Alternator which produces high output at idle

[sprung over GQ ute]

G'day, I have had issues keeping charge up to my batteries at low RPM. MY patrol is fitted with a carby chev and runs ford falcon electric fans. I fitted a new 120 amp alternator( apparently a bosch copy think its an OEX) which has now shit its self after only 12months approx 20,000kms. Has dual batteries; aux is a 120amp/hr deep cycle 87 size and start battery is multi purpose 100amp/hr 750cca, both batteries are new. At low RPM the vsr open circuits and stops charging the aux battery. This is not acceptable as my compressor fridge runs off my aux battery and I hate warm beer. Does anyone know of an alternator which produces high output at low rpm. I have considered fitting a smaller pulley to the alternator to spin it faster at idle. Would this work? Was told I might end up throwing belts if I change the pulley size.

[drivesafe]

Hi sprung and have you measured the voltage at your cranking battery while your motor is idling?

[sprung over GQ ute]

Voltage at my crank battery at idle with fans on was between 12.8 and 13.1 from memory and vsr is open circuit. Maxium voltage at start batery is 13.8 at high RPM. VSR requires 13.2v to close and at 12.8 it will open circuit again. When VSR is closed (charging aux battery) voltage will drop below 12.7v at idle due to the extra load created by the aux battery/fridge. The situation becomes a lot worse at night with lights on. I have fitted HID headlight in an attempt to help the issue which has only helped slightly.

[stockhorse]

Sounds more like you need to adjust the reg so you have higher voltage at idol, this will of course flow through for the whole rev range.13.8 at high revs is far too low , most modern batteries run at 14.4 max charging volts.

[sprung over GQ ute]

I was under the impression max voltage should be higher. The auto elec said 13.8 v was normal. Has anyone fitted a 120amp alternator which produces more than 13.8 volt maxium. If so what brand and what supplier Thanks

[stockhorse]

My stock 55 amp alt (60 seriescruiser) puts out 14.4 at 750 revs,external reg adjusted for the higher output. You can buy a thing that replaces your standard fuse and increases the alt voltage output.They sell for about $20, I think Jaycar sells them.

[sprung over GQ ute]

Did you have to remove/bypass the alternater's internal regulator or does your 60 run an external regulator. I can't understand how something which replaces a fuse can increase voltage output of an alternator. Is it a capacitor or something similar.

[stockhorse]

My 60 does run an external reg so I just bought an adjustable one.The fuse replacement thing Is I think ( could be wrong) a diode that tricks the alt into thinking it is putting out less voltage than it is so it boosts its output.
i.e alt is putting out 13.8. you add diode and alt thinks it is putting out 13.3 so increases the voltage to what it thinks is 13.8 but is actually 14.3