The Ford Barn

MikeK · 06-16-2021, 09:26 AM

The correct generator/alternator voltage for a wet cell vehicle battery is 7.15V, +or- 0.05V. This was the factory set point for generator regulators that came after the A era. Most old repair manuals for all generator era cars call for adjusting cleaned and repaired mechanical regulators to that range.

All of the aftermarket one-wire alternator regulators are set for an extremely high 7.5 or (gasp) higher output. This is done for several reasons:

1) The assumption is a car that is driven once a week or less, maybe for an hour, with a battery that has sat and discharged somewhat, and likely less than perfect electrical connections. You get an extremely high charge rate and fast recovery within that single hour of drive time.

Disadvantage: If all your connections are excellent, you drive more than an hour a week, and you start with a fully charged battery you are instantly boiling/outgassing/overcharging/damaging your battery. Think about it- 7.5V on a 6V battery is the same as a whopping too-high 15.0V on a later 12V battery.

2) The bean-counters victory.- "Wow, my lights sure are bright with the new alternator!"
Yep, that 7.5V+ set-point sells 'em fast.

Disadvantage: A bulb marked 6-8V that lasts 300 hours on 6V lasts maybe 50 on 8V. @7.5V, maybe 75 hours. Try 15V on your 12V modern wheels and see how many bulbs you go through!

Unfortunately there are NO correct 7.15V regulators for the popular 10/12SI one-wire alternators. Additionally those repop regulators have no soft-start. Instead they deliver a whopping (to my oscilloscope and your vehicle electronics) start up square edged surge. There is also no time or temperature compensation to adjust and taper the set point as the vehicle runs and warms under-hood, and the inital starting discharge is recovered. Modern alternators do these things, but not antique 10SI's with one wire conversions.

It is possible to open up a Transpo 6V one-wire 10SI regulator and change the internal voltage-divider resistor array that controls the rotor current drive transistor to give you a more realistic 7.1V set-point, but that is not a do-it-yourself job for the average car enthusiast.

06-16-2021, 09:26 AM	#22
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: Alternator Always Charging The correct generator/alternator voltage for a wet cell vehicle battery is 7.15V, +or- 0.05V. This was the factory set point for generator regulators that came after the A era. Most old repair manuals for all generator era cars call for adjusting cleaned and repaired mechanical regulators to that range. All of the aftermarket one-wire alternator regulators are set for an extremely high 7.5 or (gasp) higher output. This is done for several reasons: 1) The assumption is a car that is driven once a week or less, maybe for an hour, with a battery that has sat and discharged somewhat, and likely less than perfect electrical connections. You get an extremely high charge rate and fast recovery within that single hour of drive time. Disadvantage: If all your connections are excellent, you drive more than an hour a week, and you start with a fully charged battery you are instantly boiling/outgassing/overcharging/damaging your battery. Think about it- 7.5V on a 6V battery is the same as a whopping too-high 15.0V on a later 12V battery. 2) The bean-counters victory.- "Wow, my lights sure are bright with the new alternator!" Yep, that 7.5V+ set-point sells 'em fast. Disadvantage: A bulb marked 6-8V that lasts 300 hours on 6V lasts maybe 50 on 8V. @7.5V, maybe 75 hours. Try 15V on your 12V modern wheels and see how many bulbs you go through! Unfortunately there are NO correct 7.15V regulators for the popular 10/12SI one-wire alternators. Additionally those repop regulators have no soft-start. Instead they deliver a whopping (to my oscilloscope and your vehicle electronics) start up square edged surge. There is also no time or temperature compensation to adjust and taper the set point as the vehicle runs and warms under-hood, and the inital starting discharge is recovered. Modern alternators do these things, but not antique 10SI's with one wire conversions. It is possible to open up a Transpo 6V one-wire 10SI regulator and change the internal voltage-divider resistor array that controls the rotor current drive transistor to give you a more realistic 7.1V set-point, but that is not a do-it-yourself job for the average car enthusiast.