Modern Vespa

Was never happy having to remove the battery cover to check out the battery voltage, when engine was running and not.
I figured out where to pop a couple of 3/8" dia. holes then pressed in two wooden plugs from my junk box. Works just fine.

Or try one of these?

https://www.amazon.co.uk/d/gqf/Jazooli-Digital-Car-Cigarette-Voltmeter-Voltage-Battery/B00G0HIXJ2/ref=pd_lpo_107_tr_t_2?_encoding=UTF8&psc=1&refRID=1BPZPCDMVGX273GX4Q9F

why not just wire in a battery tender lean and have that to the outside of the cover. then you can check voltage at that any time.

I guess that's one way of doing it???
I have one of these on my MP3/250 and GTS 300 Sport.

The Heads Up Voltage Monitor contains a micro-processor that monitors the output of the charging system and is accurate to within 200 milli-volts. The Heads Up Voltage Monitor has a onboard microprocessor that performs a self-test each time voltage is applied. The single, multi-color LED (red-amber-green) visually indicates the state of your motorcycle's battery / charging system. At a glance, you will instantly know whether the voltage is normal or abnormal. Additionally, you will be notified of serious fault conditions by an attention-getting flash of the LED.

Features
Single Mult-Color LED visual indicator to mount on dash
Precision microprocessor monitoring
Automatic self-testing sequence at motorcycle start up
Auto-dimming - after 8 seconds, LED intensity is dimmed to 50%
Easy installation - can be mounted virtually anywhere
Label Included for referencing voltage colors
Voltage sampling: 50,000 times per second
Serious fault detection & notification

Voltage Table
Flashing Green Above - 15.2V
Steady Green 12.9V - 15.1V
Steady Amber 12.7V - 12.8V
Steady Red 12.1V - 12.6V
Flashing Red Below - 12V

Vendor Link: http://www.signaldynamics.com/heads-up-voltage-monitor/

It saved me recently when I had a warning about my VR that went bad on my MP3/250.

My Youtube Video after Install on MP3/250 some time back.
https://youtu.be/xa5fANUdczo

I do enjoy having the Voltage Readout on the MP3/500 Sport.

Keith,
Marietta, GA