• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Higher Voltage: Fool Your Alternator?


Well-Known Member
Most Helpful Member
Hello again,

For my old Hyundai i bought a brand new battery and brand new alternator and i found that the alternator would not keep the battery charged correctly at 14v. It had to be a little higher than that. I had to bring the battery in the house now and then and change it with a 10 amp battery charger or 10 amp power supply and set the voltage much higher than 14v. I think it was 14.7v or something like that. That way it would hold the more normal voltage over a month or two when used in the car again and with short trips with the car.
Yeah with short trips the battery doesnt get as well charged either if the charge voltage is too low.


Well-Known Member
Most Helpful Member
My newer Chev Silverado 1/2T pickup is too smart for its own good. The Alternator Voltage Regulator is controlled by one of the many on-board computers. It shuts off the alternator during start. A few seconds after start, the VR is set to ~14.8V, but the exact voltage seems to be a function of engine compartment temperature. After about 10min of running, the voltage drops to about 13.8V, and it stays near there for the duration of the trip. During the trip, as the engine compartment gets hotter, the VR voltage is further reduced.

I'm sure that the algorithm is published somewhere....


New Member
Some of the older members here will remember the 2 or 3 bobbin Regulator with a DC generator. This did 3 functions: Voltage limit, current limit and reverse current cutout. Temperature compensation was provided by the bi-metal leaf spring on the voltage bobbin's armature. Hence the higher terminal voltage on a cold startup. The current limit would be in operation until the battery started to come up, (otherwise the commutator would leak smoke and molten solder) After about 30 mins, when the under bonnet temperature rose to normal operating, the volts settled back down to about 13.8. This was an effective way of bulk charging and using the lower voltage setting (determined by the bimetal spring) to float charge (if the vehicle was running long enough) Note that the excitation of the generator's field was in a sense: PWM, by the action of the contacts in the voltage limiter, switching the field current on and off. (Some regulators had changeover contacts and 1 or 2 resistors to give a smoother control). This resulted in a somewhat sawtooth waveform (and commutator ripple). If one connected an oscilloscope to the battery, the peak voltage would be in the order of 15+ volts, Your analogue meter reads RMS so it would read about 13 or 14v. Batteries actually liked this PWM method of charging, this is why we used to get 5+ years out of a battery and never had any sulphation. And we all survived on 15 -20 amps of total charging current. I am not aware of any temperature compensation in these modern cars with alternators that output as much (or more current) than my welding machine....!!! Early alternator systems had external electro-mechanical regulators with temperature sensitive bi-metal leaf springs. These regulators were voltage limiting only as alternators were self current limited by virtue of the stator core saturating, reverse current was taken care of by those hefty rectifier diodes acting as a one way valve. MikeM: note your comments about temperature compensation, about time the vehicle manufacturers did this.

Latest threads

EE World Online Articles