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I took my 04 TB into the shop because the battery would go dead if not driven for a couple of days. Prior to this, I was experiencing some performance issues (rough idle, lower fuel mileage, shifting at higher RPM in low gears, etc). I think the battery might have had a bad cell (brand new 6 months ago). The performance problems seemed to have disappeared after the battery was replaced. Any ideas as to what may have been causing these performance issues and why changing the battery would have corrected it (resetting of faulty codes, engine working harder to charge the battery at lower idle, etc.)? I'm just curious for future reference and perhaps others are experiencing similar issues and this could be off assistance.
 

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2002 chevy trailblazer_ltz
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Thought that was normal(both TV and Equilnox) does that since new.Dies like a rock!And they told me it has a memory with the computer that shuts the battery off when it goes low and it keeps anough power to crank it over(that never worked!)
 

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2004 chevy trailblazer_lt
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Actually, the answer is simple. If the battery is really low, during cranking if the voltage ever falls below 5 volts (which is very possible on a bad battery with a broken plate) the PCM loses the stored settings for crankshaft position and goes into factory default mode. (I am assuming it can lose a BUNCH of other parameters as well.)

This means your computer has lost all the adaptability it has built up over time to compensate for your engine, your conditions and the build-up of residue on the back of the throttle body, and the engine starts and runs like crap for a while.

This is why these vehicles are heavily dependent on good batteries to run properly.

Also, when the battery gets really low, the alternator can no longer charge it. (That's what a battery charger is for.) It is actually a bit of a myth that alternators charge a dead battery; alternators actually need a minimum voltage to work, so they can't charge a completely dead battery. The engine runs on the voltage of the battery, not the output of the alternator. In simple terms, the engine runs off the battery and the alternator is only there to charge the battery. This is why most modern vehicles will not run without a battery in place.


Good luck with your new battery ...

... and by the way, your throttle body needs cleaning.
 

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2004 gmc envoy_slt_xl
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The engine runs on the voltage of the battery, not the output of the alternator. In simple terms, the engine runs off the battery and the alternator is only there to charge the battery. This is why most modern vehicles will not run without a battery in place.
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Actually I have run many vehicles with the battery disconnected. Just for testing. I never run it for long as it sends the alternator into a high output and can overheat it. From what I know, the battery is used to start the engine and to suppliment the alternator if needed. If I am wrong, when did it change ?
 

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2004 chevy trailblazer_lt
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Hey, even experts disagree on this point!

:)

As far as I know, it has been this way ever since alternators had an 'exciter' circuit built into them that required battery voltage before the alternator would start producing output.

Plus, it is not the overheating of an alternator that would kill it; it would be the voltage spikes.

In theory, diesels do not need a battery once it is started but most modern gas engine vehicles do. This is why most vehicles will keep on running for quite a while with a failed alternator (because the vehicle is running off the battery) but will die almost instantly with a dead or disconnected battery (because the alternator does not supply juice directly to the ignition system.)

Yes, you are quite correct that SOME vehicles will run with the battery disconnected for a few minutes. Not being an alternator expert, nor willing to chance ruining my alternator, I am not going to try it on my Trailblazer, but essentially it still remains that the engine runs off the battery power and the alternator charges the battery.
 

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2006 chevy trailblazer_ls
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This is why most vehicles will keep on running for quite a while with a failed alternator (because the vehicle is running off the battery) but will die almost instantly with a dead or disconnected battery (because the alternator does not supply juice directly to the ignition system.)

Yes, you are quite correct that SOME vehicles will run with the battery disconnected for a few minutes. Not being an alternator expert, nor willing to chance ruining my alternator, I am not going to try it on my Trailblazer, but essentially it still remains that the engine runs off the battery power and the alternator charges the battery.
The alternator supplies ~100% of the vehicle's power while running.

Without getting too technical:

The battery and alternator are connected in parallel. To say that the battery runs the vehicle and the alternator charges the battery suggests that current is flowing both into and out of the battery at the same time.

Once the engine is started the electrical system is at a higher potential than the battery (~14.3V vs. 12V respectively). This necessarily means that current does not flow out of the battery (physics). In fact, current flows into the battery to recharge whatever capacity was used during cranking. Once it is charged it effectively goes into 'standby' (and arguably stablizes the alternator). Only when the alternator fails does the battery start discharging as it takes over running the ignition and other systems for a few minutes.

Why an engine would shutdown with a failed battery? I'm not sure. I can only speculate that transient voltages can momentarily drop low enough to cause the alternator field to collapse and without the magnetic field the alternator cannot generate electricity; with no back up from the battery the engine shuts down.

I would argue that calling it a "charging system" is a misnomer for 99% of its duty.
 

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2004 chevy trailblazer_lt
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I bow to your expertise and am not comfortable arguing because I am not an electrical or alternator expert.

I, too, have heard the argument that the battery has a lower 'potential' than the alternator. (I think of higher potential as being like a water pipe at a higher elevation.) BUT once the vehicle is running, the battery then jumps to the same 'potential' as the alternator. (This is easily seen on a voltmeter reading. Engine OFF, battery is 12.5 volts or so; engine ON, battery is 14.7 volts or so.)

This stands to reason; the battery accepts the higher potential from the alternator but it is just like a water pipe - if one has higher pressure than the other, once they are connected together, the pressure equalizes.

The engine takes this 'higher pressure' out of the battery and uses it to run. The 'higher pressure' is maintained by the alternator. If the alternator fails, the voltage ('pressure') drops somewhat but the vehicle is designed to run even at lower and lower voltages, until it drops too low. (This may, in fact, be hours later.)

If the battery is disconnected on a running engine, some vehicles may run for a few minutes, but will NOT run for hours. The alternator is not designed to run the vehicle; it is designed to charge the battery.

But all this is pretty irrelevant anyway. Like I said, I am not an expert - and even the experts disagree on this point.:):):)
 
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