looking at my evoscan logs i noticed that under cruise conditons my battery was at 14.2 and when under WOT runs it would drop to 13.6-13.7, is this normal? I am running a buschur mini battery.

tks

 

Yes, normal

 

Indeed, its a design feature of the ecu. The ecu tells the alternator how much excitation field to apply to the alternator. At high rpms, the need to maintain battery voltage isnt really needed because WOT is only a few seconds. The ecu dumps the alternator and you lose the CEMF (counter torque caused by electrical loads inside the car's electrical system) developed inside the alternator, thus freeing HP and torque for the drive train. The A/C does the same thing, above ~5000ish rpm the ecu dumps the a/c compressor... Yep!

 

Wow I never knew that. I was checking out Mellon Tuning's site and wondered why he needed you to log Battery Voltage. I'm sure he has a very good reason why.

 

The ECU in the EVO does not shut off the alternator during acceleration. The voltage drops because the electrical demand is so HIGH at WOT that the alternator can not maintain 14.2 VDC. If the alternator were to turn off, you would see a voltage of 12.6 volts or less (likely under 12 VDC on a perfectly healthy battery). I beleive the ECU simply turns the field current off or on as well, and does not vary it under any situation.

The new BMW M3 on the other hand does vary the alternator field current and does shut down the alternator during WOT to increase available HP. It then activates the alternator on any decel event to produce a form of regenerative braking as well as being on during normal cruise conditions.

Voltage available will have a significant impact on injector and ignition performance. Mellon likely asks for it to be logged so that he can adjust the injector battery latency table for cruise conditions and to verify the car charging system is working correctly.

 

03whitegsr. check out pins 33 and 41. The ecu does infact reduce alternator excitation during high load. The Vbatt will drop at wot, but not to 12.6v. The lead acid battery being on a trickle charge the entire time your driving will not drop to 12.6v instantly. If you stayed at WOT for a long long time, you would see the voltage lower. I dont know if the ecu completely kills the excitation, but i know it reduces it. If the battery dropped that much from electrical demand, that would be crazy crazy. The engine does not need electrical power like huge subwoofers. Injectors and ign coils dont use that much power.

 

This too. Ecu is directly connected to field coil.

 

After I read this thread I just went to my last log and I noticed that the battery voltage at idle was around 13.8 and 13.7 but at WOT was around 13.5 and 13.4. Should I worry about it?

 

Sorry man, I completely disagree with you on this. Yes, the ECU controls the field, but no it does not shut it off under load. You shut off the field current to the alternator, the voltage will IMMEDIATELY drop to 12.6 VDC or lower, regardless of the electrical load.

The ignition system, fuel pump and the injectors all use a HUGE amount of power at full load and high RPM. It's actually a big problem on the really high HP cars and why many go over to high current alternators on race cars. They want to keep voltage as high as possible to avoid fuel starvation and ignition blow-out.

 

I am current on an OEM stock batt. Its readings are in good status(blue semi-circle); the lights are bright all electrical seems to be working fine. I get a reading of 12.6-12.8 max. Yet at times the car does not turn on. no tick-tick from the alternator and tapping it does not work. On three occasions I had to push start in order to get it started. Is it the alternator or the Fuse Link on the batt cable. Are these signs of a short somewhere? Any input would be appreciated.

 

The ignition system needs all the current it can get at WOT with higher boost, that would be bass ackwards for the ECU control it like that. If my car doesn't have full voltage at WOT, it misfires like mad. I would think it just drops a bit since there is more load on the charging system.