Volt meter reading all over the place

[KTMrida4life 10+ year member]

I have a 150A alt I bought from a guy on ebay (stock is 90), a Duralast gold battery under the hood, and a C&D 134ah battery in my trunk.

After I put the second battery in, my resting voltage for both (total) was 12.4 with the car off. Car @ idle when hot was 13.8, when cold in the morning 14.2.

I started it up last night to go to my friends house to pick him up, and I noticed the lights were sorta dim. Not really dim, but just not bright. This is with all accessories off, including my headunit/system. Plugged in my volt meter.. and I see 12.0 volts. WTF? So I then drive to his house.. whole way there, I never saw higher than 12.4 volts.

Today I drive to meet a guy from CACO an hour and a half away to trade him some batteries for his SAZ 3k. The whole way there, my voltage is all over the place! Doesnt matter if the radios on and bumping loud or off. Sometimes it does like normal and I see the typical 13.8 volts.. but 75% of the time its bouncing around. I'll see 14 volts, then 13.4, then 13.7, then 12.6, etc... wherever it wants to.

Car still starts and runs fine AFAIK. I checked battery connections under the hood and they are tight. Didn't check wire from alternator yet, or alternator belt yet.

Does this sound more like a bad battery or a bad alternator?

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[tommyk90 5,000+ posts]

Sounds like a bad alt to me.

Though even with the car off your battery should be resting higher than 12.4.

12.6-13.0 is more normal.

But this could be due to your alt not charging your batts properly more so than a bad battery.

 

[audiobaun 5,000+ posts]

I agree,does sound like the alternator is having issues with charging both batteries.May try a single battery at a time to rule out a bad cell in one of the batteries.Just a simple test to verify.

 

[KTMrida4life 10+ year member]

Sounds like a bad alt to me.
Though even with the car off your battery should be resting higher than 12.4.

12.6-13.0 is more normal.

But this could be due to your alt not charging your batts properly more so than a bad battery.

Well even with just the Duralast gold in, even when it was brand new (less than a year ago) its always sat at 12.4. Dunno why. Adding the other battery bumped it up to 12.4. Heres whats weird.. I noticed tonight when the car is in the ACC position (radio totally 100% off) resting voltage is 12.9, then 12.8, then 12.7, all the way to 12.1 where it finally stays. Takes a whole minute or so to go down.. but Ive never seen it do this before.

I agree,does sound like the alternator is having issues with charging both batteries.May try a single battery at a time to rule out a bad cell in one of the batteries.Just a simple test to verify.

Will do. Ive bought another alt from the same dude on ebay I got this one from and its still going strong after 3 years. The other cars engine blew up before the alt did.. this one is less than a year old. Probably 8 months. Only 5 months of that have been actual use, and only 6k miles put on it...

The battery under the hood is recent too. Still has 2 years of warranty on it. Im going to have it tested by itself tomorrow at autozone.. I bet its cooked. Ill be putting in an AGM battery, around 50ah.

 

[audiobaun 5,000+ posts]

This past year with the heat drought, alot of setting batteries have been still sold out to the public, believe it or not.So, more than likely you could have a bad cell.Good to be getting it tested and its free.I had a bad alternator from Orielys this year, and it is now that they are making products in mexico,and shipping here, so a long haul in hot semis, and rugged roads will and do damage internals as well.It took me 2 alts before I got a good one, and I even had the 2nd one tested before I left the auto store.It went on a 90 honda, so it was pretty tight to take off, and put back on.I wasnt very happy at the time.But Fixed now.But the heat this last year created some real strain on alot of peoples batteries.

 

[KTMrida4life 10+ year member]

This past year with the heat drought, alot of setting batteries have been still sold out to the public, believe it or not.So, more than likely you could have a bad cell.Good to be getting it tested and its free.I had a bad alternator from Orielys this year, and it is now that they are making products in mexico,and shipping here, so a long haul in hot semis, and rugged roads will and do damage internals as well.It took me 2 alts before I got a good one, and I even had the 2nd one tested before I left the auto store.It went on a 90 honda, so it was pretty tight to take off, and put back on.I wasnt very happy at the time.But Fixed now.But the heat this last year created some real strain on alot of peoples batteries.

Ill check it out.

I know my rear battery was load tested before install and passed with flying colors. So tomorrow Ill go to autozone, have my front battery tested.. and see what happens. If its bad, it still has a warranty. YAY.

I'll have my alt tested too if I can... If nothings loose, then its either my front batt or alt. I'll let you all know whats wrong, I just got a SAZ 3k and Im not hooking it up until my electrical is stable... my car would die otherwise. lol

 

[subzero 5,000+ posts]

We need to know what kind of alt this is. The issue seems like a bad regulator. However, it could be a bad battery having an effect on the alt. Even thou you have a new bat and a used one, the combination could end up killing everything else in the circuit. but the alt regulator seems to grab my suspicions. My alt has an adjustable voltage regulator and it was designed to charge after 700rpm. so sometimes I see it go from 12.3 to 15.2 my resting voltage is 12.75. This leads me to believe that you bought a bad regulator...

 

[audiobaun 5,000+ posts]

Ill check it out.
I know my rear battery was load tested before install and passed with flying colors. So tomorrow Ill go to autozone, have my front battery tested.. and see what happens. If its bad, it still has a warranty. YAY.

I'll have my alt tested too if I can... If nothings loose, then its either my front batt or alt. I'll let you all know whats wrong, I just got a SAZ 3k and Im not hooking it up until my electrical is stable... my car would die otherwise. lol

I am curious to find out what the results are .Let us know man,Id like to know whats up.

 

[KTMrida4life 10+ year member]

We need to know what kind of alt this is. The issue seems like a bad regulator. However, it could be a bad battery having an effect on the alt. Even thou you have a new bat and a used one, the combination could end up killing everything else in the circuit. but the alt regulator seems to grab my suspicions. My alt has an adjustable voltage regulator and it was designed to charge after 700rpm. so sometimes I see it go from 12.3 to 15.2 my resting voltage is 12.75. This leads me to believe that you bought a bad regulator...

I would assume all alternators have this regulator, right? Mine is not adjustable.

That sure sounds like it would cause the problems Im having. Sometimes I get full voltage at idle, sometimes 12v until I rev it up, when it comes back down it will be somewhere else.

Doesnt make any **** sense. lol

I'll make a video for you guys if I cant figure it out tomorrow.

 

[KTMrida4life 10+ year member]

I am curious to find out what the results are .Let us know man,Id like to know whats up.

OK. Today I hooked up just my rear battery and let the car idle. Front battery unhooked. Voltage still moving all over..

then tried just hooking up my front battery, no rear... voltage still moving

Had the alt tested at autozone.. of course when I take it there, the alt decides to work temporarily. Put out 14.0V at idle which is .2 more than it used to. As soon as I get back in my car to leave, I look down and its at 13.2. lol

Now its putting out a steady 14.0V while Im driving, but when I come to a stop at a stoplight.. it goes down. Way down.. as low as 12.3V, which is my battery voltage.

I think the voltage regulator inside it is shot. Im putting the stock alt back in tomorrow.

Could hooking up 2 different batteries to an alternator destroy its voltage regulator in 2 weeks?