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NECRO ALERT Alternator or battery?

CPA

CPA

Elite Member
Two days ago on a very hot afternoon, I jumped into my 2010 Nissan Titan. While it cranked and started, the cranking was less robust than usual and the battery indicator stayed all the way to the low end of the reading until several seconds after the engine started. This has now happened on each start. All other gauges are doing the same thing.

the last time I had an alternator issue was on a Chevy Corsica and I recall that the car would not turnover at all. Is it that the alternator is on it's last legs but still is pulling just enough amps to start the vehicle? Or could it be that the battery is dying (or maybe has a loose cable)? I haven't had any time to check anything under the hood yet, and I'd hate to go through the hassle of changing the alternator if it's most likely just the battery.

Any thought?

PS, I'm not a car guy, but I do know some basics and have had experience changing out batteries, alternators, brakes and some other small, easy items.
 
The alternator doesn't do squat until the engine is running. So if you are having trouble starting, its likely the battery.

Plus a battery swap is cheaper than an alternator, so I would start there anyways.
 
deerslayer said:
I believe most auto parts stores can test your alternator and battery.
Click to expand...

Yup, Advance will even give you a printed readout of your charging system and will re-charge your battery for free. If your alternator's shot it's going to go dead again but at least you can make it home..
 
If you have a multimeter you can test both alternator and battery. First with the car completely off, hood popped, close all doors and come back in 10 minutes before checking battery. You need to wait in order for all the computers can go into standby mode, reducing idle consumption (since you will have recently opened the doors). From here check the battery voltage. If it's less than 11.9V, it's possible it's severely discharged due to a possibly bad alternator, otherwise you could have a bad cell which means the battery is done for. If the battery voltage is 12.5v that means the battery is nearly fully charged. 11.9V =discharged battery 12.6/7= Fully charged flooded lead acid battery. To ensure you don't have a battery with a bad cell, take the battery to an autozone or autoparts store that charges batteries, charge the battery, put the battery back into the car, remove the "surface charge" by turning on the high beams for about 15s then turn everything off, check the voltage again. If the voltage of the battery is around 11.9V despite being just charged and after having removed the surface charge, you have a bad battery, end of story.

The next step is to turn on the car and once again, with the multimeter check the battery voltage. The voltage should be above 13.2V which is on the extreme low end of an alternator's output voltage. Anything lower than this, and the alternator is definitely toast.

One thing to be on the lookout for is if you hear a "whining" noise coming from the engine while idling the vehicle or even when slightly revved up. Go on youtube and look up "supercharger whine" or alternator whine. If you have that issue, that means you have a bad diode in the alternator and the alternator needs rebuild/replacement, no question.

Now back to the battery/Alternator voltage while the car is idling. If the voltage is above 13.2V with all accessories off but then sinks to around 12V when you start turning on the high beams, radio, blower fan (A/C off), you could have a bad alternator and or very corroded/bad electrical grounds. On a newer vehicle (less than 5 years old), it's most likely the alternator, though a battery with a bad cell could have this effect.

Basically the autozone on car alternator testers do exactly the steps listed above but it's more automated and a whole lot easier. However their load testing while the car is idling is a bit weak compared to doing a test with actual accessories operating which is why I prefer my method above.

Alternator charging voltage range is 13.2v-14.7V, though ideal is around 14.1V charging voltage as too high leads to overcharge and too low leads to undercharge.
 
tortillasoup said:
If it's less than 11.9V, it's possible it's severely discharged due to a possibly bad alternator, otherwise you could have a bad cell which means the battery is done for.
Click to expand...

Or, and this is infinitely more likely, the battery is simply old and needs to be replaced because they do go bad with age.

tortillasoup said:
One thing to be on the lookout for is if you hear a "whining" noise coming from the engine while idling the vehicle or even when slightly revved up. Go on youtube and look up "supercharger whine" or alternator whine. If you have that issue, that means you have a bad diode in the alternator and the alternator needs rebuild/replacement, no question.
Click to expand...

No. It means you have belt whine which is completely unrelated to the alternator.

Every engine I've ever worked on in my life has some degree of noise from the belts.

Alternator whine (really more of a whistle) from a bad diode does not come from the engine. It comes through the radio as a result of interference due to the bad diode, but it is not audible from the engine compartment.

A bad alternator bearing can cause sounds like belt whine, and does indicate a need for replacement, but a failed bearing wouldn't cause a low charge rate.

tortillasoup said:
Now back to the battery/Alternator voltage while the car is idling. If the voltage is above 13.2V with all accessories off but then sinks to around 12V when you start turning on the high beams, radio, blower fan (A/C off), you could have a bad alternator and or very corroded/bad electrical grounds. On a newer vehicle (less than 5 years old), it's most likely the alternator, though a battery with a bad cell could have this effect.
Click to expand...

The battery is wired in parallel with the rest of the circuit and a mere "bad" cell will not cause low system voltage. A reversed cell might (as it would create some amount of voltage "backwards" through the system that would have to be overcome by the alternator. Reversed cells are relatively rare, however.

If the voltage drops as accessories are switched on, then the voltage regulator is likely faulty. However, for a 2010 Titan it does not appear that the voltage regulator is available separate from the alternator so a failed VR would seem to require replacement of the entire alternator.

ZV
 
tortillasoup said:
If you have a multimeter you can test both alternator and battery. First with the car completely off, hood popped, close all doors and come back in 10 minutes before checking battery. You need to wait in order for all the computers can go into standby mode, reducing idle consumption (since you will have recently opened the doors). From here check the battery voltage. If it's less than 11.9V, it's possible it's severely discharged due to a possibly bad alternator, otherwise you could have a bad cell which means the battery is done for. If the battery voltage is 12.5v that means the battery is nearly fully charged. 11.9V =discharged battery 12.6/7= Fully charged flooded lead acid battery. To ensure you don't have a battery with a bad cell, take the battery to an autozone or autoparts store that charges batteries, charge the battery, put the battery back into the car, remove the "surface charge" by turning on the high beams for about 15s then turn everything off, check the voltage again. If the voltage of the battery is around 11.9V despite being just charged and after having removed the surface charge, you have a bad battery, end of story.

The next step is to turn on the car and once again, with the multimeter check the battery voltage. The voltage should be above 13.2V which is on the extreme low end of an alternator's output voltage. Anything lower than this, and the alternator is definitely toast.

One thing to be on the lookout for is if you hear a "whining" noise coming from the engine while idling the vehicle or even when slightly revved up. Go on youtube and look up "supercharger whine" or alternator whine. If you have that issue, that means you have a bad diode in the alternator and the alternator needs rebuild/replacement, no question.

Now back to the battery/Alternator voltage while the car is idling. If the voltage is above 13.2V with all accessories off but then sinks to around 12V when you start turning on the high beams, radio, blower fan (A/C off), you could have a bad alternator and or very corroded/bad electrical grounds. On a newer vehicle (less than 5 years old), it's most likely the alternator, though a battery with a bad cell could have this effect.

Basically the autozone on car alternator testers do exactly the steps listed above but it's more automated and a whole lot easier. However their load testing while the car is idling is a bit weak compared to doing a test with actual accessories operating which is why I prefer my method above.

Alternator charging voltage range is 13.2v-14.7V, though ideal is around 14.1V charging voltage as too high leads to overcharge and too low leads to undercharge.
Click to expand...
I was at Advance a few days ago, my mom left her lights on and ran her batt down flat, the charged it a total of 45 min, then did a "CCA" test on it, (cold cranking amps), this is a much more accurate method of testing a batt than just hooking a DMM up to it..
 
BUTCH1 said:
I was at Advance a few days ago, my mom left her lights on and ran her batt down flat, the charged it a total of 45 min, then did a "CCA" test on it, (cold cranking amps), this is a much more accurate method of testing a batt than just hooking a DMM up to it..
Click to expand...

True but you do what you can do. CCA is a good test but only does cranking amps, so the battery can have strong cranking amps but no reserve capacity and vice versa. I have a car battery that I use that fails the CCA test as it's a 500CCA battery and only does 400CCA however the reserve capacity on this battery is excellent and I have absolutely zero issue with cranking over the engine in the car that its in. Oh and the best part is the battery has just reached the 10 year mark!
 
tortillasoup said:
True but you do what you can do. CCA is a good test but only does cranking amps, so the battery can have strong cranking amps but no reserve capacity and vice versa. I have a car battery that I use that fails the CCA test as it's a 500CCA battery and only does 400CCA however the reserve capacity on this battery is excellent and I have absolutely zero issue with cranking over the engine in the car that its in. Oh and the best part is the battery has just reached the 10 year mark!
Click to expand...

What were the RC test results? Or how do you know the RC is excellent?
 
tortillasoup said:
True but you do what you can do. CCA is a good test but only does cranking amps, so the battery can have strong cranking amps but no reserve capacity and vice versa. I have a car battery that I use that fails the CCA test as it's a 500CCA battery and only does 400CCA however the reserve capacity on this battery is excellent and I have absolutely zero issue with cranking over the engine in the car that its in. Oh and the best part is the battery has just reached the 10 year mark!
Click to expand...

"Reserve capacity" is irrelevant if you can't start the car in the first place. Once running the car runs off the alternator's output, the battery is needed for mainly one thing only, to dump a bunch of current through a starter to get your engine started.
 
BUTCH1 said:
Reserve capacity" is irrelevant if you can't start the car in the first place. Once running the car runs off the alternator's output, the battery is needed for mainly one thing only, to dump a bunch of current through a starter to get your engine started.
Click to expand...
or we call that a CD= Current Dump!!
 
BUTCH1 said:
"Reserve capacity" is irrelevant if you can't start the car in the first place. Once running the car runs off the alternator's output, the battery is needed for mainly one thing only, to dump a bunch of current through a starter to get your engine started.
Click to expand...

If you have high parasitic draws and you park the car for some time, the battery will get sulfated quicker and deeper, making the problem escalate until you end up with a vehicle that needs boosting and by that time, the battery is useless even if it cranks good when charged because it has so little capacity left.

Look at it this way, a battery with good cranking power but poor reserve capacity could be cranked over, turned off about 5 times and then it's completely dead. However one with good reserve could handle probably over 30 crankings before it's dead.
 
tortillasoup said:
If you have high parasitic draws and you park the car for some time, the battery will get sulfated quicker and deeper, making the problem escalate until you end up with a vehicle that needs boosting and by that time, the battery is useless even if it cranks good when charged because it has so little capacity left.

Look at it this way, a battery with good cranking power but poor reserve capacity could be cranked over, turned off about 5 times and then it's completely dead. However one with good reserve could handle probably over 30 crankings before it's dead.
Click to expand...

So, basically, if your car is complete shit with huge electrical issues that cause more parasitic draw than any properly functioning car should ever come even close to having, and if the car is in such bad shape that it needs to be cranked over multiple times because it doesn't reliably start, then reserve capacity is important.

If huge amounts of reserve capacity were important for SLI batteries in normal cars, we'd all be using deep-cycle batteries in our cars. But we're not. Because cranking amps are more important assuming that everything else is functioning properly.

ZV
 
tortillasoup said:
Look at it this way, a battery with good cranking power but poor reserve capacity could be cranked over, turned off about 5 times and then it's completely dead. However one with good reserve could handle probably over 30 crankings before it's dead.
Click to expand...

IIRC, reserve capacity is not any sort of indicator of cranking ability.

But if your car won't start after 30 crank attempts, you probably need a fire extinguisher and a new starter...
 
LTC8K6 said:
IIRC, reserve capacity is not any sort of indicator of cranking ability.

But if your car won't start after 30 crank attempts, you probably need a fire extinguisher and a new starter...
Click to expand...

If the car has exhausted its reserve capacity, then the car will have no cranking ability. I did mention in earlier posts that a battery can have good RC but poor cranking. It can also have good cranking + good RC, poor RC but good cranking, and the best of all, poor cranking and poor RC.
 
Cars aren't rocket science I get the ones with higher CCA cause I live where -40 is guaranteed, however every part supplier knows what is appropriate for your car
 
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