Cig lighter or built-in, is not too important.
In single-digit meter this digit is rounded to either higher or lower voltage, and you don't know where. Not to mention that this digit can already be inaccurate.
Cheap meters from Amazon are amazing things. I hooked up one to very stable DC power supply and it was changing the reading on the go: 12.1, 12.15, 12.2 - while the circuit was warming up. Waited 20 minutes, it never stopped. Disconnected for a few minutes and reconnected, - it started from a different voltage this time.
DROK is a little more reliable, - mine was some other brand.
