars electrical system is never really constant. And as a result the ECU is constantly altering how much energy the alternator is putting out. The battery absorbs or lets out a little bit of energy when the system load varies, acting as a buffer for the short time that the alternator takes to respond to the requested output change.

Without the battery, the voltage coming out of the alternator could swing higher or lower than it should during these load changes and requested output changes. And if it swings too high, you burn stuff out.

I am with you, but 200 volts? Not sure how that is possible, and devices use no more amps than they are capable of drawing, unless we are talking about ultra high voltage. Interesting.

 

[willbur]

good ole google


did you get this from google too?

No. Knew this for at least 40 years- before good ol google. But what difference does this make, anyway? So easy to learn stuff these days with just a little effort.
Believe what you want regarding high transient voltages that are generated when a disconnect happens. The battery is like a surge protector. Got nothing to do with amperage.
Not gonna argue about this.

 

[iontrap]

The battery tested good at AA(CCA were a little low), but decided to replace it. No problems starting with the new battery.

 

[burbguy82]

No. Knew this for at least 40 years- before good ol google. But what difference does this make, anyway? So easy to learn stuff these days with just a little effort.
Believe what you want regarding high transient voltages that are generated when a disconnect happens. The battery is like a surge protector. Got nothing to do with amperage.
Not gonna argue about this.

great, thanks, have a good one

 

[vpr5703]

I am with you, but 200 volts? Not sure how that is possible, and devices use no more amps than they are capable of drawing, unless we are talking about ultra high voltage. Interesting.

I don't think 200 volts would be possible. But as cars get more advanced we get more electronic doodads. And they might be fickle. So while I don't think 200V would be possible, 20-40 might be. And applying voltage significantly higher than rated input can destroy sensitive parts

 

[willbur]

I don't think 200 volts would be possible. But as cars get more advanced we get more electronic doodads. And they might be fickle. So while I don't think 200V would be possible, 20-40 might be. And applying voltage significantly higher than rated input can destroy sensitive parts

Here is a very rough analogy to illustrate the principal: the ignition system uses an inductor (the "coil") to take 12 volts and produce 20k volts to fire the plugs. A driver circuit in the ECM or ign. module ("points" in old cars) interrupts the 12 volts. As the magnetic field collapses in the coil/inductor, it induces a secondary voltage, say 20k. Twelve volts now makes 20K volts.
Now, interrupt the field current flowing to the alternator by removing a battery cable on a running engine. The stator windings are the inductor and the collapsing field generates a transient counter EMF. Voila- 200 volts. If you are lucky, a clamping circuit may reduce this to protect the electronics.

I quote the 200v I remember from an old electronics book. I just read 150v on "good ol google" LOL

 

[dumkid]

My alternator only puts out 13.5v. good enough i guess because my battery is 3 years old and doesn't show signs of dying yet.

Alternator is original with 197,000 miles