I noticed I have 4-8 volts between my disconected positive battery terminal and the positive post on the battery. My buddy said it indicates a short. It seems to make sence it would be. Everything is turned off and no radio involved yet. I routely leave the battery disconected while it's idle in the shop so I haven't had a dead battery. Would the voltage reading necessarily be a short???
Hmmm It could be a short.... you can also check something by putting your meter in Amps mode, put your meter in-line with your electrical system (hot probe of meter to disconnected power wire, and negative probe to hot side of battery) and read the amperage while your car is not running. That will measure the drainage. You can test some things by turning your key on, turning your key to accessories, and even starting your car. That will show your amperage drain at different points. If you've added something recently to your car, and you think it may be draining, and your meter shows a drain, try disconnecting the power to your new addition..... I recently used this to troubleshoot my recent addition of power seats. I mocked up the mounting, ran power from a switched source (so they have power when the key is on), and hooked them up. Everything worked great, so I locked the car up, and called it a night. The next day I came back and my battery was dead!! I immediately thought of my new power seats that could be draining the battery. Even .25 amps constantly draining, could drain a battery overnight. I used the method above to make sure that it was not in fact draining, and I didn't have any leaks in my wiring...... so my determination is that the battery has a couple of bad cells in it so it won't take a full-charge, and when it gets to it's highest level of charge, it drains faster. Just some ideas, remember voltage is measure of current, and Amperage is the strength of that current. It's likely that those volts that you are reading are the volts that are stored in the battery, or a capacitor somewhere in the line, that is storing the energy. Measuring the amps would tell you if something is draining your system when it shouldn't be. Not much should be except for a radio with a clock or something like that. Hope this is as clear as mud! -Eric
Voltage is a measure of potential difference. Does not have anything to do with current. You measure a voltage with the battery disconnected because there is a difference between the 2 points. But with the battery + disconnected, there is no current flow. You can have a radio drawing 1 amp or another drawing 5 amps and the both will measure a 12v drop. You are correct in that an amp meter put between the + post and + cable will show if there is any current drain. A short is no resistance. If you had a short somewhere in your system, it would probably burn up real quick when the battery was connected due to the amount of current that would flow. Resistance is what limits current. The less resistance, the more current. One more thing, try this with the car running and you will get a negitive reading because the alternator will be charging the battery, but dont keep it hooked up this way while trying to start the car. Most meters only have a 10 amp scale. Most are fused and you will blow that fuse rather quickly. If it dont have a fuse, you will fry the meter.
(hot probe of meter to disconnected power wire, and negative probe to hot side of bat You can get away with this connection if you have a digital meter. If you are using an analog meter you need to connect the meter's red + lead to the more positve point in the circuit, which would be the + terminal of the battery.
Disconnect your positive battery cable and install your meter leads between the battery and cable. If nothing is on you should have no amp draw on the battery. If your showing an amp draw on the meter, 1st make sure everything is off and then start disconnecting different parts that have constant battery voltage. Remove the wires to the different parts one at a time until the draw on the meter goes away. Personally, I'd start by removing the hot wire at the alternator then try the voltage regulator. etc.
My mistake Thanks for clearing that up Dennis. Need to get out all of my old books and refresh.... -Eric
I hope I didnt sound like I was slamming ya cause thats not my intention. Its just after Navy schools and 30 years in the business there are a few things I remember.
No No. No Dennis, that's not the way I took it...... you've got some ahem, "experience" on me, and I appreciate you clearing it up. I was able to use some of what I remembered in diagnosing my dead battery. -Eric
If you do have current draw you can sometimes unplug fuses one at a time and figure out what circuit is the problem.
amp test sounds like the voltage alone dosen't necessalily indicate a short. I'll try the amp test. I've added a few extras recently like an electric chock and electronic ignition but thier switched at the ignition. Also removed the neutral safety switch and looped the plug closed, but again, it should be switced with the ignition. I'll let you know after i amp test it. Thanks!!
