If your battery test good with a good amp draw test and hydrometer. Then check voltage draw on your battery as stated before. Connect an ammeter in line and check to see if the amp are over 2-3 .
Most batteries can handle 1-2 amps draw overnite ,most electrical systems don't have that though,most are miliamps.2 amp draw on a 60 amp hr battery gives you 30hrs with 2amps drawing on it if my math is correct...
Or less depending on the battery state...
Clocks,Radio memory,ECU memory, and any other device that has to have a power source for memory need miliamps only.
But if your battery has a bad cell it can drain down over nite or long periods, if you can check the cells with a hydrometer do it, if not the voltage readings and amp draw test will only test it so far. TO be 100 % sure you need to check with hydrometer.
You do have to have 12.2 volts in the battery to test it,charge it if needed,it it doesnt have the 12 volts it aint gonna start the car ,period. The current draw of the starter roughly 150-180- amps will bring the voltage down to 7-8 which wont be enough to run the electronics ,or ignition for that matter if not an efi motor.
With drops in voltage from old wiring and relays and battery cables you could loose a lot of cranking power over time even with a good battery. But if that battery is good ,you have base to start at.
Then check
Like seabass said:
Check curent draw first ,with a amp meter or voltage meter if you dont have a an amp meter , not a test light it isnt accurate enough,if it is above 1-2 amps you have a problem, start pulling fuses to systems inside the cab ,then if it is still there disconnect the alternator as stated before .
It could be something as simple as a out of adjustment brake lite switch , or alt diode bleeding back.
I have seen headlight relays stick as well ,just enough to draw ,but not light the lights...yep seen a lot of wierd shit when it comes to electrical.
Good luck
Gary
