I have '95 toyota corolla that's been giving me some grief lately. I haven't done any real troubleshooting yet, but I will tonight. thought I would get some pointers before I begin. When I press the brakes or turn on the headlights there is a temporary dip in voltage (I assume, no voltage indication on the dash) that causes my radio turn off and the "battery" idiot light to come on. sometimes it's so severe that it kills the car. This makes me think that while running, it is operating on the alternator alone and recieving no help from the battery. When I go to start it, sometimes it won't start. When it does start, it starts just fine, no clicking. Once it wouldn't start and I got it to start by wiggling the battery connection. I thought "so that's what the problem was, a loose battery connection" so I tightened the heck out it, but still having the problem. battery termianls are clean and tight, no corrosion. I am pretty sure the alternator is charging, because I drove it home 500 miles like this and the battery is still charged. I also think the battery is fine because it still starts just fine (when it starts). Yesterday I turned on the headlights and it died; wouldn't start again, and I heard buzzing in the dash. Sounded like high frequency relay chatter.

 

Probably first to check is Earths, Battery to engine, battery to vehicle frame, Some times there are multiple earths from loom to battery & or to vehicle shell. Poor earths cause weird problems.

 

One item that will get you is if the ground strap from the engine to the firewall is missing, broken, loose or corroded. If that happens, then the electrical connection from the alternator's ground to the chassis ground is through the drive train and whatever metal might be between the engine and the chassis, like fuel lines, transmission linkage, etc. - but those are relatively poor conductors. If you can't find a ground strap, it won't hurt to add one.

Many vehicles don't have an engine to firewall ground strap. Instead, they have a 2nd wire from the battery negative terminal that runs to the chassis. If that wire is intermittent, broken, corroded, missing, etc. you'll wind up with the exact same problems. It's the ground wire for all of the chassis accessories; lights, instruments, etc.

Another possibility is that one of your battery terminals is broken inside the case. This can happen if someone bangs on the terminal with a tool or applies excessive twisting force to the terminal. If you gently try to rotate the terminals using the wires from the battery connectors and the terminal moves, you'll need to replace the battery.