Normal Brake Lights Voltage Drop

[Philo]

What would I expect to see as a normal drop in voltage when stepping on the brake pedal ? I do have the brake light upgrade kit installed that a member was selling years back so I basically have four rear lamps and the center light in the rear window. The bulbs in there are P21W, so I guess five total if the center rear window light uses the same bulb.

On my super cool ShawnD gauge set I'm seeing almost a full one volt drop when I hit the pedal. The drop happens only for half a second and then climbs back to 13.8V ish.. Is this normal ? I'm also getting voltage drops when the RPM drops a bit low, an issue with the S54 that many many owners complain about online. When lifting the RPM drops below the idle target to around 600 RPM. The target is 850 RPM.

Anyway, just wondering if the voltage should drop and then rebound when hitting the brakes.

Thx.

[Mike W.]

I'm surprised you don't remember this, it used to get talked about quite a bit. It's common.

What's going on isn't the car or the battery or the alt losing voltage, it's the brake/radio circuit. After going thru the mickey mouse 8A GBC fuse, thru a small wire and with 4 brake lamps, which is nominally about 7 amps, running, there's a substantial voltage drop. Actually probably not so much while you're foot is on the brake as the initial surge thru the bulbs, the exact number I don't know, but it's a whole lot more, very briefly, until the filament heats up, you get a big drop. Stock M5s I believe have the premium sound system with an amp in back which is a further draw.

There's a bunch of ways around it, first at least check and clean the fuse and fuseholder. That might help, maybe. I'd grab a big capacitor and put a diode in front of it just before the radio, but I doubt many would do that. Some have used that little 2 fuse aux fusebox, one side of which is ignition fused IIRC. You could pick up power from something else that's ignition acc switched, or even just off an ignition powered circuit, but then you'd have to have the ignition on, not in Acc to listen without it running.

But it's not just you, it's common.

[Galahad]

... Actually probably not so much while you're foot is on the brake as the initial surge thru the bulbs, the exact number I don't know, but it's a whole lot more, very briefly, until the filament heats up, you get a big drop. ...

How fun I get to do some math!

Let's ignore all the following: internal resistance of the battery, resistance of the fuse, contact resistances at all the different connectors, the wire being old and having more resistance than when it was new

Incandescent bulbs are something like inrush ~ 5x operating according to the data sheet I'm looking at. If you have 5x 21w bulbs, each is about 1.5A, so you're pulling ~35A briefly when you hit the brakes. Half second to come up to temperature sounds about right.

ETM says a 1mm^2 wire from the fuse box to the check relay in the back, that's the same as 17AWG, which is about 5.5 ohms/1000ft. The wire is probably 10', so we're looking at 55mOhm if the wire is good.

At 35A you're dropping about 2V just in the wire, so the brake lights are only seeing 11.8V.

That doesn't actually address the voltage drop you're seeing since the meter is almost certainly looking at something very close to the battery, so let's look at the battery internal resistance, going for very rough numbers here.

Assume a car battery is actually 12V, CCA is the number you get when you short a car battery with a wrench, and a wrench has no resistance (I measured a 18mm wrench at ~3mOhm so that's close enough).

One of my car batteries is rated at 600CCA, at 12V that gives an internal battery resistance of 20mOhm. That's a minimum value - short current is going to be higher than CCA, as is the initial battery voltage. I googled it, and some guy on the internet making a combat robot said his LiPOs were about 20mOhm so it seems like my estimate is in the right order of magnitude.

20mOhm at 35A is a drop of 0.7V, so the 1V drop you're seeing sounds pretty reasonable.

Alternators don't make anywhere near the rated current at idle, they're not spinning fast enough. Not surprising the alternator can't compensate for the inrush load.

I'd get one of those giant capacitors people use when they have too much sound system, that would probably fix the 1V drop at the battery. Nothing to do about the drop at the lights other than rewire the entire harness.

[Mike W.]

... Actually probably not so much while you're foot is on the brake as the initial surge thru the bulbs, the exact number I don't know, but it's a whole lot more, very briefly, until the filament heats up, you get a big drop. ...

How fun I get to do some math!

Let's ignore all the following: internal resistance of the battery, resistance of the fuse, contact resistances at all the different connectors, the wire being old and having more resistance than when it was new

Incandescent bulbs are something like inrush ~ 5x operating according to the data sheet I'm looking at. If you have 5x 21w bulbs, each is about 1.5A, so you're pulling ~35A briefly when you hit the brakes. Half second to come up to temperature sounds about right.

Very good, I'm impressed, you're much more of an EE than I am.

However... I'm seeing values of ~12X for inrush current doing a quick search. Plus 1.75A nominal for each 21W lamp which is almost like a sixth lamp compared to 1.5A. Now this is all using 12V instead of the nominal 14V that the alt puts out and 12V lamps are often rated at 13-13.5V. But the distance between the battery and the brake lamps would actually work to the advantage of the radio since it's much closer. But if we use that 12X multiplier and 1.75A per lamp, we now have an inrush of 105A, but probably less as the wire would act as a resistor with that much current.

But, the voltage at the lamps is not the real factor or problem, it's the voltage at the stereo. While rated at 12V, my guess is it would keep working down to somewhere around 6-8 volts, suggesting a much larger voltage drop.

Long ago when I had an E3 Bavaria I couldn't get rid of alternator noise on the stereo. I finally got a I dunno, maybe 10K cap, put it in next to the stereo and that fixed it, no more noise. Which is what I'd do in this case, next to the stereo, who cares about momentary drop on the lights, with maybe a diode in there before it so it wouldn't be trying to power the lights, just the stereo. Something like this, https://www.amazon.com/BOJACK-25Voltage ... 291&sr=8-6

[Galahad]

Very good, I'm impressed, you're much more of an EE than I am.

However... I'm seeing values of ~12X for inrush current doing a quick search. Plus 1.75A nominal for each 21W lamp which is almost like a sixth lamp compared to 1.5A. Now this is all using 12V instead of the nominal 14V that the alt puts out and 12V lamps are often rated at 13-13.5V. But the distance between the battery and the brake lamps would actually work to the advantage of the radio since it's much closer. But if we use that 12X multiplier and 1.75A per lamp, we now have an inrush of 105A, but probably less as the wire would act as a resistor with that much current.

But, the voltage at the lamps is not the real factor or problem, it's the voltage at the stereo. While rated at 12V, my guess is it would keep working down to somewhere around 6-8 volts, suggesting a much larger voltage drop.

Long ago when I had an E3 Bavaria I couldn't get rid of alternator noise on the stereo. I finally got a I dunno, maybe 10K cap, put it in next to the stereo and that fixed it, no more noise. Which is what I'd do in this case, next to the stereo, who cares about momentary drop on the lights, with maybe a diode in there before it so it wouldn't be trying to power the lights, just the stereo. Something like this [25V 2200uF Electrolytic]

Having multiple EE degrees makes it hard to turn off, haha.

Oh it's quite possible the inrush is much higher than I quoted - I tried to be pretty conservative with my numbers and still demonstrate the problem. Working with 12x, 1.75A, 13.5V, the bulbs cold are ~0.62 Ohm. 5 in series would be 0.125 Ohm for the system. The bulbs alone would pull ~105A, but the wire is half the resistance of the bulbs so the system minimum is 0.180 Ohm, bringing it down to 75A. Still way too much even for the torpedo fuses, but there's more resistance other places. Lower current will just make the bulbs take longer to warm up (and put more heat into whatever connections are corroded).

If the voltage at the stereo is the only issue we're trying to address, a cap and a diode would solve the problem. Downside of that approach is everything in the car sees a dip in voltage, not just the stereo. It probably doesn't hurt anything, but you'd hate for something to degrade a bit in the ECU (solder joints?) and suddenly have your engine cut when you hit the brakes.