I still use a couple of the older models, as well as a rather dearer CTEK MXS5 though mine was considerably cheaper than this link
Along side these I was recently recommended an Ancel BM200 battery monitor.
It’s small, cheap and connects via Bluetooth to your phone.
Suitably impressed, I bought a couple more, one for the Marlin and another for Jane’s KA, which keeps on suffering from a flat battery, die to lots of short shopping trips.
The Disco now has the larger BM200 Pro version, which in some ways is more limited, but more convenient.
That has a separate wireless display, with USB-C and solar panel charging.
Neither the KA or the Marlin have anywhere to fit the remote display.

N.B. shop around as the prices for the Ancel monitors vary hugely
Bill