How to test your battery, alternator, and diagnosing parasitic loss

[OutToWinPAHC]

BATTERY AND ALTERNATOR DIAGNOSIS

So your car won’t start or it dies out on you and you think it’s an electrical issue? Try these steps.

Testing the battery.I suggest using a load tested and decent multimeter, but you can also do it with the starter and a meter as well. But AutoZone and other stores will charge and test it for free, so it may be easier to take the battery to them. If you have a charger, multimeter and optional load tester you can test it at home.

PRETESTFirst check the battery voltage. If it’s under 12.5 volts charge it with an auto charger. If your charger doesn’t turn off when the battery is charged you should throw it out. But if you’re desperate after a few minutes check the voltage while charging. If its 14.5Volts or higher turn off the charger.

While the battery is charging check to make sure the terminals are not corroded. Make sure the cables are also not corroded or brittle. Make sure the connection is tight but not over torqued. DON’T CRUSH THE POST.

TESTINGA - Connect the meter to the battery on the volts DC scale, if the meter has min/max function use it (this will capture the high voltage peak, and low crest and you can view it). Connect your load tester and flip the switch and hold it for 3-5 seconds. Check the load tester meter and make sure its in pass or check your meter reading and make sure it didn't drop under 9volts. If it drops under 9 or straight to 0-3VDC, the battery is bad.

B - Connect the meter to the battery on the volts DC scale, if the meter has min/max function use it (this will capture the high voltage peak, and low crest and you can view it). Next step is to crank the engine over if it cranks. If the voltage on the meter drops below 9 volts the battery is bad, if it goes straight to zero this indicates an open cell and it also a bad battery.

WANT TO TEST THE ALTERNATOR? Without removing it and taking it to get tested?

Simple start the car and turn the lights on, AC on, radio on, wipers on. With an electrical glove on do it one of two ways.

A-If you have a battery fuse right in line with the positive battery cable… remove itB-With a glove and insulated wrench disconnect the either battery cable, I like positive, but if your not using an insulated wrench go for negative.

If your car dies right then and there….. Your alternator is not supplying enough current. When the alternator does not supply enough current it takes it from the battery and run it down.

BOTH THE BATTERY AND ALTERNATOR CHECK OUT GOOD, BUT AFTER A FEW DAYS MY BATTERY DIES?

Parasitic battery loss is the technical term for this problem. It is caused by two root factors.

A – A circuit or device in the car that stays on drawing current. All cars have circuits that do this such as the ECU, clock, and headunit memory. Those items are not issues usually, unless something breaks down in these devices causing them to draw more current. But if you have added new devices like amps, caps, engine electronics, they could be wired incorrectly causing them to stay on all the time. The other thing that can cause this to happen a nick in a harness shorting to ground. It may not be a hard short due to a painted surface, but if there is a current loss you will drain the battery.

B – The other thing that causes this… A leaking battery. When a battery drips or leaks it leaves a trail of electrolyte down the side of the battery. The chassis is grounded so if the battery is leaking a little (like a single drip that runs down the side of the battery) it will make a trace to ground. Kind of like a tiny solder trace on a board. Current flows down this trace to ground causing a small but stead discharge of current. If you battery is leaking replace it. If you have wet mark around the terminals or green fuzzy looking stuff known as copper sulfate on the terminal replace the battery and cable.

[slipnfall]

Perhaps the 240 electronics are a little more resilient to this sort of thing, but generally speaking it's not a good idea to remove either positive or negative cables from the battery while running (especially ground). Most sensitive electronics do not take kindly to losing their reference, which can cause device terminals to float outside their maximum range. Remember the +V will try to seek out *any* lower potential. Also, the battery may be drawing significant current trying to charge - after all you're diagnosing alt/batt ; popping a cable off is going to make a nasty little arc. Electronics do not like back-emf.

Short version: if you car is to the point that it is running, take it friggin' autozone or whatever and let them test it for free. They'll diagnose the battery and alt in one shot.