A good battery will sit at 12.6 volt roughly, as soon as ignition switched on thers voltage drop. so 12volt aint enuff , that will drop to 11.5v right away,, when recharging it will go up to 13+volts,, whitch is good,,it will settle back to the 12.5 area,
Here . this will do yeh,,, mines is model up about £18.00,
http://www.ebay.co.uk/itm/DIY-DIGITAL-LCD-MULTIMETER-DC-OHM-VOLT-AC-BATTERY-OPERATED-ELECTRIC-METER-13190-/320595309882?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item4aa4f8353a