Battery voltage drop

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mattmatthew

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Hi,

I've had to charge my battery twice in about 3 weeks. At full charge it's about 12.42v, and at idle it seems to be dropping to 12.10. It's been a day since I charged it and already the current voltage is 12.10, 12.00 or less at idle.

When I increase the throttle, voltage increases, but I have to open it up quite a bit for it to exceed 13v.


CAR INFO:

It's a 1971 Slant 6 225 block in my 1965 Plymouth Valiant.

I recently changed the alternator. The one NAPA sold me was the 35amp squareback style. I noticed it has 2 field tabs (as opposed to my previous that only had one). I connected the green wire to the smaller tab, but wasn't sure what to do with the other. (According to the 1970 service manual, the blue/white goes to the ballast resistor?)

AdPqwMi.jpg


Also just FYI, the car still has the 1965 electrical components... The only thing that was upgraded was installing an MSD box and coil with electronic ignition.

I've been running this alternator without the second field tab hooked up to anything for months. Only recently I've noticed significant battery drain.

I read a previous post from another person that had a voltage drop issue... and ran through some of the steps to test voltage drop and grounding suggested by 67Dart273:

- unhooked the green field wire, voltage did not change with rpm and < 13v
- had battery tested yesterday, shop said it's still good
- tested ground ... one stab into the neg post and the other into the voltage regulator mounting... read 2.2mV

the voltage drop test i did not do because i was missing that 2nd field connection. am i sabotaging myself by not having this 2nd wire hooked up?
 
Yes you need the second field connector connected to something. If you are using the old style regulator then that tab needs to be grounded. If you are using a new electronic voltage regulator you need to have the blue wire extended from the ballast resistor and connected to the second tab. The regulator cannot sense the battery voltage unless the alternator is connected correctly. If there is no voltage sensing happening there will be no charging even though all components are good.
 
mopardude62 said:
Yes you need the second field connector connected to something. If you are using the old style regulator then that tab needs to be grounded. If you are using a new electronic voltage regulator you need to have the blue wire extended from the ballast resistor and connected to the second tab. The regulator cannot sense the battery voltage unless the alternator is connected correctly. If there is no voltage sensing happening there will be no charging even though all components are good.
Click to expand...


Sheese, I hate it when someone posts something stupid without fully reading the prev posts and I just did exactly that. :D

Nice explanation of the voltage sensing requirement, blue wire, and field ground.
And that is correct, that it will not charge without both fields connected.
If the currently connected feild wire is putting voltage into the alternator and you ground the currently open terminal, the alternator will put out 100% charge voltage.

This means you can verify that the system charges, and then deal with the regulator.
 
This is the style I'm currently using...

Voltage Regulator:

yxK4FIB.jpg


Ballast Resistor:

5SRQlMu.jpg


I know I haven't changed either of these, so that means I'm sending the 2nd field tab to ground, correct? Can I ground it to the block?
 
mattmatthew said:
This is the style I'm currently using...

Voltage Regulator:

yxK4FIB.jpg


Ballast Resistor:

5SRQlMu.jpg


I know I haven't changed either of these, so that means I'm sending the 2nd field tab to ground, correct? Can I ground it to the block?
Click to expand...

You can not just ground it and leave it like that, becuase it'll burn up your battery (and quite possibly other electricals in the car)
I meant just long enough to make sure it is capable of charging.
If it is capable then you should hear it drag down the engine a little and the alternator sound will change.

If it does charge, then after checking all the connections and grounds I would replace the regulator.
 
so if i change the regulator, i shoudn't ground the 2nd field tab? just leave it without a connection?

or did you mean:

if the regulator is bad, keeping the 2nd field tab grounded will drain the battery;

if it's good, then keeping the 2nd tab grounded is ok
 
TrailBeast said:
You can not just ground it and leave it like that, becuase it'll burn up your battery (and quite possibly other electricals in the car)
I meant just long enough to make sure it is capable of charging.
If it is capable then you should hear it drag down the engine a little and the alternator sound will change.

If it does charge, then after checking all the connections and grounds I would replace the regulator.
Click to expand...

I think if your regulator is working then yes you need to ground that second lug and leave it that way. It will do one of two things, it will charge normally if the regulator is grounded and working properly. If the regulator is bad then it will probably not charge at all. From the picture it looks like that regulator is a newer version but definitely the older style that uses the grounded lug on the alternator.

TrailBeast is correct that if you were using the newer voltage regulator (which you are not) you would ground one lug of the alternator TEMPERARLY to see if the alternator is working.
 
Wouldn't it be simpler or smarter to go back to the part store that sold you the wrong alternator. sure there are ways to make what you have work but... correct parts don't require gerry rigging.
 
No need to take the alternator back unless you want to; it is common for the parts remanufacturers to just put a ground tab at the outside of this 2nd field terminal anyway for thre older charging system configuration. With the older style VR, it is 100% correct to ground the 2nd field connection. That is the way the older alternators were configured; the 2nd end of the field winding was grounded at the inside of the alternator case. It does not matter which end of the field winding is grounded.

I would ground the 2nd field connection to the alternator case. Not being grounded with the older regulator is almost certainly the reason that the car is not charging.

Once you get this connection grounded, use your voltmeter to check the charging voltage on the battery. It should read 13.7 to 14.3 volts at fast idle and warmed up across the battery + and - terminals. Any significant deviation from this indicates another problem.

if you do not get that battery charging voltage, then put your voltmeter on the green wire at the ungrounded filed connection. if should read a few volts under nomral conditions. If the system is not charging OK, then report your voltage reading at this connection.
 
RedFish said:
Wouldn't it be simpler or smarter to go back to the part store that sold you the wrong alternator. sure there are ways to make what you have work but... correct parts don't require gerry rigging.
Click to expand...

yu don' live in montana, do ya?
 
nm9stheham said:
No need to take the alternator back unless you want to; it is common for the parts remanufacturers to just put a ground tab at the outside of this 2nd field terminal anyway for thre older charging system configuration. With the older style VR, it is 100% correct to ground the 2nd field connection. That is the way the older alternators were configured; the 2nd end of the field winding was grounded at the inside of the alternator case. It does not matter which end of the field winding is grounded.

I would ground the 2nd field connection to the alternator case. Not being grounded with the older regulator is almost certainly the reason that the car is not charging.

Once you get this connection grounded, use your voltmeter to check the charging voltage on the battery. It should read 13.7 to 14.3 volts at fast idle and warmed up across the battery + and - terminals. Any significant deviation from this indicates another problem.

if you do not get that battery charging voltage, then put your voltmeter on the green wire at the ungrounded filed connection. if should read a few volts under nomral conditions. If the system is not charging OK, then report your voltage reading at this connection.
Click to expand...

do this above
 
Grounded the 2nd field tab to the alternator case as suggested.

Starting with 12.15v on the battery with power off,

At fast idle, voltage read 12.95v.

Put my voltmeter to the green field tab at normal idle, i'm getting 10.25v, which seems high...

Since there's two field tabs maybe i have them reversed? How do I know which tab the green goes on and which tab should be grounded?

On the upside, when I put my idle back down to normal, when the motor was warmed up it was at least reading higher voltage than my starting, 12.55v... good sign?
 
It does not matter which field tab is grounded and which is fed by the VR; all it needs to do is set up the rotating magnetic field one way or the other inside the alternator. At least you now have it charging.

Did you run this for long? One thing going on is that your battery is not at all charged. In fact with it down at 12.0v it is at least half discharged. Under those circumstances, the outputs will be low, especially with a 35A alternator, because the alternator is doing all it can just to charge the battery.

The 10.25v on the field indicates that the regulator is commanding as much field current to the field winding in the alternator that it can. It may be due to the battery discharge or may be that the regulator is not seeing full voltage; I would suspect that this is due to a discharged battery at this point. When things are normal and well charged, the filed voltage will be a lot lower.

I would do this so we can get good measurements to see if something else is going on: Charge the battery overnight. Make sure that the battery is near to full charge; you can tell this if it reads 12.6 or 12.7 v after you have pulled the charger off for a 30 minutes or an hour. Once charged, put the battery back in, and then retest the battery charging voltage test to see if it is good.
- Check the battery charging voltage for the 13.7 to 14.3 volts at fast idle, alternaotr warmed up.
- Measure the VR voltage into the ungrounded field terminal like you did and let us know.
- Check the power feed into the VR (the blue wire) and report that too. If you have voltage drops in the harness, bulkhead connector, and/or ignition switch, then the VR will see too low a votlage and react by commanding the alternator to too high an output voltage to try to compensate. But that is not happenning (yet).

But, right now, with a partly discharged battery, the voltages will be off. BTW, if you choose the drive the car instead of using the charger to re-charge the battery, then you need to drive for a half hour or more to get a good charge into the battery.

I would also let the battery set disconnected for several hours after charging to see if the 12.6 or 12.7 volts holds for several hours. If that voltage drops disconnected, then the battery has a problem. Or, take it to the auto store for testing. A partly shorted battery cell will also hold down the charging voltage.
 
After full charge: 12.8v
After sitting before install: 12.5v
After fast idle: 14.3v
Ungrounded tab at normal idle: 10.5v
Grounded tab at normal idle: 2.2mV

Ran the car for about 15 min in total.

The auto store claims the battery is still good. Had it checked a couple days ago. Unfortunately, because this is my daily driver, I took it off the charger at 100% last night before I went to sleep, and checked voltage this morning before I put it back in before going to work...
 
OK, well the battery is not in bad shape; with a 12.5 volt reading after a partial charge and sitting all night, it is decent. And the fact that you get a 14.3 battery charging voltage after the battery charge says that that the alternator is putting out decently.

Now the final thing to check out is why the VR output to the field is at 10.5V, which is high for fast idle operation. But that may be fine at low idle, because the alternator cannot put out much at low idle and the VR will push more juice into the field trying to compensate for that. Measure it again after driving to and from work today at both fast idle and low idle. Also measure the voltage to the VR's blue wire at both low and fast idle.

While you are at it, check the screws that hold that VR to the firewall and make sure they are clean and making a good ground. The VR case needs to be well grounded to chassis for it all to work right. (Some of these older VR's have a separate ground lug on the case that can be grounded but yours does not look to have that.) The same goes for your battery to engine ground and for the engine to firewall ground.
 
Wasn't able to get any readings on the way to work, so I took them after I made the drive home:

Power off: 12.5v
High idle: 13.9v
Ungrounded tab on high idle (green): 1.3v
Grounded tab on high idle (blue): 2.3mv

numbers seem better after a real drive, rather than just running the car for 15 min after a cold start. i was on the road for a good 30-35 minutes.

screws mounting the VR to the firewall are pretty clean. so are the other grounds.
 
Those readings sound pretty darned good; looks like it is fixed! The slightly lower 13.9V charging voltage at the battery when it is warmed up is normal; alternator voltages normally drop a bit hot versus cold. The much lower field voltage says that the battery is taking very little charging current, so is well charged.

12.5v for the battery off is a tiny bit low; I would take some readings on sevaral consecutive mornings to see if it holds at that voltage. And, your meter could be 0.1v low. If you want a poor-man's meter calibration, check the voltage on an unused 9V transistor battery new out of the pack; it should read 9.6v or 9.7v.

Congrats!
 
No, thank you! And all I had to do was hook up that wire lol... ugh

Well no celebration yet. Time will tell, I'll be watching this like a hawk over the next week or so.
 
Some weird numbers today. Granted it wasn't after the haul home but after I had stopped and grabbed dinner.

12.37v at rest
12.86v at high idle (it wouldn't go any higher than this)
10.89v ungrounded at high idle
2.2mv grounded at high idle

do you think the numbers are this way because i let the car rest instead of just taking a sampling directly after my commute?
 
Almsot sounds like a shorted cell in the battery. That would drain it down when you stopped and the alternator coudl charge as hard as it could and would not bring the votlage up properly. The 10.89V at the field brush is saying that the low battery votage is prompting the VR to command the alternator to full output.

The other possibilities are:
1) alternator gone bad again
2) something in the wiring between alternator output and battery. Exmaine the alternator output wire carefully for breaks and bad connections, especially where it goes through the firewall. Wiggle each connection and along the wire whiel watching the bettery charging voltage at fast idle and see if it jumps upas you wiggle it anywhere.

Has your ammeter been acting OK, showing 'charge' (needle towards 'C'). It should show some 'C' after starting and then gradually settle back towards the middle as you drive it a while. Constantly being heavily towards 'C' indicates that it is always charging the battery which is typical of a shorted cell.

Disconect the battery and let is set, and take readings every hour and in the morning to see if it is dropping. If yes, then suspect a bad cell. If not, then the battery is not being charged in the car. Re-charge the abtteyr and start over in the troubleshooting process.
 
Glad you fixed it. 67Dart273 has answered this question a gazillion times, so search his posts (many photos). Keep the square-back alternator since better and easier to repair. It might help to clean up the ground connection on your Vreg (the sometimes rusty sheet-metal screws).

If your dash ammeter works, that is a great way to monitor the charging system. A slight discharge at idle is OK, as long as it goes positive after starting and at higher rpm. I also use a cigarette lighter voltmeter in all my cars (Amazon, real cheap LED one at HF). My newer cars have you flying blind and when the battery gets low, they go crazy. In my 2002 T&C, when the serpentine belt was slipping, I knew the battery was low when the windshield wipers started actuating on their own, the electronic transmission display started showing all gears, chimes going off, total car voodoo. Started using a voltmeter on trips since then.
 
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