How to methodically find the battery drain in our E46–and fit an Optima battery, too | Project BMW M3

David S.
Update by David S. Wallens to the BMW M3 project car
May 22, 2021

How do many of us deal with electrical issues? Ignore them. 

Our E46-chassis BMW M3 had been shedding some electrons–not enough to leave us stranded, but we knew there was an issue. 

In fact, we knew it was a problem when we bought the car because the seller–and old friend–told us. But, he added, it wasn't bad enough for the battery to die overnight.

Our solution for however many years: Just keep the car parked on a CTEK battery tender. That seemed to keep things in check, but we knew it wasn’t a long-term solution. Time to fix it. 

We started with some detective work. Okay, really, we called Carl Heideman because he knows all. We also reread a tech piece that he wrote for us about chasing electrical issues. 

So, we followed Carl's instructions, but added in a step: Rennie from Redline Bimmer Performance told us to allow the car to fall asleep. That takes about 16 minutes. 

Then we placed a test light between the negative battery lead and the negative battery terminal. It lit up. 

Okay, we had a draw. But how much? The car has some electrical draw when sleeping, right, so did we really have a problem? 

First step: How much was the draw actually hurting us? 

So we measured the battery’s voltage every day. The car was not plugged in or driven during this test. We checked the battery voltage every evening. 

Start: 12.8 volts
Day 1: 12.60 volts
Day 2: 12.45 volts
Day 3: 12.34 volts
Day 4: 12.17 volts
Day 5: 11.98 volts
Day 6: 11.86 volts
Day 7: 11.62 volts

At this point, it was obvious that we had a problem. 

One thing we knew: The car wasn’t doing anything weird or obvious. When the car was parked and sitting, we didn’t notice any fans running, lights turning on, or the like. Even the trunk and interior lights seemed to behave. The HVAC wasn't acting up, and our car doesn’t have any aftermarket electronics. 

The (all-knowing) internet gave us a few clues. A popular one: just replace the "hedgehog," also known as the final stage resistor, as it’s a likely culprit. BimmerWorld has them starting at about $35.

But we’d rather be more methodical instead of just throwing parts at a problem. Plus, for what it's worth, we knew that the previous owner had replaced that part. And our HVAC didn’t display any issues.

But we still noted the fuse that controls the final stage resistor: 50.

We also cruised the internet for other likely suspects and made a list of fuses often associated with battery drains: 63, 50, 37, 41, 62, 49, 52, 60, 57, 7, 41 and 67.

Now the fun part: Testing each circuit.

We could have continued to the test light but small issue: Even when the engine is off and the car is sleeping, the M3 still draws some current. According to Rennie, it’s about 50 milliamps. 

So we need to test with the multimeter.

We disconnected the battery’s negative lead and placed the multimeter between the lead and the battery's negative post. Then we set the multimeter to measure DC current. We saw about 2.7 amps of draw. 

Then, one by one, removed each fuse on our list. 

Did the current draw change? If not, we reinserted the fuse and went to the next one.

We started with fuse 63. No change as we still saw 2.7 amps. 

So onto fuse 50. No change.

And so on. 

Yeah, we were methodical. 

We saw some minor variations but nothing that caused alarm–until we got to fuses 49 and 52. Pulling each of these dropped our draw from about 2.7 amps to about 40 milliamps. 

A clue!

According to the fuse chart found in the glove box, both of those fuses are associated with the central locking system. Some internet sleuthing helped narrow it down even further:

Fuse 49: anti-theft, central locking, interior light, windshield washer system, front windows.

Fuse 52: central locking, glove compartment light, hand lamp, interior lights, passenger compartment/trunk lighting, windshield washer system. 

After checking all of the circuits on our list, we circled back to fuses 49 and 52. We started by checking out the easy items on the list of systems tied to those two fuses. 

The interior, glove box and trunk lights all turned off when the car went to sleep. 

The windows seemed to operate normally.

We couldn’t find any issue with the headlight washers.

Our trunk lock actuator had always seemed a bit flakey, though. Sometimes it didn’t seem eager to unlock.

We unplugged the actuator and tested the draw. No change. Bummer. 

So we checked the rest of the central locking system. 

The doors seemed to lock and unlock on cue. 

But then we noticed something: Our fuel door wasn’t locking and unlocking. While the rest of the car locks close with authority, our fuel door actuator sat silently. 

We pulled back the trunk interior lining so we could unplug the actuator. 

When we did so, the fuel door actuator's little plastic sleeve (BMW part No. 51258151536) that fits inside the rubber boot (BMW part No. 51258151799) popped loose and landed on the ground. Doink. 

We measured the draw on the battery: The draw had dropped to about 75 milliamps. 

We plugged in the actuator, and it worked–well, kind of. Now we could hear it move, but it still didn’t lock the door. 

Was it jammed? Was it just unhappy? It wouldn’t answer our questions, so we left the actuator unplugged and again checked battery voltage over a few days. 

Start: 12.90 volts
Day 1: 12.89 volts
Day 2: 12.88 volts
Day 3: 12.88 volts
Day 4: 12.87 volts
Day 5: 12.86 volts

So still some drain, but not nearly what we saw before. We continued to leave the car off the charger just to see how it would do. We also ordered a new fuel door actuator because we’re not savages. 

Mission accomplished?

Well, not so quickly. 

A few days later, the M3 greeted us with a dead battery.

Womp, womp.

Battery issues? According to the paperwork that came with the car, ours was pushing six years old. 

So we took the car to the local auto parts story for a test. Their tester said that we had a bad cell.

At this point, we figured that maybe we should replace the battery–solid foundation and all that. 

The parts store replacement batteries ranged from $149 to $199.

Could we do better? Maybe.

So we pinged BimmerWorld’s James Clay, our hand-holder with this project. Plus, doesn’t he fly a particular battery company’s logo? 

James recommended an Optima battery yet, at the same time, the company's site doesn't list our application. 

We want the Optima H6 model, he said, and BimmerWorld sells it for $294.99.

What's so special about the Optima?

Unlike some of the batteries offered across the local parts counter, the Optima is an absorbed glass mat battery. “It’s a more effective approach than a traditional flooded lead-acid battery,” he notes. “The lead is in constant contact with the acid component.”

All Optima batteries are 100% virgin lead–that is a big thing,” he continues. “Not recycled–we do like recycling, and Optima batteries can be recycled, because recycled lead is pretty good–but did you want pretty good or pure?”

His last selling point: “Finally, and I swear this is huge: Optima batteries have cast-on straps. I've been through the plant and seen the operation–it's wild. Any other battery has welded straps–like internal wires to route the power. 

An Optima has a cast lead bus bar–thick and beefy. But more than that, instead of being welded onto the plate that is in the acid of a normal battery, they actually flip the battery upside-down late in the manufacturing process into a mold and then pour the molten lead in. It forms the straps and melts into the AGM plates or cells–depending on model–so there is no weld to fail. 

This is the common failure point of a battery and it is just totally eliminated with this manufacturing process. That's what allows an Optima to get beaten to death like we do on the track without failing.”

The Optima also offers more cold cranking amps: 800 vs. 770 for the $199 option. 

James recommends the Optima H6 for our M3 as well as several other BMW models. It's smaller than the stock H7/94R battery but, he notes, the car’s battery mount is drilled for both sizes. The Optima has the posts in the correct spots, too. It also has the proper vent.

The Optima weighs more that the batteries back at the parts store, though: 54 pounds vs. 42 to 47 pounds.

The Optima is heavier “because they pack it more efficiently and it has more lead than the comparable battery of the same size,” James notes, “so it's more effective.”

Lighter alternatives exist. “We have U.S. group 35 and 51 batteries, and the mounting kits to make them a direct fit in BMWs,” James continues. “But we typically use these where weight is more of an issue than durable street performance. 

For my race car, a D51R Yellowtop is my go-to: light and durable for racing use. But in my street car, where the added 25 pounds of lead and other materials isn't going to kill me and I want a long life on the street with low hassle, the Optima H6 is the easy toss-it-in-and-forget-it option.”

One last caution from James: “And once we get rid of the whole strap weld failure, your new weak spot of the battery is oxidation. If the car is going to sit, especially with a draw from an issue or just new car stuff, like an E9X, it needs a maintainer.”

Since our M3 can sit for weeks at a time, we ordered an Optima Digital 400 Performance Maintainer and Charger as well. (True story: Someone might have recently done something bad to one of our CTEK maintainers.)

The Optima maintainer features beefy clips, pulse conditioning and an actual screen that shares actual info. 

Battery installation is simply reverse of removal—just don’t forget to reattach the vent tube. Super bonus: We found a 10mm wrench down there. 

Bonus: Watch these videos for talk about battery mounting and care. 



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View comments on the GRM forums
Colin Wood
Colin Wood Associate Editor
5/18/21 3:06 p.m.

Came for the project car update, stayed for the Metallica.

David S. Wallens
David S. Wallens Editorial Director
5/18/21 3:08 p.m.

In reply to Colin Wood :

Thanks. Always try to over-deliver. 

bluej (Forum Supporter)
bluej (Forum Supporter) PowerDork
5/18/21 7:08 p.m.

Timely for me, nice!

David S. Wallens
David S. Wallens Editorial Director
5/18/21 7:26 p.m.
bluej (Forum Supporter) said:

Timely for me, nice!

Thanks. It's basically the article that I wish I could have found. So I wrote it. 

This process will work for any car. Have a drain? Here's how to (hopefully) track it down. 

Bonus Metallica, too. 

Jerry From LA
Jerry From LA SuperDork
5/19/21 11:08 a.m.

New cars can take about an hour before they go into "sleep" mode so wait a bit to obtain a solid result.

CAinCA GRM+ Memberand Reader
5/19/21 11:30 a.m.

I had an 84 Mustang 2.3L that was draining the battery on a regular basis. I put my meter in line with the positive cable and could see that there was a couple amp draw even when the car was off. I noticed that every time I made the connection from the battery to the positive cable that I could hear a click under the dash. I made a remote switch out of a length of speaker wire and climbed under the dash. I kept making and breaking the connection until I finally felt a relay click with my fingers. I called the dealer and found out it was for the ignition system. That was the first time I had ever heard of something failing "on" instead of "off". 

LopRacer Dork
5/19/21 12:59 p.m.

I agree this is the article you need when you have a draw. Just finished playing this drain game with my Cr-v and behold I had mis-wired my new driving light relay to be energized off and de-energized on.  (swapped the 87 and 87a pin locations in my pre-wired relay connector.) The coil of the relay was enough of a draw to bring down my old battery over 5-7 days. I also lost a cell in the battery along the way, so new battery and now I have an acceptable 500mA draw but wish it were closer to 200mA. My main draw now is the radio/acc circuit, which is to be expected.

David S. Wallens
David S. Wallens Editorial Director
5/19/21 1:51 p.m.

My hope was to get through this without having to replace the battery, hence the womp, womp when it died. Both Carl and James said the same thing: Get the battery tested, but don't be surprised if you need to replace it. So at least I went in prepared for the worst. But, hey, we all got an article out of it. 

noddaz GRM+ Memberand UberDork
5/19/21 2:12 p.m.

Great!  But can you leave the car un-attended for several days without a battery maintainer to keep the battery charged?

And I cannot believe that no one has replied "I wondered where I left that wrench".

Scott (Who needs to go back outside and finish putting an axle in his Jetta)

David S. Wallens
David S. Wallens Editorial Director
5/19/21 2:33 p.m.

The new battery just went into the car, but after Amelia I'll chart the battery over a week or so. 

noddaz GRM+ Memberand UberDork
5/19/21 3:03 p.m.

Good enough.  

And the axle is done.


David S. Wallens
David S. Wallens Editorial Director
5/20/21 3:00 p.m.

Considering the drain has greatly been minimized, I have good hopes. 

GCrites80s HalfDork
7/11/21 7:56 p.m.

Any drain news?


David S. Wallens
David S. Wallens Editorial Director
7/12/21 2:42 p.m.
GCrites80s said:

Any drain news?


Sadly, no, no news on the drain. I haven't had a chance to let the car sit for a week or two. (I need to move the car in order to get out the lawnmower or get up into the attic.)

GCrites80s HalfDork
7/13/21 7:59 p.m.

Another reason I'm curious is that I have a 0.01v per day drain (as of day 4) but the battery is 6 years old. I did the little load test that multimeters come with but I don't know if that strains a car battery enough. It tested good (green). I need to get around to doing the amperage draw test but I have 2 other projects going on.

David S. Wallens
David S. Wallens Editorial Director
7/15/21 12:34 p.m.

I had a few friends who know more than me suggest the test at the parts store. So that's what I did, and their tester said that we needed a new battery. It was a few years old. 

GCrites80s HalfDork
7/15/21 1:52 p.m.

Yeah I'm used to seeing the store lights dim when they test batteries and alternators at the FLAPS laugh

GCrites80s HalfDork
8/13/21 8:15 p.m.

I got a new battery, which I needed. The old one was even older than I expected at 2014. After that:


Here's a 6-day breakdown



12.51 (I got in and out of the car a few times after the previous reading)

12.55 (notice it went UP)



Still lighting up the test light. So I gave it the amp draw test... 0.01 mA. Sometimes 0.00. Is something intermittent happening while the car sits? With the stock fuel pump the pump used to cycle every once in a while for a few seconds as the car sat. It was fairly loud when it did it too. Fuel pump went bad and was replaced with a Walbro 8 years ago. I have never heard it cycle since even with spending hours in the garage. Now the horn button is a little sketchy and corroded which makes the horn only buzz weakly when you hit it. I am swapping steering wheels hopefully this weekend so that sketchy connection will be out of the loop.

Anyway, is the battery draining by .01-.02v a day when it's 90+ degrees out really that big of a deal? It may not be.

GCrites80s HalfDork
8/15/21 8:39 p.m.

12.50 today after skipping a day.

Note that this radio's memory only keeps track of clock time and what song you were on. All the rest of the memory functions are burned to flash memory which is demonstrated by the fact that the car sat for 2 weeks with the fusible link blown and one week with the battery disconnected. All other settings were not forgotten such as tone shaping, Bluetooth syncing, balance/fader, speaker size and levels. I'd Imagine a lot of head units made in the past 5-10 years also use flash memory and aren't real drains.

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