Last week we discussed run-down batteries. This week we’ll talk about a car’s charging system. That system consists of the alternator and the voltage regulator. The alternator puts out current at a higher voltage than the battery in order to satisfy the car’s electrical demands while keepin the battery fully charged. The regulator is basically a switch that enables and disables the alternator very quickly—tens to hundreds of times per second—so that, on average, the voltage going back into the battery is about 1 to 1.5 volts higher than the battery’s resting voltage. That is, a fully-charged battery at rest should read 12.6 volts, but when the alternator is charging the battery, it should read about 13.5 to 14 volts.

    Those numbers went by a little quickly, and they’re incredibly important, so let me repeat them:

    • The battery’s resting voltage is 12.6V—that is, if you take a multimeter, set it to measure voltage, and put the two probes across the battery terminals, it should read about 12.6V. If the reading is substantially less, like below 12.2V, your battery is dead, and needs to be tested and recharged or replaced (see last week’s piece).
    • The charging voltage measured at the battery with the engine running should be about 1 to 1.5 volts higher than resting voltage—between about 13.5 and about 14 volts. So, after you start your car, with a multimeter measuring the voltage across the battery, if it still reads 12.6 volts and not 13.5 to 14 volts, it means that the alternator is not charging the battery.

    These two steps constitute a charging-system basic health test. It is trivially easy, and it’s the most important thing you can do for your car’s electrical system health. Remember that if your battery is dead, something caused it to die. The four most likely somethings are 1. a dead alternator, 2. the age of the battery, 3. an electrical device such as a radio or headlights accidentally left on, and 4. a parasitic drain caused by a device you’re not aware of, such as a control module that’s not going to sleep. You need to rule out #1 before you jump-start the car and drive off down the road.

    Note that on vintage cars like the 3.0CS, 2002, or Bavaria, there is a fifth possible cause: Beginning in the mid-1970s, most cars, including BMWs, began using regulators that were integral with the alternator; but in vintage cars, the alternator and regulator are separate. An equally likely possibility to the alternator being dead is the regulator being dead—dying in the “open” position so it never switches the alternator to charge the battery—or the wires connecting the alternator to the regulator breaking. There are three wires (D+, D-, and DF) with three push-on terminals at each end. That’s nine things to go wrong that aren’t there on a modern internally-regulated alternator, which is why so many people with vintage cars retrofit a modern one.

    The battery also is part of the charging system, because it acts as a filter, keeping spikes in both supply and demand from harming your car’s electronics. Back in the day, people used to test whether an alternator was charging the battery by starting the car, then disconnecting the negative battery terminal. The idea behind this test was that if the car continued to run, it meant that the alternator was working and supplying voltage to the coil to fire the spark plugs. Do not perform this test! In the first place, it’s likely to pop the diodes in the alternator (in which case, if your alternator was working, it ain’t now). And, from a practical standpoint, it’s completely unnecessary, as the charging system basic health test described above tells you what you need to know.

    There are two other charging system facts you should be aware of:

    • Alternators are designed to keep healthy batteries charged. Contrary to popular belief, alternators are not designed to recharge dead batteries. It is true that when a car is driven, the alternator will keep the battery charged; but if the battery is dead—particularly if it’s really dead, like sub-11.5 volts dead—it must be recharged with a good three-stage charger, then tested, and replaced if it’s no good.
    • Modern (post-1990-ish) cars with electronic control modules are very sensitive to voltage levels, and absolutely hate it when you jump a dead battery and expect the alternator to charge it and clean up the mess. In this circumstance, because the alternator is working hard to compensate voltage-wise for the drained battery, the voltage levels seen by the control modules may be low, which may create odd symptoms, such as dashboard indicator lights flashing on and off, the car bucking or running poorly, or the car dying when you hit the brakes.

    For these reasons, if you have a modern car with a deeply discharged battery, jumping the battery may start the car, but it is likely that you’ll need to replace the battery with a new or fully-charged one for the car to run right. Thus, depending on where you are and how far you need to drive, simply jumping the car and proceeding on your way may not be the thing to do. that I use a rule of thumb: If I turn the key and hear sluggish cranking or “click,” and the dashboard lights are nice and bright, jumping the car and driving it is probably okay; but if I turn the key and hear nothing, and the dashboard lights are dim or out, I’ll procure a fully-charged battery before driving the car more than a few hundred feet.

    An even more acute circumstance occurs when the cause of the dead battery is the fact that alternator hasn’t been charging it. If the battery has slowly been running down because the alternator is dead, the above symptoms of poor running, bucking, and odd indicator lights will likely manifest themselves until the car just dies altogether. If AAA or a good Samaritan jump-starts the car, it’ll simply die again half a mile down the road, perhaps someplace less safe than where it died the first time.

    So, if you find that your modern car has a dead battery, after you jump-start the car or replace the battery, it is imperative that you test the health of the charging system. With the engine running, measure the voltage across the battery terminals. You can augment the test by revving the engine up to about 2,500 rpm; the voltage should increase slightly. Then turn on a number of electrical loads (for example, headlights and blower). The voltage may dip slightly, but it shouldn’t drop dramatically, and it definitely it shouldn’t come back down to the 12.6-volt resting voltage. If it does, it’s possible that the voltage regulator is bad. As I said, on vintage cars, the regulator is external, but on most cars built since the mid-1970s, the regulator is part of the alternator; the regulator and brush pack usually form an integral, easily replaceable unit.

    By way of example, years back, when I had the E39 wagon, Maire Anne and some friends drove it to New Hampshire to go hiking. On the way back, the car began running horribly, the dashboard lights began flashing, and then the car died. She called AAA, they jumped the car, and it died again shortly thereafter (something for which I’ve barely forgiven them). She called me and described the symptoms. I suspected that the alternator had died, but one of her friends absolutely needed to get home that night and there was no easy way to procure one in time. So I drove up to New Hampshire in another car with a charged battery and tools in tow. I checked the voltage on the E39’s battery and found that it was pretty run down, less than 12 volts. I installed the charged battery, started the car, did the charging-system basic health test, and found that with the engine running, the battery voltage stayed at 12.6V, indicating that the alternator wasn’t charging the battery. If I only needed to drive ten miles, it probably would’ve been fine to drive on the fully-charged battery, even with the alternator running it down, but it never would’ve made it the 150 miles home. I left the car with a local mechanic—probably the only time in the past decade I’ve paid someone to fix my car—and drove everybody home, returning a week later to pick up the wagon.

    One very cool thing you should know is that for less than ten bucks you can buy a cigarette-lighter-plug-in LED voltmeter. These things are so cheap and handy that you should have one in the glove box of every car. They come in voltmeter-only models or with integrated USB ports. They’re not as accurate as a good hand-held multimeter, but they don’t need to be, because you don’t really care about absolute accuracy. What you’re most concerned about is whether the reading increases by 1 to 1.5 volts when the car is running, and they’re plenty accurate enough for that. If you’re having a starting or a charging problem, plug it into the lighter socket. Note the reading—12.6 volts, 12.4 volts, whatever. Then start the car. Jump it if you need to. If the voltage on the little plug-in voltmeter doesn’t increase up by 1 to 1.5 volts, the alternator is not charging the battery, and you know that even if you jump-start the car or put in a charged battery, it’s going to die again. Neat, huh?

    To recap:

    • The battery resting voltage (engine off) should be about 12.6 volts.
    • The charging voltage (engine running) should be about 13.5 to 14 volts, it should increase slightly when you rev the engine to 2,500 rpm, and it shouldn’t dip egregiously when you turn on a stiff electrical load.
    • You can easily measure the voltage with a multimeter at the battery or with a cigarette-lighter-plug-in voltmeter.
    • Don’t drive a car far if the alternator isn’t putting out charging voltage. It will strand you.
    • If the alternator is putting out charging voltage, don’t try to use your alternator to recharge a dead-as-a-doornail battery, particularly in a modern car. It will run weird. and may strand you.

    Go forth and—charge!

    (And, yes, there’s a lot more about all of this stuff in my electrical book.)—Rob Siegel

    Rob’s first book, Memoirs of a Hack Mechanic, and his new book, The Hack Mechanic Guide to European Automotive Electrical Systems, are available through Bentley Publishers, Amazon, ECS Tuning, and Bavarian Autosport—or you can get personally inscribed copies through Rob’s website: www.robsiegel.com.