How to check a car battery with a Multimeter?

Hi,





How exactly do you do the above, does the battery have to be connected to the car or disconnected, does it need to be fully charged?





Thanks in advance.|||Hi





Heres a couple or three checks you can do with a volt meter, but the real test is to have it checked by an autofactor with a proper amp tester.





Read the voltage across the battery and then start the engine and read it again with the engine running.





If both readings are more or less the same then swap the meter to read ohms, switch off the engine, and take an ohm reading between the battery negative terminal and a good earth point on the engine and also a good earth point on the car body. If there is any significant resistance at either test then check the main earth strap(s) for damage and security. If there is no significant resistance then read on:-





Change the meter back to volts and connect the positive probe to the battery positive terminal and the negative probe to the other end of the battery positive cable at the back of the alternator. If the reading is anything other than a 1, -1, or zero, then the battery positive cable needs to be changed.





If all of the above checks out and the voltage with the engine running is around 13.5 volts to 13.9 volts, or maybe a little more, then switch on the headlights, the heater blower onto full, and the heated rear window, then raise the revs to 2500 and check the volts again. If the reading remains in the 13.5 to 13.9 volt range then all is good in the charging system, otherwise the alternator is at fault.





If all of the above checks out and the battery is not holding a charge or not taking a charge, then the battery is very highly likely at fault.





Phew!!!!





Very best regards





Geordie|||That's a nice change, everybody's giving good advice here! I agree with all the answers given to your question so far. The only real way to know how much power a battery holds is to charge it right up, flatten it and see how much power you got out. As everyone has said, there is little difference between the voltage across a flat (good) battery and a charged one. One problem is that as batteries age, they fill up with a sludge (lead sulphate) which coats the plates and fills it up from the bottom. This means in effect that the battery is getting smaller, but the voltage across a small charged battery is the same as a big battery so you don't really get any meaningful information that way. Also, the voltage will be higher than usual for some hours after you last run the engine. Well, you've got some extra info from people also, so you can test your charging system too. Batteries often fail in the winter, that's when they get the hardest time.|||A multimeter won't tell you the true state of a battery, unless it is badly discharged. You can read close to 12 Volts and the battery may still have problems. If the battery is fully charged you can get some indication of it's strength by reading the voltage while turning on the hi beam lights or by cranking the engine without allowing it to start. If the battery is good it shouldn't drop more than a couple of volts. Best bet is to take it to Auto Zone or one of the orher parts stores and have them load test it.|||Depending if your multimeter is automatic just touch the red lead to the positive terminal, the black to the negative terminal. If it is a manual one set it to DC. It makes not difference if it is connected or not. Check it in the morning it should be between 12.5- 12.8, You can also check the output of your alternator, with the car running connect the leads the same way and the reading should be between 14.1 -14.7|||You can check its voltage with a multimeter. You don't have to disconnect. If it is fully charged it will be about 14 volts.





But this isn't going to tell you much. A battery can be at full voltage but lose its charge very fast if it's bu99ered. The only way to really test it is to put a hefty load on it and see how long it lasts.|||Its really not a reliable way unless the battery is totally hosed. Sears will check it for free and have the proper equipment to test it under load which is the only way that you'll know for sure.|||Some good answers already. If you are worried about battery drainage problems, check out this read: http://www.ohmcheck.com/carbattery.htm Some good tips.