bought a brand new battery for my 318. Works/starts great been using it for a month. recently, I noticed little specs of ruined paint in my engine bay, I thought it was brake fluid leaking from somewhere and splattering all around the engine bay.. turns out one of the vents/caps at the top of the battery is leaking, and the battery is all pushed out and looks like it's about to explode. I stopped at the parts store and they tested my battery. With the engine off, the battery was fulled charger. At engine idle, the battery was getting 14.24 volts and was 90 degrees, and they said the alternator was good. I noticed over the past few days that the needle in the alternator gauge in my dash goes way past the 'C' mark, like the battery is getting over charged. Does this sound like a battery problem or an alternator problem?

voltage regulator problem..it happened to me..

put a voltmeter on it..im betting the alternator is charging at 14.5+ volts.

I just had. an overcharged problem. The regulator wasn't ground well. I cleaned up the all the parts and screwed it back on

As said bad reg or no reg ground. Less likely would be a grounded "green" field wire or alt field is grounded on the green wire side (brush assy). ASAP! pull the connector off of the reg till you get it solved. Deepcycling the batt by letting it run down with no charging is FAR less damaging than driving it with it full fielded like it is right now. That'll cook the batt/damage wiring & terminals/stress bulbs & accessories that are on as they ain't designed for 15+ volts

sounds like a classic bad voltage regulator

where is the voltage regulator and what does it look like?

ok I just looked at a picture, Ill track it down

Might be the battery, if the volts are under 14.6v warm at say 2500 rpm the charging system is fine.

Replace the Voltage regulator as a start.

Do not drive it with the charging pegged to the C. The wiring in these old cars won't handle high current and when the alternator is putting out the max it has the same effect as a shorted wire. Too much current flow is too much current flow. I had to rewire my Satellite because of a bad VR that I ignored for too long.

You say they tested the voltage at the battery at idle and it was at 14.25v? If so, that voltage is fine...

15v+ would cause your problem. 14.5v is ideal charging voltage, so your problem may be the battery. Unless something else goofy is going on

these alternators are not very good. if you have 14.5 volts at idle...what kind of voltage is it putting out when you have say 2500 RPM?!

As others have said, test your voltage at higher RPM. It's possible a bad battery could cause your ammeter to read "charge".

Just to throw one more thing into the mix

Some time ago I put a new alternator on my Charger and the lead wire stud rotated in the alternator causing a direct internal ground

Had the exact same symptoms as you describe so if the voltage regulator does not cure your problem or you have messed with the alternator recently it's another place to look. You can test it with an ohm meter from the lead wire stud to the case.

Ok so began by replacing the voltage regulator, and making sure the the ground was metal to metal on both sides. Using my FSM engine wiring diagram I went over all the associated wires and made sure they were hooked up right, and replaced/repaired all bad connections, it was a mess in there hidden by electrical tape. About to start it up, but I noticed my choke wire isnt hooked up. It needs 12V when the engine is on, should I hook it to the power side of the ballast resistor?

Yes that will work. The factory used the blue field wire (same circuit anyway) going to the alternator/from VR but both should get battery voltage when the key is in the 'run' position.

I would go and get a new engine harness, if the funds are available. It looks like the factory wrap was replaced with scotch 33 after some 'investigating'. There seems to be poor connections and radio shack crimps to try and get by, but I'd recommend a new harness after looking at your pictures.