Battery drain, charging system diagnosis

[Cordell]

With the many people who keep bringing this up I wanted to put together the basic guidlines to check out battery and charging system issues.

 

First thing that needs to be done if your battery is dead or the vehicle cranks slow is to not only test the battery but to attempt to charge the battery. The most common issue with a vehicle's electrical issues is the battery so I can not stress enough to check the condition of the battery. You cannot accurately test a discharged battery, the majority of battery testers are voltage based and only give a calculation of capacity so when a battery has low voltage most of these testers will prompt you to charge the battery and that is what you need to do.

 

When charging a battery do not just hook up a charger and walk away. If the battery is bad it may overheat and needs to be checked periodically. I am not refering to battery maintainers or trickle chargers on a known good battery. If the battery leaks it is no good and needs to be replaced, if the case appears swollen it is not worth saving and is a bad idea to attempt to charge. Use some common sense, the termials need to be clean and tight, the battery cannot be physically damaged. Side post batteries are prone to leaking around the side post and its not always obvious, check it, if its wet, replace the battery.

 

Once you know that the battery is fully charged and in good condition its time to verify the rest of the charging system functions properly. Start the engine, does it crank normally? If not this could be a couple things but with a known good battery you only need to look at the condition of the cable to the starter, the starter, and the mechanical condition of the engine. Once the engine is running you can now check the output of the alternator. You should have roughly 14 volts, this will vary from car to car depending on how the alternator is controlled. Basically you are looking for a voltage over 13V. Ideally you would want to load test the alternator to see how many amps it can create, this is not easy to do with a typical digital Volt Ohm Meter (DVOM). If the alternator is questionable find a place that can properly test it.

 

The other thing to consider is the condition of the cables and terminals. This needs to checked for loose terminals, frayed wires, corrosion, and brittle insulation. Most cabling issues can be identified with a visual inspection, however checking voltage drop and resistance may become neccesary. If the alternator's voltage doesn't reach the battery, or the battery's power doesn't reach the starter these basic circuits will need to be diagnosed, and if you beleive this to be an issue start a tech thread.

 

Now once you have done complete accurate checks of the battery, alternator, and related cableing you can see if your vehicle has a draw on it. This type of test requires the use of a DVOM that is accurate down to at least 5-10 milliAmps. The oldschool way of testing with a test light is perfectly fine on an oldschool vehicle without modern computer modules and other things that will create small "normal" draws on a battery and that method will not be discussed. For the purpose of this thread is to discuss modern testing to provide accurate results. Keep in mind a proper draw test will take at least an hour if nothing is wrong, and much much longer if there are issues. Ideally having a battery disconnect in the negative terminal of the battery will make this test much easier. What has to happen to set up the test is to connect the Ampmeter in series between the negative battery terminal and the negative battery cable. Once you have done this taking steps to put all the modules "to sleep" needs to happen. Since you will likely need access inside the vehicle later in this test bypassing the door switch so the vehicle thinks its door is closed is a good idea (varies with vehicle). Make sure if it has an underhood light that it is disconnected. Now once the car is off, and all electrical loads are bypassed or turned off (so the car is just like it would sit overnight) let the car sit for 10-15 minutes and see what your draw is. Some vehicle may need to sit longer others not so long, keep in mind this is a generalized process.

 

At this point you should have a good idea of what the draw is on your vehicle. Correct current draw depends on the vehicle, for years I was taught that having a draw less the 10 milliamps was what you wanted. However with cars today having multiple modules and various forms of "keep alive" memory there is no set amount of draw that is considered "normal". Keep in mind that the higher the draw the less time a battery will have in reserve. I could look up the calculation but for the point of this discussion it simply needs to be understood that a realatively low draw, under 20 milliamps, should maintain plenty of reserve capacity in a battery that is properly sized for the vehicle and in good working order for at least a week. So if your battery is draining overnight, expect a large draw.

 

If you have gone this far and actually have a draw now comes the hard part, actually figuring out what it is. First thing I recommend is unplugging anything aftermarket and rechecking. Then you need to pull fuses one by one rechecking the draw with each single fuse removed. Once you figure out which fuse that contains the largest draw you need to know what all is on that circuit. Then you can process through that circuit to see what the culprit is. Keep in mind modules that are supposed to go to sleep and don't, lights staying on (like the trunk or underhood), power door lock buttons sticking, and radios and their amplifiers are all possibilities. This part of the process can be quite the headache and it may be worth it to let a pro tackle these issues when they start to push your skill level. As a technician who deals with these things on a normal basis even I can spend a few days isolating issues like these.

 

 

 

Meant only to give general guidance, and if you feel I missed something or have something you feel needs added send me a private message. I wish to keep these type of threads free from clutter.

Edited January 22, 2013 by Cordell