my stereo a few weeks ago, started staying lit dimly but not coming on properly, just red display and lights. I finally got round to pulling the fuse for it. That put it off at last obviously, and when re-inserted, worked perfectly afterwards.
It would be an idea to pull the fuses one by one that supply everything that works with the ignition off, thats interior light, immob, sidelights, stereo, and brake lights - there may be more but cant think of any right now. Anything that still works with the keys out can be the culprit first, remember that if you switch off then hit the indicator stalk getting out the sidelights for that side will be illuminated and may flatten the battery.
To measure what is being drawn from the battery when the car is off, you need a multimeter that measures DC amps and set on the 20A range,
the test leads often need moved to another socket on the meter as the AMPS range is a virtual short circuit so NEVER EVER put a meter ACROSS the battery when its set to AMPS, you will blow the meter to bits and **** your pants in the process.
Now, the negative lead is the safest to remove from the battery, and you need to connect the meter in SERIES with the battery, that is meter BLACK lead to BATTERY NEGATIVE and meter RED lead to CAR NEGATIVE (BLACK) Cable that you detached. When you do this, there may be a small spark, as the ECU draws about 2 Amps when power is applied, then settles to about 1.6A after about 20 secs, then may drop even lower (< 1A) after 5 minutes as the ECU goes to hibernation mode. If the meter shows a higher reading than 2 Amps (2000 mA if thats the scaling on your meter) then something is drawing excessive current. Thats when you need to pull selected fuses one by one until the current drops away to normal (< 2A) - then you have found the faulty circuit.
To test the alternator charging circuit, charge the battery, fit it, start the engine and connect the meter across the battery terminals (REMEMBER TO SET THE METER TO DC VOLTS 20V RANGE AND PUT THE LEADS BACK TO THE NORMAL V/OHMS SOCKETS FIRST!!!) - get a reliable connection, and put all the lights on, full beams, rear demister, fogs etc and check that the voltage is between 13.8V and 14.8v at 2,500 RPM
Any lower than 13.8v and the alternator rectifier pack is ******** or the fan belt is very loose, or the battery has a buckled cell (It wouldn't take a charge on a home charger if the battery is faulty like this or low on water/acid) - if its higher than 14.8v then the voltage regulator is ********, and its part of the alternator on Bosch units, and a seperate black lump on the rear of Lucas / AC Delco units.
DON'T TOP UP OR EVEN OPEN A BATTERY WHILST IT IS CHARGING, IN THE HOUSE OR ON THE CAR! The sulphuric acid and hydrogen gases present are extremely dangerous whilst charging!
To check a battery's individual cells, assuming its not a sealed type, remove all caps and while the car is off or the battery is disconnected, place the meter on 20V DC scale again, connect the black meter lead to the NEG terminal, then dip the RED meter lead into each cell's liquid one at a time, working away from the NEG end, or measure each pair of liquid filled holes next to each other. There are 6 holes representing 6 cells x 2v each, making 12v (roughly) so any hole that measures less than 2v to its neighbour is bad and will make the battery seem ok measured overall (10v say, but it wont charge properly or give any useful capacity.
Hope this is clear and helps mate, any more clarification needed just ask.
You cant get a shock from touching a car battery terminals even both at the same time as the voltage is way too low to feel, but if you short it out with a low resistance conductor like metal or wire then it will dump massive amounts of current into the short with a loud BANG - approx 400 AMPS, which will vapourise a wire or meter wrongly connected.
