Yes.
There are tables available for lead-acid battery voltage versus state-of-charge. They're pretty accurate if you use them correctly, but several things can foul them up.
- The main one is for use when the battery is unloaded or lightly loaded (and has had at least a few minutes of light load after the last charging to remove "surface charge").
- A second one is available for use during moderate charging.
Expanded scale voltmeters are available for hanging across a floating wet lead-acid battery bank either continuously or with a pushbutton. They have one section showing approximate state of charge when the battery is discharging, one when it's charging. Available at boating and trailer supply stores.
Tables and expanded-scale voltmeters will give inaccurate readings if:
- The battery temperature is significantly above or below room temperature (correction factors are available).
- The battery is charging and sulfated.
- The battery has BEEN charging but isn't charging now - it will have a "surface charge" until it has been moderately loaded for a few minutes.
- The battery is under significant load.
- The battery has within the last few minutes been under extreme load (i.e. cranking) and is now under even minor load (it will be "polarized" - have a layer of depleted material near the plate, and this makes even minor loads make it read low).
Hydrometers are always right, but a pain. An expanded scale voltmeter (and a table or state-of-charge markings) is a good device for a quick check.
