Found a good looking add on volt / amp meter
 

Not sure how accurate but good enough to monitor the charging system on a car. The meter overall dims are 1 inch x 2 inches so the display will be kind of small but good enough. At some point I want to add one to my battery charger.

If you got another shunt and a selector switch, you could monitor glow plug current and get an instant indication that one is burned out. ( If the GP are in parallel )

$ 18.00 now, on sale for an unknown period of time $ 13 . They have lots of other meters and much more in the way of electronic goodies.

https://www.mpja.com/Mini-Panel-Meter-Dual-Red-LED-Display-30-Volts-100-Amps-DC/productinfo/32063+ME

Just a warning...they don't like add-on meters here. Sorry.

https://www.peachparts.com/shopforum/tech-help/379019-every-single-one-you-needs.html

Oh boy! Marlin P Jones is still out there kicking! Almost 30 years ago their operation was once located right next door to a warehouse condo I owned in Riviera Beach. Great people and a great operation, they always had some of the coolest stuff if someone is a tinkerer or an inventor type. I'm still using a tube of Tef-Gel I bought to use on aluminum boat stuff all those years ago. Small world.

that MP Jones meter is certainly cheap! I would not hesitate to put it in a motor home or HUMVEE or some military vehicle but to me its a bit too much for a Mercedes at least any that I drive. I do like the idea of monitoring GP current though!
That said, I use something to monitor battery voltage that seems to be NLA, at least I couldn't find any after a thorough search.
It is a small device with a couple of cigarette lighter outlets that is hardwired to 12V (I connected in parallel with the tilt-out lighter socket in the W210 car.

Of note is the feature it has where three small LED's indicate the state of charge: green, yellow and red.
These indicate analog values and are a quick indicator of whether your battery voltage is changing or low. A lot easier then reading a seven segment LED or LCD display.
And I could have used one in my '87 300D recently when my alternator started going out, I never saw the dash light come on but gradually voltage would drop to the point the car would be hard to start then a short trip and it died on me.
I attribute the early death of the 3 y.o. Bosch battery to a possible intermittent brush contacts. I am going to pull the alternator and will upgrade to a 115A one and then I'll take the regulator out and examine it to see if my theory is correct.

I mounted my nice charge indicating cig lighter outlet on the side of the console near the radio, where a laptop charger could be plugged in as well.
(Pics to follow)

Some newer cig outlets even have a USB port. One I saw featured a bright blue LED that lit up for whatever reason - this is not what I recommend, driving at night with a bright blue light in my face is not my cup o' tea!

There is something about the European cig lighters in Mercedes that leaves me hating them because I have seen them not make good contact to available cigarette lighter plugs.
Anyone else had that problem?
DDH

MPJ has some volt meters that plug into the lighter socket, I didn't look for the type you describe.

Also, mid / late 90's MB have a function to look at HVAC data that includes battery voltage. With the HVAC blower running and key on, press and hold " REST " , the display will change to a " 01 " then flash a value. Use the fan " + " and " - " button to scroll through the data. I think battery voltage is 06 or at least in that area. Voltage will be a few tenths below actual battery due to voltage drop through the wiring but it is close enough.

The do sell an analog / digital meter. Yep,a light or analog display is better in getting your attention and digital for an actual reading.


The euro and some newer domestics use a shallow socket, use an older plug with the ground contact strap pretty high up and it will fall out. I remember seeing an adapter to cure this. Newer lighter plugs have a longer ground strap to fix the problem.

Yep, there are still a few electronic surplus / goodie stores around. Have a look at Sciplus sciplus.com for goodies. Have a look at the old hand drawn product pics, I expect them to go away at some point.