Discovered the other day that our boat's battery voltmeter reads about 0.2V high, at least compared with my (mid- $$ range) DMM. Got me thinking, how can I easily check the DMM's accuracy? Need accuracy of at least 0,05V for it to be any good the check battery state of charge.
Most batteries' open circuit voltages depend on state of charge (except mercury ones, but they're a bit hard to get these days!)
I can construct a voltage regulator (say 10V) circuit running off the boat's 12VDC, which are quite accurate, but before I do, any bright ideas on something simpler?