[ap72]

My charging system on this 78 280z I just bought it acting up. Its registering 13.5V instead of the 14.70v I should see. What would cause this? I charged the battery overnight and it shows 12.3v (a little low but close enough).

[oldbogie]

Originally Posted by ap72

My charging system on this 78 280z I just bought it acting up. Its registering 13.5V instead of the 14.70v I should see. What would cause this? I charged the battery overnight and it shows 12.3v (a little low but close enough).

The amount of charge on the system is dependent upon the state of charge of the battery. If it is well up, the amount of charge should fall. Since the battery shows 12.3 volts with no load or input, a charge voltage of 13.5 is still showing the regulator is feeding charge back into the battery.

A 14.7 volt charge rate is actually quite a bit and would indicate that there is a loss somewhere the regulator is trying to make up for. this could be the result of a high withdrawl of power or an electrical problem somewhere, a poor ground back to the alternator is a common problem.

Batteries change their charge requirement through thier life for a number of reasons the common ones result from the effects of charge and discharge cycles. Since charging a battery is actually a reverse plating process, so is discharging for that matter, metals or their oxides are actually being moved from one plate to another along with free electrons. Plating of metals is not a uniform deposition process, there will be high and low spots and that includes places where fingers develop that may short one plate to another. There is also a conductive sludge that settles to the bottom which if it touches plates shorts them out. Since this is a dynamic situation over a battery's life it's rate of needed charge, beyond that to simply replace electrons used in starting, etc., will change the amount of current needed from the alternator/regulator to maintain a the state of charge.

At this point I don't see a cause for concern. That of course could change, but as long as the charge rate is above the steady state of the battery and the battery is hanging not below 12 volts this looks OK.

Bogie

[chucksrt]

13.5v is normal for most imports!

[joe_padavano]

Originally Posted by chucksrt

13.5v is normal for most imports!

It's right around normal for most "12v" cars, period. Actually, most run about 13.8v, but the difference is lost in the meter accuracy. 14.7 is way too high and will prematurely burn out bulbs and possibly damage the battery by overcharging.

[willowbilly3]

Rule of thumb is check batt volts, start the engine and turn everything on, should see one volt more than batt volt at rest. So for example if your fully charged battery showed 12.5 then at idle 13.5 would be normal with all accessories turned on.

[DanielC]

Does the car have an internal regulated or external regulated alternator? I do not know if it originally had an external regulated alternator from the factory, 1978 is about the time when things were changing.

My 1971 Datsun 521 has a ground wire from the frame of the alternator to one of the screws that hold the voltage regulator to the inner fender. If your car was originally set up like this, and had a later higher amp, internal regulated alternator put in, it is possible this ground wire is missing, or not hooked up.

Make sure your grounds are good. Battery to engine. Engine to body of car. The frame of the alternator to both.

On my 521, the headlights have a separate ground wire to the voltage regulator mount. But the other smaller lights, and accessories use the body, and frame for a ground. If the ground connection from the alternator to the external regulator goes bad, the electrical system will try to ground through the throttle linkage. This will cause problems. Wacky, hard to diagnose problems. Check your grounds. What was a good ground in 1978 may have deteriorated by now.

13.5 volts is OK. Do the load test mentioned above. If you are above 13.5, I would not worry about it. Datsun (Nissan) external regulators are temperature compensated. They regulate to a higher voltage when cold.

I would like to see a little more voltage in the battery if you used a charger on it overnight. How old is the battery?

[T-bucket23]

You need to see if the alternator is putting out the rated current under load. You cant test an alternators function with a volt meter. It could be putting out 14 volts but only be capable of 10 amps.
A voltage test is good to prove if it isnt working, it will not prove proper functionality.

To answer your original question, 13.5 is in the normal range for voltage. As someone else stated 14.7 would be too high. The alternator output voltage will be higher with a fully charged battery as the current requirements get lower the voltage will get a little higher.