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What's normal alternator output?

16K views 4 replies 3 participants last post by  Red 
#1 ·
I ask because I'm seeing 13.2 around town, with 14.1-14.2 only right after startup, and the battery is around 12.2. So either the alternator is being "smart" and just leading the battery by one volt (which is a modern charging logic) or the alternator is weak, and that's killing the battery.
 
#2 ·
Battery & Alternator Voltages

A typical wet cell battery is considered fully charged at around 12.55 to 12.65 volts. Your alternator output appears to be normal--refreshing the charge with higher voltage after start-up, then tapering off to what amounts to be a trickle charge.

Check the age of your battery--holding at 12.2 volts seems to be low and could be a sign of pending demise (if it's original). True way to tell is to check the specific gravity of each cell. Typical float bulb tester will show which cell(s) may be weak (all all OK). Find it at any auto store.
 
#3 ·
Thanks, Bay. I know battery chemistry and charging systems in general very well. I just don't know which logic Nissan used in 2009.
The old "classical" rules for charging haven't been universal, or best, for a decade now. A full battery in theory, ignoring the alloy blends used, would be 2.2 volts per cell times six cells, or 13.2 volts. In practice the alloys used (except for TPPL pure lead cells) tend to be 12.6-12.8 volts when charged, with Optima claiming 13.2 for some of their lead-tin alloys.


This is just a generic battery and as I said, it is the second one in the car, not the original. Screwing around with wet acid and hydrometers certainly works, if your clothes are worthless. These days, a voltmeter and knowing how to use it can tell you just as much if not more, even if it doesn't allow for cell testing.


The best modern chargers will "lead" battery voltage by a fixed amount, and put all the rest of their power into increased amperage. This provides more efficient charging, because there is less boiling (microbubbles) in the electrolyte. I don't think the Murano has that technology, and I don't think it would reduce to 13.2 volts for a 12.2 volt battery out of intentional intelligence. I think the charge controller, which may be part of the ECU rather than a traditional integral alternator/regulator, may be at fault.


But there are so many variations on the market these days, it is impossible to make any reasonable guess at fixing things unless one knows what the factory system's normal behavior is.


For instance, most chargers and alternators in the past 15-20 years will not put out ANY charge if the battery drops below 10.5~11 volts. This is to prevent some dummy from trying to charge a really dead battery, resulting in some of the 10,000+ battery charging explosions in the US every year. But hook up a second power source to "show" 12 volts? And the dead battery can be charged with no problem. Usually.


The old 2, 3, or four stage dumb chargers? Treasure them. The new ones are way too clever for many purposes, good at others like not overcharging. Sometimes.
 
#4 ·
Don't forget that a lot of vehicles built in the last 10 years incorporate a battery temperature sensor that directly affects the charging rate of the alternator as well so as not to over charge the battery. They can go bad and cause over and undercharging scenarios as well, just one more layer of complexity to add to the systems that most don't know about, I have to say batteries do last a lot longer with this system but folks do not know about it. Not sure if the Nissan has it or not but something to check into. I personally have not seen a vehicle without one in recent years.
 
#5 ·
Interesting. I know that's common in some other markets but wasn't aware of any cars using it. I didn't see anything like that on the Murano, just the usual charge and sense wires. A battery temp sensor is usually an extra "tab" bolted onto one of the terminals, so it reflects internal temperature.


Audi has a system that uses an explosive bolt (pyrotechnic charge link) to cut power to the alternator and starter after any crash that deploys the air bags, to ensure the potentially unsafe vehicle cannot be started again until after it has been $erviced. I suppose that's progress, at least VW group has stopped using a positive ground--as the Beetle had.(G)
 
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