Voltage alone only gives a ball park figure as to battery state of charge, and can be very misleading if large loads or large charging sources were recently employed
Amps flowing at voltage gives a much better idea.
When one can see AH removed from what hopefully was a brimming full battery, one can have confidence, and notice when battery performance degrades and failure will come as no surprise.
I make use of these RC watt meters:
They have their limitations, like only counting to 64AH, and only counting current
in one direction with a load and source side, and their 12awg aluminum
leads might be super flexible, but I would not pass more than 20 amps through them continuously.
I replaced the 12awg with 8awg, and use 45 amp anderson powerpoles on them, and will pass 40 amps through it for an hour with minimal heating
They are not 100% accurate and currents under 0.8 amps become progressively less accurate, but on one of the 3 I have used it was accurate does to 0.01 amps.
If there is interest I can elaborate more on these inexpensive tools which are so much better than just a voltmeter.