Ah, the parasitic drain—the thing that drains your battery when you haven’t driven your car in a week. On a vintage car, these aren’t usually a big problem. If they occur, they’re usually something dumb, like a power amp or other aftermarket sound-system component that’s accidentally wired to be always on, or a power antenna that’s always trying to go up or down (or, in a 1980s BMW, the battery in the rechargeable flashlight in the glovebox gone bad). If a problem does rear its head, the time-honored solution is to set a multimeter to measure amperage, put it between the negative battery post and cable, look at the amperage reading, pull fuses until it drops, then see what devices are on that fuse. If you still can’t figure it out, you unscrew the fuse box and look at what’s actually wired to the fuse, because you never know when (or why) someone got creative with the wiring on an old car.

But in a newer (post-OBD-II) car, parasitic drains can leave you pulling your hair out.

In the first place, there are so many more things that can cause the problems: all of the actuators in the central locking system, heated door locks, alarm systems, and the myriad of control modules themselves. They’re supposed to go to sleep after the car has been locked, but if any of the little switches on the door, trunk, or hood locks don’t register closure, the modules stay awake, putting a bigger drain on the battery.

Second, unlike the six or twelve fuses in a 2002, a newer car can have dozens of fuses, and the system causing the drain may draw current from more than one of them. Because of this, instead of pulling fuses, you’re supposed to leave them in place and use a multimeter to measure the voltage drop across the back of the fuse, and use that to infer the current passing through it.

For these reasons, as long as the drain is minor—that is, if it doesn’t deplete the battery between when you park it at work and when you go home at the end of the day (if that happens in a BMW and you didn’t leave the lights on, it’s usually the Final Stage Resistor Unit (FSU), often called the “sword”), many people just live with the problem via one of two workarounds. They either install a cut-off switch on the negative terminal of the battery and disconnect it when they park the car, or they use a trickle charger.

So that’s the background against which I present to you the case of Jeanette, my 95-year-old next-door neighbor.

Ironically, Jeanette is the widow of Hank the Crank, the guy who made my life hell for two decades as he watched me like a hawk, chewed me out if any of my cars left a drop of oil on the street in front of his house, and called the cops on me if any of my registration or inspection stickers were out of date. But time heals most wounds; Hank softened when I began snow-blowing his sidewalk when he got old and sick, and Jeanette has turned out to be a lovely lady.

Jeanette has a 2012 Honda Accord with 21,000 miles on it. At age 95, she still drives it to the hairdresser and to visit her 102-year-old brother in the nursing home. The car has behaved as a 21,000-mile Accord should: It has needed nothing.

That is, until late last year.

If she did not drive the car for between four and seven days at a time, Jeanette would come out to find that it wouldn’t start. She’d call AAA, and they’d jump it and advise her to drive it more; she wouldn’t, so of course it kept happening.

Eventually she used up her AAA benefit. She took the car to the Honda dealer, and they apparently gave it a quick look and found nothing wrong. They also told her to drive it more, specifically telling her to take it up onto I-95. She told me all of this, confiding with a little embarrassment, “I don’t drive on the highway.”

She called me one morning when the car wouldn’t start before her hairdresser appointment. I put the battery on charge for half an hour and told her that if the car wouldn’t start when she was done with the appointment, I’d come and jump-start it (the hair place is only a few miles away). I thought about just offering to drive her there and back, but if she wanted that, she would’ve asked. You don’t mess with someone’s independence.

But about five days later, not surprisingly, the same thing happened again. This time her brother had fallen in the nursing home; she wanted to go and see him, the car wouldn’t start, and she called me in tears, not knowing what to do. Her niece came and picked her up while I had a more thorough look at the car.

Whenever a car runs down its battery, I advise stepping through it like this:

  1. Was it operator error? Did you do something careless? Were the headlights, parking lights, or any interior lights left on? Did you or a family member leave something plugged into the cigarette-lighter socket? (A Facebook friend told me the story of troubleshooting a drain in a customer’s car and finding that they had left a heating pad plugged in.) Or is it some visible malfunction, like the light on the makeup mirror on the visor not going out when the lid is flipped closed on it?
  2. Is the alternator charging the battery? I’ve written many times about this, but while the engine’s running, the alternator and regulator should be charging the battery somewhere in the 13.5 to 14.2-volt range. If instead the voltage reading is the battery’s static voltage of 12.6 or less, then anything you do, whether it’s driving the car or letting it sit, is going to drain the battery.
  3. Is the battery itself bad? This is a little tougher to diagnose unless you have some sort of tester. For about eight years I’ve had a little Cen-Tech digital battery analyzer; these days you can buy something similar on Amazon for as low as thirty bucks. Instead of doing the load test of the old-school carbon-pile testers, these perform an inferred resistance reading. On the Cen-Tech, a reading of less than 5 milliohms means that the battery is very good, over ten means it’s bad, and in the middle it’s a gray area. It also tells you what percentage of the battery’s cold-cranking amps (CCA) it still has. Over the years, I’ve learned to trust it.
  4. What is the measured parasitic drain? When you actually put a multimeter configured to measure current between the negative battery terminal and ground, what does it read? The oft-quoted industry-standard threshold for a parasitic drain on a modern car is 50 milliamps (0.050 amps). This is still higher than I’d like to see—10 to 20 milliamps is a more comfortable number—but the point is that if you see hundreds of milliamps—or worse, actual amps—then you know that there’s a significant problem.

I applied this to Jeanette’s Accord. I found no lights left on and nothing plugged into the cigarette-lighter socket, so there were no smoking guns (or accessories).

I jump-started the car and put my meter across the battery terminals and measured 14.3 volts, so there was nothing obviously wrong with the charging system.

I fully charged the two-year-old Honda-branded battery, then tested it with the Cen-Tech battery analyzer, and the resistance reading was 6.45 milliohms. It also reported that the battery had about 90% of its 490 CCA rating. So while the battery wasn’t like new, it wasn’t obviously bad, either.

The best way to measure the drain on any car is to install a battery disconnect switch, because this lets you direct the current through the multimeter without risking damage to it. The problem is that when you disconnect and reconnect the negative battery cable, there can be a surge of start-up current, because the always-on systems in the car—particularly in a modern car—spring to life. You may even see a little spark at the battery when you reconnect the negative cable.

Many, if not most, multimeters have a ten-amp fuse in their amperage circuit. Ten amps is a lot of current, and while it would be unusual to see this much when you reconnect the battery, well, you are doing this because there’s some sort of a drain. For this reason, it’s better to install the disconnect switch, set it “on” (meaning current flows through the switch), connect the multimeter with one lead on either side of the switch as shown below, then flip the switch to “off” to force the current to flow through the meter.

Note how the multimeter’s leads are connected on either side of the switch.

The drain reading on Jeanette’s Accord fluctuated around 46 milliamps—just below the rule-of-thumb threshold where it’s considered a problem.

Now, the way you’re supposed to further diagnose this on a modern car is to go through the steps to make the control modules fall asleep. This means closing the hood and locking the doors, which obviously makes it impossible to pull fuses (or, more correctly, to measure the voltage drop across the back of the fuses). So if you’re going all-in, what you need to do is to fool the car into thinking that it’s closed and locked, either by rolling the hood and door latches closed, or using jumper wires across the microswitches. I didn’t have space in my garage to do this, and it was cold out, so instead I carefully arranged the multimeter probes so shutting the hood wouldn’t cut them, put the meter on the ground, carefully closed the hood and locked the doors, and watched the meter. In under a minute, it settled down at around 46 milliamps. I left and came back 40 minutes later, and it was at about the same reading.

So, was there really anything wrong? If the battery hadn’t been running down, and if the behavior hadn’t only surfaced in the last few months, I would’ve said no. I couldn’t easily commit the time and the space needed to go deeper, and I wasn’t sure that finding a shop to properly trouble-shoot it was worth the money. (To get an idea what’s involved in doing this correctly, watch this video where a pro troubleshoots a drain in a late-model Civic, isolating it to one of the door-lock actuators.)

If this had been my car, I would’ve said that the problem was already solved: I’d already installed a battery-disconnect switch. However, it wasn’t reasonable for me to expect a 95-year-old woman to pop the hood, lift it up, fumble with the hood rod to hold it up, throw the switch, then close the hood every time she parked the car in her driveway, and then go through the routine again when she wanted to drive it.

The next-most-obvious solution was to set her up with a trickle charger. After all, I’d just used the electrical outlet on the lamppost at the top of her driveway to charge the battery. I thought that if I bought her a battery maintainer that plugged into the cigarette-lighter socket and combined that with a flat extension cord that the door could easily close on, we’d be done.

I found, however, that there were two problems with this approach. The first was that the cigarette-lighter socket on the Accord isn’t active until the key is turned to the accessories setting. I could get around that by wiring a short cable directly to the battery with a connector hanging out the front grille.

But of greater concern was the tripping hazard. During the short period I was recharging the Honda’s battery, Jeanette came out to watch what I was doing, and I saw her nearly trip over the cord. The idea of an extension cord, the necessity of her having to disconnect a plug, the slightly downward-sloping driveway, and winter weather all conspiring to send her tumbling made this a hard no. I was not going to impose a solution that carried with it a chance of her falling.

Create a mix with a 95-year-old woman, this slight downhill slope, winter weather, and electrical cables? I think not.

So instead I installed a new battery.

Costco sells common models of Interstate batteries at a discounted price. I went to costco.interstatebatteries.com and found that an Interstate 710 CCA battery with advanced glass mat (AGM) construction was reportedly $180 at the Costco tire center. Unfortunately, when I called, I learned that for some reason Costco no longer sells AGM batteries, so instead I ponied up $230 at an Interstate dealer—the most I’ve ever paid for a battery (to be clear, she reimbursed me).

My hope is that the combination of the battery’s newness, the 45% addition capacity from the increase from 490 CCA to 710, and the AGM construction will buy reliability, or at least longer times between needed recharges. I’ll go over next week and check the battery voltage to see how it’s doing.

This was NOT inexpensive, but it should solve most of the problem.

And I’ll also think about a very interesting suggestion a friend of mine made: buying a battery-charging solar panel and laying it on the Accord’s back deck under the rear windshield. The idea is that this solution has zero cables for her to trip on, and even if it’s not guaranteed to keep the battery topped up, as a proper maintainer or trickle charger would, it would still be better than no augmentation. If the car’s cigarette-lighter socket was active without the key turned, I’d already have done it. Without that, I’d need to wire it to the battery, and I need to think about that.

So I guess I’m Jeanette’s battery maintainer. I can live with that.—Rob “Human Battery Tender” Siegel


Rob’s newest book, The Best of The Hack Mechanic, is available here on Amazon, as are his seven other books. Signed copies can be ordered directly from Rob here.




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