I recently bought a volt meter/usb that plugs into cig lighter in my SE and it got me thinking: How reliable is the battery warning light in the instrument cluster in case I lose charge for whatever reason?

I've read of people who had a bad Regulator and yet the light didn't come on and their engine stalled (Gas not diesel).

It's worthless .

Good deal on the charging indicator, I have a few different types, the newest is smaller and has two USB ports for the cell 'phone and the Foster boys game when we travel plus it hsd a numeric display and color changes & flashes to announce problems ....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

If the brushes wear away so they no longer contact the slip rings, the light won't light up and the battery won't charge. If the regulator just takes a dump for whatever reason, the light will complete a ground path through the regulator and illuminate. It's largely meaningless.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The purpose of the light is always misunderstood. It's not really there to detect any and all alternator failures. The real reason for the indicator light is to let you know the key is in the on position, but the engine not turning. That's the historical purpose, believe it or not. I permanently installed a voltmeter in my 190d:

The battery light is very useful. If it is on, the alternator circuit is not working. If it flickers, the brush(es) is worn out. If it is off the alternator circuit is working but it doesn't tell you the alternator may be overcharging the battery. However, you can't expect a simple on off light to tell you everything. It is on every car, don't know about EV, so it is there for good reasons.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I agree that it is useful. If you turn your key and you don't see the light you know a charging issues is likely going to happen or the bulb is burned out.

If you see the light while you are driving it is time to investigate.

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84 300D, 82 Volvo 244Gl Diesel

This looks like something that might be useful to me.
If you're willing, could you please elaborate on the part(s) that you installed and how you did the wiring? Thanks-

Very nice Frank .

The deal with the charge lamp is : your alternator can fail and it won't tell you so you drive on the mechanical fuel injection then the next time you try to start it, nothing .

BT, DT no fun, good thing i wasn't 75 miles down a bad Desert road with no cell service .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I've lost charge in a variety of diesel and gasoline Mercedes without so much as a flicker from the light.

I have a cigarette lighter phone charger that has a voltmeter display if I want to check charging.

This really isn't a big deal. Any generic voltmeter with a suitable range will do the job. Drill a suitable hole and mount it in the dash. The one in the photo came from a direct-from-China seller on Ebay. This is a similar example:

https://www.ebay.com/itm/LED-Digital-Display-Voltmeter-Car-Motorcycle-Voltage-Volt-Gauge-Panel-Meter-12V/382624102408?hash=item59162c7808:m:mjOteNyJ32wi3AcLHzv4Mzw

A voltmeter has two wires: one goes to any ground, the other goes to any switched 12V source, I think I used the lighter wire for this installation. If you want to be anal, you could run wires all the way to the battery, but it's not really necessary.

As far as reading it. A fully charged battery reads 12.5V. Above 13V means the alternator is charging. Bosch alternators are usually regulated to 14.25V @70F. They will vary from 13.5V to 15V depending on operating temperature. If the reading drops below 12.5V, it means the battery is discharging. There are extreme conditions when this may be ok: idling for a time with the headlights, wiper and heater on, for example. So much more information that you get from an idiot light.

Truly, I sincerely appreciate your willingness to share not only the details of the kind of part and wiring, but also the proper way to read the information.

This will be a nice project for me. Thank you!

Thanks everyone for your responses.

I wonder how hard it would be to either buy or make an electronic circuit that would trigger a buzzer or a warning light when voltage drops below 13V ?

would work better than a voltmeter because you wouldn't have to look down and you would know instantly.

If you are computer saavy I see a lot of neat stuff done with Arduino.

I saw somewhere online a W124 Brabus multi gauge setup with its display in the outside temperature spot in the cluster, and controls built into a modified headlight switch bezel. I think that was a neat design, not a fan of gauges that look out of place.

Thanx Frank ;

I don't see what size hole it requires....

The colors thing is nice but, blue isn't so easy to read at night when there's ambient light around...

BT, DT and wasn't pleased .

Too bad they don't make this round one in 6 ~ 24 volts, it'd be very handy for vintage vehicles and ex military with 24 VDC systems .

-Nate

Any opinions about the usefulness of a display that in addition to current (voltage), also displays amperage?

Thanks-