I received a bunch of e-mail about last week’s E39 battery story. Several people said that, with their club discount, they got a genuine BMW battery from their dealer for their E39 for about $200. Considering that Autozone’s price is $143 for their basic battery, and $207 for the “platinum,” that’s not bad.

    Since there was so much interest in the battery piece—not surprising, since batteries are probably tied with tires as the most commonly replaced item on a car—I thought that I’d delve into some battery-related topics in a bit more depth. I could just say, “Buy my electrical book,” but you folks are my friends, so I’ll let you in on the inside skinny for free!

    If you turn the ignition key and hear that dreaded “click,” or “click-click-click,” or experience sluggish cranking, the odds are overwhelming that either your battery is dead or the connections at the battery cables are corroded. Here’s what to do: First, take a multimeter and put it across the terminals of the battery. Don’t have a multimeter? Go to Harbor Freight or any hardware or electronics store or Amazon and buy one. The cheapest ones at Harbor Freight cost $5.99. Those are non-autoranging multimeters, however, and what you really want is an autoranging meter. The least expensive autoranging meter on Amazon is about fifteen bucks. The point is, go and buy yourself a multimeter. Don’t hold the purchase hostage because you want an expensive Fluke meter. (Yes, Satch has a Fluke, but that’s because Mark Jon Calabrese gave it to him for nothing.)

    Any multimeter will work fine for this measurement. (There’s a lot more about this in the book). Set the meter to measure voltage, and put the red multimeter probe in the center of the positive battery terminal, right on the post—not on the battery cable, on the post. Do the same with the black lead; put in the center of the negative terminal, right on the post.

    For a fully-charged battery, the meter should read 12.6 volts, because a car battery is actually made up of six 2.1-volt cells, and six times 2.1 is 12.6. That’s right; your 12-volt battery is actually a 12.6-volt battery.

    With every drop of 0.2 volts, the battery has lost about 25% of its charge. By the time it dips below 12 volts, it’s fully discharged. People argue about this, saying, “I tested the battery, and it reads 12 volts, so it can’t be bad.” Don’t argue, just believe it.

    If you turn the key and the car goes “click” or cranks sluggishly, and you measure the battery voltage and it’s substantially less than 12V, you’ve found the culprit: Your battery is dead. (Note that if you turn the key and you hear nothing at all, and the dashboard lights remain good and strong, the problem could be that the starter isn’t being engaged at all due to an interlock or a faulty connection.)

    Of course, finding out why the battery died is another issue. If the battery is over four years old, it may have simply died a natural death. But if it’s a recent battery and the voltage is low, something drained it. There are two strong suspects: The alternator isn’t charging the battery—and we’ll get to that in a later piece—or a parasitic drain is drawing current from the battery while the car is sitting. (Again, there’s a lot more about both of these in the book.)

    If the battery is recent, and the voltage is only slightly low, then it will probably charge back up fine when you use a good high-quality three-stage charger. But if the battery voltage is really low—like below ten volts—then the battery may well be toast. You can try charging it, but discharging a battery down to the single-volt level often damages it past the ability to recharge. I’ve had good luck with Interacter chargers. They’re American-made, and not cheap, but they’ve occasionally rescued batteries that I thought were goners.

    One problem is that the charger may cheerfully report that the battery is fully charged, but the battery may still not be capable of delivering the kind of amperage needed to start the car. So how do you independently test a battery? You can take it into an auto-parts store and have them test it for you for free, but they will use one of the two methods:

    In the old days, you used a carbon-pile tester, which put a large electrical load on the battery—sort of the battery equivalent of a cardiac run-on-the-treadmill-till-you-drop stress test. A good carbon-pile tester will put a load of about a third of the cold-cranking-amp (CCA) rating on the battery for about fifteen seconds and test whether the voltage falls below 9.6 volts.

    However, a new generation of relatively inexpensive conductance testers is replacing the stress test with a simple measurement. Last week I mentioned the Harbor Freight battery analyzer (see http://www.harborfreight.com/digital-automotive-battery-analyzer-66892.html). I’d like to go into a bit more detail about it. With the ubiquitous 20% off coupon, these cost about $65; if you have a lot of cars, you have a lot of batteries, and the ability to easily triage them into good and bad is well worth the money.

    I’ve been very happy with my battery analyzer. Note that the readings of the analyzer are the most accurate when its clamps are attached directly to the battery posts, not the battery-cable terminals. You’ll find reviews of the analyzer that say that the wire connections to the clamps are a little fragile; they are, but I reinforced mine with a couple of small zip ties.

    The analyzer tells you three things. The first is battery voltage, which is nothing special, since you can measure that with a multimeter, and presumably you already have. The second is the percent of battery capacity. The tester has you enter the battery’s cold-cranking-amp (CCA) rating, and shows you both graphically and numerically what percentage of the CCA the battery actually has left.

    However, it’s a bit misleading. There’s a very thorough video here (https://www.youtube.com/watch?v=kNyiigm7x14) which explains that the problem is that CCA is measured at 32ºF, so unless it’s freezing outside, the reading will be optimistic. Given that optimistic nature, however, if, next to the bar graph, the analyzer displays “NG” for “no good,” it’s probably—you guessed it–no good.

    But the third—and most useful—thing the tester tells you is the resistance of the battery, which is a measurement of the sulfation of its plates. If the battery tests at 5 milliohms or less, it is in excellent condition. If it’s much over 20 milliohms, the battery is probably weak, capable of only a few starts before it drains. If it’s in the middle, the battery is okay but not great. I’ve probably tested twenty batteries with the tester by now, and it’s been very reliable. If a battery has had trouble cranking a car over, and I charge it and then test it with the analyzer and it tests at less than 5 milliohms, and I put it back in, the battery has been fine; but if one tests at over 20 milliohms, usually, when I put it back in a car, I’ll get a few cranks out of it and then it’ll die again—even if, after charging, it reads 12.6 volts. You can see why this is so useful.

    If a battery is fully charged, with voltage around 12.6, and the analyzer says that the battery is good, but the car still goes “click” or cranks sluggishly, odds are strong that there is corrosion on the posts preventing good current flow from the battery into the cables. Go to an auto-parts store and buy one of those post-and-terminal cleaners; they have a female brush socket that slides over the battery posts, and a male brush that slides into the battery cable terminals. Undo the negative terminal first, then clean it and the negative post. Don’t reconnect it yet. Put a rubber glove over it to make sure it doesn’t accidentally touch the battery post. Then take off the positive terminal, clean the post and the terminal, and put it back on and tighten it. Then put the negative terminal back on and tighten it.

    Now try starting the car. Often, if battery voltage is good, cleaning the posts is all that it takes.

    I recently had a “click” problem when starting my Z3. It has the same battery-disconnect switch I’ve installed in many of the cars so the battery doesn’t drain as quickly when the car is laid up and not driven; you attach the switch to the negative battery post, then attach the battery cable to the switch. These inexpensive switches, available at Harbor Freight and Amazon, are like a battery-cable terminal; they clamp directly to the battery and have one end that is round, like a battery post, so that you can attach the original negative battery cable to the end of the switch.

    I tested the battery voltage in the Z3 and it was fine. I cleaned the terminals and still had the same problem. I threw the battery on a charger overnight just to be certain, then tested it with the analyzer, and the resistance was less than 5 milliohms, indicating that nothing was wrong with the battery. On a hunch, I removed the disconnect switch, and the car started instantly, indicating that the problem was likely poor electrical contact at the faces of the switch itself where they mate with the battery and cable.

    I guess I need to look for a source of higher-quality switches.

    Let’s recap. If the car goes “click” or cranks sluggishly when you try to start it:

    • Measure the battery voltage at the posts with a multimeter.
    • If it’s much less than 12.6 volts, re-charge the battery. If it’s really low, like less than ten volts, the battery is probably a goner and should probably be replaced. The question of why it’s low will be the subject of another article.
    • Clean the battery posts and terminals.
    • Try to start the car again. If it still goes “click” or cranks sluggishly, test the battery with a battery analyzer. If it reports that the battery is bad, replace it.
    • If the battery tests out as good but the starter still spins sluggishly, if at all, you either have a bad connection somewhere other than at the battery terminals, or the starter itself is bad. Read up on voltage-drop testing to learn about how to locate a bad connection that is sapping voltage and causing current to drop.—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.