 An ESR meter known not to have diode protection will give a reading of internal resistance for a rechargeable or non-rechargeable battery of any size down to the smallest button cells which gives an indication of the state of charge.To use it, measurements on fully charged and fully discharged batteries of the same type can be used to determine resistances associated with those states.The cost of an ESR meter makes it uneconomic for measuring battery voltages as its only function, but a meter used for cheeking capacitors can take on the additional duty.