I think the ECU is the culprit here. I believe the ECU constantly monitors voltage - like you mentioned - if the contacts inside the ecu have failed (the ones that are supplied with voltage) it could trigger your charge light. If you have a spare ECU laying around i'd switch it out and see if it fixes it. Otherwise there are usually quite a few available in the for sale section for pretty cheap prices - especially the h3z model (if you have an auto)
Good thinking, but wrong. The battery light in the SHO is incredibly simple. There is no circuitry whatsoever involved; it is a simple opposing ground circuit. There is a wire that goes from the battery positive terminal to one leg of the light, and a wire that goes from the alternator to the other leg of the light (the plug on the alt with two wires on it - one wire goes to POS to tell the alternator how much to charge, the other goes to the idiot light). If everything is working right, the voltage at the POS terminal and the voltage being output by the alternator will be equal, thus allowing no voltage flow across that light. If either one is higher or lower than the other, that will allow voltage to flow, making the light turn on. If it is a small imbalance, the light will be dim. If it is a large imbalance, the light will be bright.
This means either the battery can be dying (not accepting alt charge and producing a lower voltage) or, very rarely, producing more voltage than the alternator can handle. This almost never happens. Also, if the alternator isn't putting out as much voltage as the battery (<~12v), the light will turn on. If the alternator is putting out more voltage than the battery can handle (>~15v), the light will turn on.
Basically, the light turning on means either your battery is ***** or your alternator is *****. Or, the wiring going to the idiot light is *****. It does happen sometimes. I saw the wire from the alt plug short out to ground once, creating a bright-ass light. Freaked the owner out pretty good. Anyway, if the battery is fine (and this is easy to check with a load meter), that means the alternator is either not putting out enough voltage, or putting out too much. You need to take it to an AAP or AZ and get the charging system tested. It's free and pretty quick, if the tech knows what they're doing.
If im not mistaken the light will come on when the alternator is putting out too much voltage as well.It is a common issue for voltage regulators to fail in alternators.
This can cause all sorts of issues like melted wires and fried batteries.
In short,have your alternator checked at a parts store.
This.
