I should just add for informational purposes a follow up on this:

- Problem was never sussed out, we never found out what was the direct cause of this BUT I have a theory.

- Battery draining problem no longer exists!

Since we converted the car to manual that is. Note that the sound of the starter motor changed dramatically from a loud whine to a quiet crank (though this may be due to a heavy flywheel instead of the automatic flywheel. Heavy = larger rotational inertia = larger force it puts on the starter).

The battery draining problem may have been due to the way the automatic neutral safety switch works. What it does is has some sort current running to the starter motor which prevents the starter gear from extending (and thus cranking the flywheel) whilst the transmission in and gear other than Park or Neutral. Since installing a manual box this is no longer needed, the current preventing this gear from extending also is no longer needed and so this reduces the needed load on the battery.

Still the only mystery is why this load doesn't show up on an ammeter, though we had someone do that for us and I'm not sure how they checked the load.

So bottom line, those who own Automatic Sunnys with starting issues take this into account, keeping in mind we checked EVERYTHING we possibly could to try and suss it out.

sounds like its not charging if you check the output it should read approx 14 volts plus / mins 0.5 volts.
The external regulator are mechanical type and are prone to failure. if its too low the battery wont charge and will go flat after a fews days of driving.

Sorry to all for taking a while to reply to suggestions, but I have uni exams atm . My last one is on thursday, I'll have time to solve this problem.
The battery is still going flat even after the earth to the sterio was fixed. It takes 3 days for it to be struggling to start the car. I really thought that the sterio earth was the problem coz when the car has no sterio it is fine and can sit around for 3 weeks at least. It is not the sterio itself coz I've had two different one's and the problem was there for both. And they were both wired by different people.
The battery is sitting on the tray, so I shall also get a rubber mat to put under the battery.
I'll get back to u all later on the progress.

I had a similar problem,I took it a sparky he had one look at the alternator pulley and said the belt is loose.
I said how do you know simple he said the pulley is rusty from the heat of the belt slipping and burning the paint off.
one new belt correctly adjusted and all was well.
You may want to put a rubber mat under the battery I have heard from a few people that leaving a battery on the ground will speed up it going flat same rule may aply sitting directly on the battery tray.
ade

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.

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

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. )

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.

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?

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.