The following tips are given only as a guide to understanding the electrical systems fitted to a large range of motor vehicle's. Fault finding technics using simple test gear and procedures still require health and safety concerns. You carry out any testing only at your own risk to yourself, or damage to the vehicle.

Well, I am writing this as it comes to mind (much like giving a talk to a local motoring club) and hope to add more info on difference electrical system fault finding tips as time permits. So lets make a start, first OK vehicle battery flat? seems a good place to make a start so what do we check and what do we assume, well we don't assume anything lets get that right first. Quick test, turn on head lights and see if they work this will give us a work load test on the battery, if they don't work at all then then you have no power ie; amps/volts, and the battery probably is discharged. OK whats caused it to have no power, well if its over four years old its due for a change and can discharge with out much warning at this age, spongy lead coatings on plates fall to the bottom of the battery with age and can short out the plates and drain battery to zero. OK the battery was changed only last week so maybe we have a drain on the battery like over night, again quick test remove a battery terminal, best to remove the negative terminal, and connect a simple test light between the terminal and the lead you removed, if any drain present it will light up, notice how bright the light is, this is the power of the drain amps/volts its ability to do work. Cause you still have to find where the drain is, if the drain is large ie; sparking as you put the battery lead back on it could be the diodes in the alternator or faulty electric door locking, or relay contacts stuck. Note! some vehicle electrical systems will make a spark when connecting the terminal, this is normal and not a drain. So connect both the battery terminals, and with your test light connect the clip end to the - terminal lead and the screwdriver end to the - battery post and then remove the - battery lead with the tester connected, it should not light, (really need a picture but just think about it and it will make sense). If its small drain say 0.25 amps its probably a single light bulb staying on ie; boot, glove box,radio internal fault etc. As a start try to identifier which fuse are permanently live, (without turning anything on) and remove one at a time and see if the test light goes off. You will need owners car manual, or sometimes on fuse box cover to known what circuits they are protecting. If test light goes off then the drain is on that circuit.  Stay you have a 40 amp/hr battery and the drain is 1A the  battery will drain flat in 40hours, 0.25A = 160hours. This of cause means completely flat, to turn a engine first thing on a cold morning (small petrol engine)needs about 150A and no less than 9Volts, 9x150=1350 Watts, remember the 1000W electric fire used in homes, lots of heat there.It you have a multimeter and start with a 10A setting you can read the current drain, (turn down to 1amp setting if drain low) connected as the test light. If you can start the car a simple test again is (if you have no meters to test) let the car idle below 1000RPM and look at the reflection off a wall or stand near the head lights, and then take the RPM to say 2000RPM and notice if the lights brighten, this will tell you you if the alternator is charging the battery. Meter reading should be around 14v to 15v max, and hopfully maintained with load on, head lights etc. If the volts go down on load check battery negative to engine block, should be no more than 0.25volts. Same test to body of vehicle. You can also test the positive side by connecting the volt meter to battery + and the other end to alternator main output terminal should be lower than 0.25volts. This test both engine/body earthing and alternator main wiring to battery, so if charge reading still low probably alternator internal fault, stator/ rotor weak fields etc.Check ignition warning comes on full brightest and goes out fully when engine running. If IGN/warning flicker, usually alternator brushes worn not making fully to rotor slip rings, if goes to half brightest then suspect main rectifier diodes or faulty stator phasing. Phasing? alternator spinning rotor as six north and six south poles, fixed stator as three coils, 3x12=36 phase output ripple. This is smooth enough for 12v DC output. By the way do make sure that the alternator is bolted metal to metal, can be rubber mounts and earth wire from alternator case to engine block..........