Yall are smart! Battery issue or something else?

[skadude016 5,000+ posts]

Ok here is the deal. So I was out last night and was leaving cranked the car and it felt like i was cranking off of 9-10 volts. Just a real slow crank if ya know what I mean.

I get home pop the hood and test the batt car off. 12.8 so thats cool. I then unhook the rear battery from the front and get this. All the lights in the car go out!

So with the rear battery unhook I test voltage again. 12.7 and of course im beyond baffled. The main front batt is sitting at 12.7 volts no lights are on and im unable to crank the car. But I hook the rear batt up and all lights come on and it will crank.

Any ideas?

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[bonesninja 5,000+ posts]

wow...you must good...i mean to be able to feel you cranking off 9 or 10 volts

 

[wu501 10+ year member]

from what you wrote it seems obvious that something is wrong with the front battery.

 

[billjones42 10+ year member]

are you using a battery isolator? If not, there's your problem--your batteries are killing each other. If you are, is it hooked up wrong, so that when your car is off, the car is running off the rear battery?

 

[CarAudioGuru 10+ year member]

When you have the rear battery disconnected, will the car "crank" at all? Not start, just crank?? Read the voltage when you are trying to start the car.

What is it then??? If you hook up just the rear battery to the car/starter does it start???

IF voltage is still good, even when starting (and anything greater than a .7 volt drop while starting is not good! BTW) Then you probably have an issue with the starter solenoid or stater itself.

Batteries should be paralleled, even of you use a "isolator" (not the fake-o solenoid deals, but the big *** heatsink "Shoe box" size kind with 3 - 1/4 inch posts sticking up out of it)

Sounds like you don't have the batteries separated. Then in reality both batteries are starting the car. And if you have dissimilar batteries front and rear, whichever is the "healthier" kills the other battery.

But until you do a couple more tests, its "Ouija Board " time

 

[skadude016 5,000+ posts]

When you have the rear battery disconnected, will the car "crank" at all? Not start, just crank?? Read the voltage when you are trying to start the car.
What is it then??? If you hook up just the rear battery to the car/starter does it start???

IF voltage is still good, even when starting (and anything greater than a .7 volt drop while starting is not good! BTW) Then you probably have an issue with the starter solenoid or stater itself.

Batteries should be paralleled, even of you use a "isolator" (not the fake-o solenoid deals, but the big *** heatsink "Shoe box" size kind with 3 - 1/4 inch posts sticking up out of it)

Sounds like you don't have the batteries separated. Then in reality both batteries are starting the car. And if you have dissimilar batteries front and rear, whichever is the "healthier" kills the other battery.

But until you do a couple more tests, its "Ouija Board " time

no the car will not crank on just the front battery. Also this has been in out of happening for about 3 weeks now.

 

[skadude016 5,000+ posts]

are you using a battery isolator? If not, there's your problem--your batteries are killing each other. If you are, is it hooked up wrong, so that when your car is off, the car is running off the rear battery?

I have had the 2nd batt for 2.5 years I doubt this just would of started cuz I dont have an isolator

 

[Ed Lester 10+ year member]

how does the connections look on the front battery?

 

[groundpound4200 5,000+ posts]

I would guess that something is wrong with the connections on the front battery. Did you leave all of the stock wiring in place when you started putting together your system?

 

[Ed Lester 10+ year member]

Is the front battery grounded?

 

[jrwalte 10+ year member]

You don't need an isolator and it has nothing to do with this problem. It sounds like you have a problem with either a positive or negative wire on your front battery. Trace the routes of the wires coming from your front battery. Make sure all connections are tight and not corroded.

You should have a positive wire running to your alternator and at least 1 negative to chassis. But also check to make sure your engine block has a negative running to the chassis or your front battery.

 

[Mike465 10+ year member]

Sounds like your front bat could be bad. If you have to have boths bats in for the car to crank than you front bat could be bad.

 

[MECHMAN 10+ year member]

It is a problem with the connections on the front battery cables plain and simple.

If the front battery where bad, it's resting voltage would not be 12.7 volts.

You do not need an isolator or relay, just leave them wired in parralel, and trace down the corrosion or bad connection that is preventing the front battery from getting current to the starter.

 

[kovemaster559 5,000+ posts]

their is a diode somewhere but yeah or some other shit

 

[gunz4me 10+ year member]

Could be some bad cells in the front battery. Years ago I had one that would read 12.4 volts on the DMM but it failed a load test at the local auto parts store. Mine would allow the radio to play, power door locks to work, etc. but would give me nothing but a series of clicks when I tried to start my car.

ETA: If you ruled out the connections, try bringing the battery to an Autozone or Walmart to have them test it for free.