Dead batteries before, during or after repair are a very common problem when a vehicle has been in an accident. Something as simple as door misalignment or as complex as a damaged wiring harness or module can be at the root of the problem. In a mechanical shop environment, we learn about these problems when a customer finds out they have a dead battery. In a collision environment, you can find out at almost any time throughout the repair process. I have some simple advice before we dive into a couple of tests you can run to nail down the source of a parasitic drain on the battery. Let’s start with a few vehicle feature considerations that could make your diagnosis difficult or completely unnecessary depending on the situation.
Many, if not most, late-model vehicles have some sort of battery saver that will disconnect the battery from the vehicle electrical system when say your 10-year-old leaves the door open in the garage all night. Instead of a dead battery 30 to 45 minutes into a steady load on the battery, the smart fuse block or body control module will just shut everything down. This very useful technology can also create a head scratcher when after a few days in the shop and usually about the time you are ready to move it the battery is unable to turn over the engine. Go back to that door misalignment issue or an interior light that got turned on during the accident. We could have a fairly substantial battery drain that reactivates when a door is opened and then disappears after half an hour, overall taking a few days to drain the battery from having enough energy stored to start.
But don’t panic yet and start searching for a parasitic drain. First completely charge the battery and then make sure all lights are out when the doors are shut, trunk and glove box are closed, hood is down or any other feature that turns on those 2-5 amp drains. Before we go on, let’s talk about charging batteries because the best of us get in a hurry and do it wrong.
Battery charging is not a quick operation. If it were, the electric car would probably be a much more viable means of transportation right now. SLI — Starting, Lighting and Ignition — batteries take some time to charge to full potential. These are the batteries we use in our cars and are not the same as the batteries in hybrid and electric vehicles. They are used because they can dump large amounts of current very quickly. Keep in mind that most cars require about 120-200 amps to the starter to start the engine and that most alternators/generators create about 85-140 amps. SLI batteries do not like to be recharged at 85-140 amps. They like something in the 60 amps or less range, depending on their state or charge (basically how far they are down from fully charged). The rest of that alternator output is intended to be the power station for the rest of the vehicle while always maintaining the battery at a full state of charge (SOC).
So how do you know when a battery is at a complete SOC? For an SLI battery, it should be sitting at around 12.7 volts with no load against it and no charge going into it. A battery at only 11.89 volts is dead, according to BCI, who sets standards for batteries. Without belaboring the point too much, if you jump start a system with a battery at 0 percent state of charge and then pull the cables, the alternator is going to exceed not only its intended load, but will be over charging the battery as well. You will also have no idea if that battery is any good or not.
When a battery has reached a discharged state, you need to use a quality battery charger that will limit current and/or voltage, depending on the type of battery being charged. There are two common SLI-type batteries in use: the familiar lead acid battery and the AGM (absorbed glass matt) design. They require different approaches to charging, and many modern battery chargers have different settings, depending on which type you choose. The single best way to charge these batteries is to use a fully automatic charger. Once you program in the kind of battery, cold cranking amps and where the charger’s cables are placed, it will determine if the battery is good or not.
Batteries are often damaged in collisions, but the damage is not visible. If the battery has lost its ability to store energy, you might spend time looking for a parasitic drain when, in reality, you just don’t have adequate energy stored on board anymore. It is also bad form to send a customer home with an alternator that has been damaged by repeated jumpstarts on a dead battery.
If you suspect a system drain, get the battery cables disconnected by removing the negative first. In most situations, it makes sense to repair the vehicle before you look for the cause of a parasitic drain. There are numerous reasons for a module to stay awake and drain a battery following a collision, and it seems likely that the repair is going to resolve these issues, or at least uncover them. While it makes great sense to gather diagnostic trouble codes prior to “putting the vehicle to sleep for surgery,” doing any diagnosis of those codes or a potential parasitic drain should be left for last and only performed with a fully charged and proven battery/charging system.
Once repairs are complete, connect the vehicle and clear any DTCs present. In many vehicles, you will also need to clear adaptive memory (depending on the work performed) and complete any programming required. At this point, if you suspected a parasitic drain but did not find a smoking gun, you will want to test for loss. Again, I think you will find that if you have a charging and starting system tester, you will be able to determine if a drain still exists fairly quickly. Of course, we are talking about late-model cars, so it is not as easy or quick as we would hope. Once you have test driven the vehicle and are ready to test it, you need to leave it alone for a while. There are many modules that stay awake for a while after a vehicle is shutdown to perform their tasks, such as evaporative emissions systems and body control modules. This has been common since the late 1990s in many vehicles. Once these systems all power down, you can get a pretty reliable parasitic reading, and it should be under 40 mA (milliamps). Again, I am taking a different approach than in a mechanical environment where we start in with the low amperage amp clamps and break the battery connection right away to get “in the circuit.”
If you find that you have a parasitic drain greater than 40mA, but under, say, 500mA, there are numerous detailed articles on diagnosis of those problems in our sister publication Motor Age. These are more difficult problems to find and often supervisors should be involved with the additional time that may be required and special tools that are probably not lurking in your toolbox. For larger drains of, say, 1 amp or more, you are generally talking about a non-ignition switched circuit, unless something has gone terribly wrong. You would probably know that before you got this far.
The easiest way to narrow down these problems is to remove the positive battery cable and place a DMM (digital multi meter) set to 10 amps (connecting the leads properly to the DMM so you don’t blow the fuse inside it) in line with it. You will probably have to wait awhile for the modules to go to sleep again. To simplify your work, you may want to remove the driver door interior light switch to avoid adding new current to the drain. Be very careful not to turn on any large drain, like the ignition, or you will blow the fuse in your DMM. Think about the systems that are battery powered, like fuse blocks — did an engine compartment fuse block get damaged and is feeding B+ power into a circuit that it would not normally? That will be fun to find, but consider the damage from the accident and go over electrical components in that area with a fine-tooth comb, particularly those with large-gauge wires, since they will almost certainly carry a live battery (B+) connection that is activated by a relay or internal component switching. Consider lighting circuits and switches that you may not be able to see. Were there any modules that got hit and had visual or superficial damage? Did the alternator or bracket get hit? Pull your alternator voltage regulator connection (if it is internally regulated and not PCM regulated) with the engine off and see if the drain goes away. Pulling large fuses first will help to narrow down the section of the vehicle where the drain is occurring, at the risk of waking a module you will have to wait on when you put the fuse back in. Pull small battery-powered fused circuits first to get there quicker, and have a wiring diagram of the power distribution circuits on hand so you can make logical choices.
There are no easy shortcuts to finding battery drains. The key is to be sure you actually have one. A very thorough inspection prior to diagnosis for damage will help uncover battery drains that may have been hidden or overlooked during the initial blueprinting of the repair.
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