This is a Casing for a 12V Voltmeter of the Onboard-Battery in our Mia Electric car. https://de.wikipedia.org/wiki/Mia_(PKW-Modell

The used Voltmeter is from CPTDCL. Multifunctional LCD Power Meter for Lead Acid Batteries, Voltage Meter with Temperature Display, Battery Display, Voltage Display, 12V

There is one version with a holder of a loading Connector Adaptor for an Ctek Battery charger which is connected to the 12v car battery. Ctek CTE-56260 Ctek Direct Connector Adaptor (6.3mm)