I've been chasing battery gremlins for a year. Put in a new battery a year ago.
1. If my car sits for a week it won't turn over, dead battery.
2. I was out of town for a week so I left it on the charger. Got back, won't turn over, dead battery.
3. I replaced the battery yesterday.
4. Finally got out the multi meter today. Just under 10 volts on the new battery. Started right up.
5. With the car running 15.9 volts coming out of the back of the alternator. 15.8 across the battery.
6. With the car off, 12.8 across the battery.
7. Any ideas what I'm dealing with here?
1. If the car sits for a week and the battery was fully charged at the start of the week, then there is probably a good drain on the battery even though the ignition is off and all things you can see like a dome light, trunk light, parking lights and so on are not illuminated.
2. Leaving it on the charger for a week and then returning to a dead battery with the battery still connected to the car it could be a couple of things. If the drain is let's say 10A and the charger has a 5A output, the draw is higher than the charger can put out and eventually the battery will become depleted and now whatever the 10A draw is, the 5A charger is supplying what it can but will not keep up.
3. You replaced the battery and then a day later you had 10V which is low but during that one day between putting it in and measuring it, if there is a drain on the battery with it connected to the car it might have started of at 12.7V the day before but now a day later with the drain over 24 hours or so, the battery voltage is at 10V.
4. 10V may be enough to start over a motor.
5. With the car running, you could be seeing 15.9V at the back of the alternator and 15.8V at the battery which "could be high" but this is depending on how the voltage regulator is working and/or how well connections and/or wiring are.
6. You then shut the car off and now the battery went from 10V and is now at 12.8V, and this is from the alternator pumping 15.7V into it which will raise the voltage up.
7. What are you dealing with ?. I think possibly a few things.
One is I would do is disconnect the battery from the car and measure the voltage and then put the charger on the battery for let's say a day. When you first put the charger on it, it will take time for the voltage to get built up into the battery so maybe in a few minutes, turn the charger off, disconnect it from the battery and measure the battery voltage. Write this number down. If the voltage started at 10V before the charger was hooked to it and now a few minutes later with the charger not hooked to it the battery voltage is let's say 11V, then this is good so now reconnect the charger and let it sit charging the battery for around a day (and it may be less time than this depending on the charger but I would use the lowest amperage setting output on the charger so the battery is slowly brought back to it's potential).
Now after a day of the battery being on the charger, take the charger off and measure the voltage. It may be at 13.2V but let it sit a while and the voltage should slowly drop to about 12.7 and then stabilize at this voltage. This number or voltage after sitting a while after being charged will be the "state of charge". Look at some charts online concerning tis and it gives you an idea as to how well yur battery may be perfoming. What you are doing by letting the battery sit without it being connected to the car or the charger is letting the surface charge come off of the battery and eliminate and further drain you might have by having it connected into the car.
NOW, you can do some tests. The first test will be checking for current draw with the vehicles doors closed, the ignition off, and entries closed that may have mercury switch activated lights such as trunk lights and/or an underhood light. If you have an aftermarket stereo, you can leave it connected. Basically when checking for current draws that drain the battery dead, you want everything you can to be off or the results will be skewed.
Now hopefully your meter can measure DC Amps or Amperage, or DC Current AND you know how to set the meter up and use it.
To measure current, you first connect the positive battery cable to the battery positive post. You now should still have NO connection on the battery negative post and then also have a negative battery cable still not attached to the battery.
Since we do not know how much current is being drawn off of the battery AND how much current or amperage your meter can handle is to try a known fuse and use it as a test to see if the draw is less than what your meter can handle. In other words, if the unknown draw is possibly 15A and your meter can only measure 10A, the meter may be damaged trying to measure this 15A as it was not designed for so measuring that much and with a basic test here with a fuse can help prevent this. Let's say the meter can measure UP to 10A. By taking a 10A fuse and placing it between the negative battery post and the negative battery cable, if the fuse blows, then we should be glad we didn't connect the meter to measure a draw it could not handle and possibly prevented damage to the meter. If the fuse did not blow, then now you can connect the meter and measure your current draw.
The current draw if a 10A fuse did not blow and now you have a meter capable of measuring up to 10A is now connected, the readings can be anywhere from 0.00A up to 10.0A and if it's anything above 0.02A, then we need to find this drain. If the car has an alarm or aftermarket stereo or a digital clock, then this 0.02A is within reason and this minor draw will not drain a good battery dead from 12V down to 10 volts over a weeks time frame.
Let's say the current draw is 2.0A then this is a major draw and you have to look at things that might be causing this. It could be one thing or multiple things but now with a baseline number of 2.0A we can do things to see if this number can be reduced or eliminated.
Now disconnect your meter to where you have NOTHING on the negative battery post and now take the nut off of the back of the alternators output stud and remove the wire attached to it. Tape the end off or position it so it CANNOT hit anything and just needs to be out there in the air.
Now, reconnect you meter to measure amperage. If the number is still the same, then we need to look elsewhere so now disconnect the meter to where again,NOTHING is on the negative battery post. You still need to leave the alternators output stud wire disconnected and now, try unplugging the voltage regulator and then position this plug to where it hits nothing and then reconnect your meter. Now take another reading and see if the number is still the same.
If it's the same, we need to press on further and slowly move further and further away from the battery to find the issue or issues noting at each step if the draw is the same or slightly reduced.
Eventually you will find the drain. Once it gets corrected then slowly bring everything else online and have things reconnected again noting at each point if there is any change in the readings on the meter.
WOW, that's a lot of typing.
Another thing too is if you can post back if you also have your original points style external voltage regulator. The one on my 68 was adjusted by someone that didn't know what they were doing and it was telling the alternator to output almost 16V but I never had issues with battery drain. Once I adjusted the regulator using the factory manual, it's been fine and may last another 50 years.