Just bought a 2006 Jetta TDI. Jumped it to start, no problem. Drive 20 miles home on the highway, runs great. Take the off ramp near home and come to a stop. Engine dies. Try to crank, get a few faint clicks. LSS, had it towed 5 miles home. Check the battery and it's at 4 volts. Put a charger on it and it's at 7 volts max. Usually a good battery will charge at 15V with this charger. OK so battery is toast, sticker is L2 which I assume is December 2012, over 6 years old. Now to the question. I thought the alternator puts out enough juice to keep the car running? Especially a diesel which doesn't have coils and spark plugs sucking thousands of volts. Heck, I've seen videos where a car is running with the battery removed from the car. Why couldn't this car run at least with a very weak battery?
Minimum battery voltage to keep engine running?
- Thread starter atikovi
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I wonder if too many cells shorted and the alternator can't keep the voltage high enough.
It might be a diesel but the ECU will pull plenty of current. Plus if voltage gets too low it might just decide voltage is too low, a low voltage lockout, where it just won't try.
It might be a diesel but the ECU will pull plenty of current. Plus if voltage gets too low it might just decide voltage is too low, a low voltage lockout, where it just won't try.
I'm far from expert in automotive systems, but your battery had probably gotten to the point that it was a heavy load on the alternator even at low voltages...like something was shorted or at least providing a unusually low resistance path inside the battery. I wouldn't be surprised if the car could have kept running, but some internal monitor picked up that things were way out of whack and shut the car down to be safe.
You might have been better off without the battery once you got the thing started!
You might have been better off without the battery once you got the thing started!
MolaKule
Staff member
The alternator has current limiting so if the load is too great, like shorted battery cells, it folds back the current output to keep itself from overheating and burning up.
An ECU barely pulls any current, under 1A is typical and over 2A, very unusual.
Odds are that your battery died because your alternator died. Dead battery alone will almost always start out as hard to start, not stopping while running. It's not common at all for a battery to start a vehicle and have several cells simultaneously fail within a single drive to the point where it tops off at 7V.
There might be some shutdown circuit but you are correct that an alternator alone can power a car fine EXCEPT that it may have a damaging amount of ripple if there is not a battery, or some alternate filtering circuit added in its place.
You might have some other problem that contributed to the battery draining so rapidly. Now you need to monitor the vehicle closely with a new battery, see if it has excess parasitic drain and whether the charging system is keeping it at 14V before risking driving much in it.
Unless you find another cause, I would proceed under the assumption you need a new alternator or there is a blown fuse to it, or dirty contacts, something keeping it from doing its job.
Odds are that your battery died because your alternator died. Dead battery alone will almost always start out as hard to start, not stopping while running. It's not common at all for a battery to start a vehicle and have several cells simultaneously fail within a single drive to the point where it tops off at 7V.
There might be some shutdown circuit but you are correct that an alternator alone can power a car fine EXCEPT that it may have a damaging amount of ripple if there is not a battery, or some alternate filtering circuit added in its place.
You might have some other problem that contributed to the battery draining so rapidly. Now you need to monitor the vehicle closely with a new battery, see if it has excess parasitic drain and whether the charging system is keeping it at 14V before risking driving much in it.
Unless you find another cause, I would proceed under the assumption you need a new alternator or there is a blown fuse to it, or dirty contacts, something keeping it from doing its job.
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Originally Posted by Dave9
Odds are that your battery died because your alternator died. Dead battery alone will almost always start out as hard to start, not stopping while running.
Yep bad alternator. And now that the battery probably has been drained a few times it probably wont last to much longer either.
Odds are that your battery died because your alternator died. Dead battery alone will almost always start out as hard to start, not stopping while running.
Yep bad alternator. And now that the battery probably has been drained a few times it probably wont last to much longer either.
Bad alternator would illuminate the battery light. Wouldn't have expected a 6 year old battery to last much longer anyway. Probably froze when it got down to 5 or 10 recently.
New car purchase? Any Wty? Start with a new battery before any parts is how I`d do it.
Originally Posted by atikovi
Bad alternator would illuminate the battery light. ...
That might be true in some or most cases, but when a brush wore out in my Mazda's alternator, the idiot light did not come on, because the light was connected in series with the brush. Tricky! The light did come on with a good brush whenever the ignition was on with alternator pulley stopped.
Bad alternator would illuminate the battery light. ...
That might be true in some or most cases, but when a brush wore out in my Mazda's alternator, the idiot light did not come on, because the light was connected in series with the brush. Tricky! The light did come on with a good brush whenever the ignition was on with alternator pulley stopped.
Originally Posted by Dinoburner
New car purchase? Any Wty? Start with a new battery before any parts is how I`d do it.
HAHA. 322,000 mile Jetta.
New car purchase? Any Wty? Start with a new battery before any parts is how I`d do it.
HAHA. 322,000 mile Jetta.
If the alternator is ECM-controlled this could get amusing.
I would expect at least a CEL for low system voltage, in addition to the alternator light.
Did you notice anything else weird electrical, like the HVAC fan running slow, the radio being wonky, etc?
Put any good battery in it, start it up, and see what your normal running voltage looks like, should be around 14.
I would expect at least a CEL for low system voltage, in addition to the alternator light.
Did you notice anything else weird electrical, like the HVAC fan running slow, the radio being wonky, etc?
Put any good battery in it, start it up, and see what your normal running voltage looks like, should be around 14.
Originally Posted by Warstud
Yep bad alternator. And now that the battery probably has been drained a few times it probably wont last to much longer either.
Not so fast. The Jetta might have a battery excited alternator. MANY vehicles use them now days. The alternator simply won't produce if the battery is too low on charge or dead. At 6 years old the battery is very likely toast and will need to be replaced anyway. Check the alternator output after the battery is replaced, you will have your answer.
Yep bad alternator. And now that the battery probably has been drained a few times it probably wont last to much longer either.
Not so fast. The Jetta might have a battery excited alternator. MANY vehicles use them now days. The alternator simply won't produce if the battery is too low on charge or dead. At 6 years old the battery is very likely toast and will need to be replaced anyway. Check the alternator output after the battery is replaced, you will have your answer.
Originally Posted by eljefino
If the alternator is ECM-controlled this could get amusing.
I would expect at least a CEL for low system voltage, in addition to the alternator light.
Did you notice anything else weird electrical, like the HVAC fan running slow, the radio being wonky, etc?
Put any good battery in it, start it up, and see what your normal running voltage looks like, should be around 14.
Alternator does have a decoupler pulley. Just checked and there is 14.2 volts at idle with a good battery.
If the alternator is ECM-controlled this could get amusing.
I would expect at least a CEL for low system voltage, in addition to the alternator light.
Did you notice anything else weird electrical, like the HVAC fan running slow, the radio being wonky, etc?
Put any good battery in it, start it up, and see what your normal running voltage looks like, should be around 14.
Alternator does have a decoupler pulley. Just checked and there is 14.2 volts at idle with a good battery.
Back in the day, 9 Volts is what the battery needed to hold during cranking, and if it was at least nine it was considered good, so the vehicle could run on 9. of course, during the start-cycle the ballast resistor was bypassed, giving the IGN a little boost. IME if the ballast resistor was left in the loop and not bypassed, they'd still start. 7 volts under crank is when I'd see the older vehicles not get it. ECUs then seemed to be good down to 9-10 in the 80s when the electronics entered the fray. in my mid 2000s vehicles, they'd run fine on 11. Get into the 10s and the fancier brands would start hiccuping at anything below 11.5.
Some misinformation in these posts. Failing batteries do not present as huge loads. The accept less and less charge. So the statement that failing batteries overtax an alternator is not true. Most younger shade tree guys don't see this because the electronic chargers don't give us this information.
A vehicle running with a weak battery does not demand any more power than with a strong battery. Either way, with a healthy or weak battery, the alternator provides, one way or another, all of the power to recover the battery from a start and maintain all loads while underway. The battery's depth of capacity is really only relevant at starting the vehicle. Yes, newer systems rely on the battery in waves by boosting and cutting charge rate (by voltage) based on drive cycle, but one way or another, the alternator provides all the power, period. Alternators are also getting larger - consider how much juice EPS requires now....
The stalling issue comes into play when we consider that the alternator requires electricity fed into it to generate current, and NVH considerations along with avoiding voltage spikes biases the alt to be quick to cut output (avoiding a spike when a major load turns off) and gentle to raise output (avoid over-compensating which can create a secondary spike) and NVH by sharply tugging on the drive belt. as RPMs raise and lower, and loads turn on and off, it therefore is quick to reduce and gentle to increase. During that gently increased delay, if there's an immediate demand, the battery will catch it for that 1/3 of a second. If the battery is weak, the alternator then ends up starved with enough power to operate, and the vehicle goes undervolt and quits. So the battery develops a larger roll in this case, which is becoming more prevalent with MPG programming, and is why we see shorter battery life and reduced battery warranties.
-m
Some misinformation in these posts. Failing batteries do not present as huge loads. The accept less and less charge. So the statement that failing batteries overtax an alternator is not true. Most younger shade tree guys don't see this because the electronic chargers don't give us this information.
A vehicle running with a weak battery does not demand any more power than with a strong battery. Either way, with a healthy or weak battery, the alternator provides, one way or another, all of the power to recover the battery from a start and maintain all loads while underway. The battery's depth of capacity is really only relevant at starting the vehicle. Yes, newer systems rely on the battery in waves by boosting and cutting charge rate (by voltage) based on drive cycle, but one way or another, the alternator provides all the power, period. Alternators are also getting larger - consider how much juice EPS requires now....
The stalling issue comes into play when we consider that the alternator requires electricity fed into it to generate current, and NVH considerations along with avoiding voltage spikes biases the alt to be quick to cut output (avoiding a spike when a major load turns off) and gentle to raise output (avoid over-compensating which can create a secondary spike) and NVH by sharply tugging on the drive belt. as RPMs raise and lower, and loads turn on and off, it therefore is quick to reduce and gentle to increase. During that gently increased delay, if there's an immediate demand, the battery will catch it for that 1/3 of a second. If the battery is weak, the alternator then ends up starved with enough power to operate, and the vehicle goes undervolt and quits. So the battery develops a larger roll in this case, which is becoming more prevalent with MPG programming, and is why we see shorter battery life and reduced battery warranties.
-m
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