Motor Vehicle Maintenance & Repair Asked by A concerned basshead on March 4, 2021
I recently installed an Eagle High 250amp H.O. alternator in my 2015 Altima.
Immediately upon starting the engine my battery light came on.
I hooked up my OBD-II scanner and it shows that the CPU voltage with the car running is @ 15.6v.
Using a Fluke DMM I tested at the alternator and it is in fact charging @ 15.6. This is all taking place in single digit temperature, but I doubt it would have that drastic an effect on charging voltage.
I’m running 3 batteries, 2 of which are only months old; suspecting that the older of the 3 might be the issue I removed it from the system but the charging has not decreased at all. I’m running an Optima redtop 35 under the hood and 2 Kinetik HC800’s in the trunk.
I installed this alternator to feed 1700w RMS worth of amplifiers, but my subwoofer amp goes into protection mode as soon as I start the car now. I asked the alternator manufacturer and they assured me that 15.6v is within their operating specs. So, I guess what I’m asking is; Is there a device on the market that will step down 250amps to 14.4v? All I’ve been able to find is the XS Power 993, but it is only rated for 50 amps. Any help would be appreciated
Alternators do intentionally vary the output with temperature, but a typical coefficient is about 10mV/degF, so that seems excessive.
It's seldom worth converting high power supplies when there's an easier way, and with alternators that's always the case. On the back of the alternator, there's a small box, that contains the regulator, and usually the brushholder integrated into it. The regulator switches the field current, typically a couple of amps, to hold the voltage at the output terminal within its intended range, compensating for the variation you'd get with load.
Many systems now have the regulator as part of the ECU, which in addition to being able to do some neat diagnostics e.g. intentionally varying the output to estimate that battery state of health, also have temperature compensation, usually based on either an ambient temperature sensor, or one built into the battery tray. For aftermarket alternators, and I'm fairly sure you original alternator, the compensation is done by the temperature of the regulator itself. As it charges, it gets hot - far hotter than the battery, so the temperature coefficient used on these is generally higher. If you have batteries remote from the alternator, that's probably not a good thing.
If it is excessive, you can probably find a replacement regulator, most of these aftermarket alternaotrs are based on the mechanical parts of common manufacturers, Delco, Visteon and for yours, probably Denso, so it's possible that you can find one to swap in.
Answered by Phil G on March 4, 2021
You definitely need to get the voltage down.
The easiest way to do this is to change the regulator,The regulator functions by adjusting the current to the stator in the alternator and is separate from the alternators diode(rectifier)bridge.
The charge regulator do often have a type of labeling showing the intended output voltage and this depends on the intended use of the alternator.
Answered by trond hansen on March 4, 2021
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