Hi. My B310, A12 has a problem with the battery going flat. My neighbour had a battery problem with her stanza and said changing the alternator fixed it. I checked the alternator today. It was putting out 14V max. when it first started, but a few minutes later it only put out 13.something at idle and did not change if I reved the engine. The alternator has had the bearings and brushes replaced. Could it be that the rectifierin it is not working properly. Has anyone else had this problem or know how to solove it?

This is indeed an interesting problem...

We'll see if someone can shine light on this, because it's a tough one.

Good luck getting it working there.

If we have already eliminated the obvious, like, you left the interior light on etc, then we can, for the moment, turn our focus to the altenator.
The altenator uses six diodes to enable it to provide "full wave rectification" These Diodes are one way electrical valves & they are used to allow electrical current to pass out of the altenator, but they prevent battery current from passing back into the altenator when the engine is off.

If one or more of these diodes have gone faulty, you will have a leakage of current back into the altenator that will eventually send the battery flat if left unused for long enough. Any auto electrician that paid attention in his first year of apprenticeship should be able to quickly diagnose this condition. It's also fairly easy to fix too. I'm guessing that this is what was wrong with your neighbours Stanza too.
This is a well known condition in the trade & is no great mystery to those that fix these things for a living.


When you start your engine, particularly in winter, the starter drains a lot of current from the battery & the regulator makes the altenator pump out a high output to quickly top up the charge level in the battery. After a short time, as the battery's charge state is restored, the regulator cuts back the altenators output, & voltage, to a level that maintains battery charge & also runs all the electrical devices in the car. This is why you have noticed a higher voltage when you first start it, which then drops back after a short while of running.
It's all completely normal,... nothing to see here, move along now, move along,...to your friendly auto sparks for a diagnosis & repair.

Its winter, its cold, batteries show their true condition in the cold. You really should charge your battery and then have it load tested first.
How do you start the car when the battery is flat? do you charge the battery or jump start the car?
If you cant get to an electrician to test the battery, I recommend you charge your battery, check the car starts, then disconnect the battery and leave it overnight and in the morning see if the car will start. If it doesnt start - you know its the battery. If it starts - you need to find whats draining the battery.


Hey Dodge, what happens if its a 9 diode alternator?

Thats 50% more technology than my brain can handle, but whatever the diode count, it takes only one crook diode to cause a problem.

The way i remember it, there are only six in the altenators that we most commonly see on our A series Datsuns. Hell, i still remember how to overhaul Lucas C40 generators blindfolded. It still gives me nightmares. Brrrr.
We used to do about three a day, every day, & that was just in our own fleet.

Thankyou for ur replies.
As for the problem being a faulty battery. I am pretty much 100% sure it is not. This is my 4th or 5th NEW battery I've put in this car and they shouldn't all be faulty.
Hopefully I can find a GOOD auto-electrician to help coz the ones I've seen so far have all said there is nothing wrong. Being a female also makes it harder to convince them that there is.
Anyway, I shall look into the rectifier idea. Is that a part u can buy separate or is a new alternator needed?

The diodes are realy one of those things that need to be done in a suitable auto electrical workshop if you don't have the knowledge & tools to do it yourself. In most instances the job involves diagnosing the fault to ensure that it really is a diode problem, then removing the altenator & dismantling it. Depending on the make & model of the altenator, it may be necessary to unsolder each diode in turn to test it untill the faulty one[s] are found, then installing a replacement.

Another method, after determining that the altenator is actually faulty, is to install an exchange reconditioned altenator. Most auto electricians will have a supply of the most popular units ready to go & sitting on their shelf just waiting for someone who needs a quick turnaround.
Do NOT buy a "new" altenator.

The first priority is to determine the ACTUAL caause of the problem, then act on that.

OK so battery 100% ruled out.
Now to test whats actually draining the battery.
The easiest way to do this is with a test light.
* Disconnect the -neg battery terminal only.
* Now take the test light and attach one side of it to the disconnected battery lead and the other end of the test light to the actual battery terminal.
* If something is draining the battery, the test light will glow. Brightness will depend on how big the drain actually is.
* If the test light is glowing, you now need to disconnect electrical devices one by one until the test light goes out.
* Easiest thing to do is disconnect each fuze individually. (If the testlight remains on when you remove a fuze - simply place the fuze back in and continue to the next fuze)
* If you find the test light goes out when a fuze is removed - you now need to find what that fuze is supplying power to. ( This could be numerous things and each will have to be disconnected individually )
* As Dodgeman stated, the alternator is a likely cause. To check it, do as above with the test light. And you need to disconnect the wiring from the alternator (there is no alternator fuze in the fuze panel)
* If the alternator is externally regulated, you should check the regulator also by unplugging the wires from it.

If when doing the above you find the test light doesn't glow at all, you will have to look elsewhere.
Does the car have an alarm?
A common problem in cars which gives symptoms of a flat battery, is a poor earth (-neg lead) connection to either the battery, the motor or the body of the car. (All these connections need to be tight and clean contact. )

I tried the test light idea and found that the sterio was not earthing properly. I also got the alternater checked by another electrician and he said it was ok. He checked it separately, and I watched him do it so I think that we can say that it is good. The battery is still going flat, but it could be that another battery has finally given up. I am getting the battery checked atm. Hopefully the bad earth is the cause of the problem.
I'll get back to u later on the results

Datsunny80, i was having this recharging problem for sometime (3 batteries ago), even with a 55amp internally regulated alternator! I found it last weekend while tinkering (stupidme) it was basically a grounding problem. My car would have trouble producing 14volt at any revrange. The problem was that when i swapped the motors I redid some wiring around the battery (also put in elec dizzy) and forgot to properly ground my wiring harness to the battery AND chassis. I also put in extra grounds directly to the alternator and one to a main power supply line for accessories. Now she sits on 14.2 at idle and 14.6-7 while over 2k revs. *relief*

i also noticed that the weakly grounded device (when turned on) would dramatically decrease the voltage in the car. eg i noticed it in the headlights cuz it was wiring harness problem i had.

goodluck with it. 1 thing i know, power problems suck.