your services fail really quickly and tests on the 1/2 that you're testing reveal nothing then you've identified 2 good batteries. Then switch to the good batteries and test the other pair independently. Thanks for both of these replies. I'm not a sparky and must admit I'd not considered either splitting the bank or using a hydrometer. I'm pretty sure I could borrow one. What range of reading would suggest a problem and what reading would suggest a good battery as opposed to the other sort? I'm hoping to identify no more than 2 duffers, because I can use the 2 starter ones that I know are okay. I'd do this because 1/ I won't need the starter batteries 'til spring and 2/ Although my engine is industrial it is still only 2250cc, so I don't need twin 110AH batteries either You need to separate the 2 halves of the house battery. As said use just one half after charging. Then use the other half. It will become obvious that one half is no good. (unless they are both knackered. When on discharge if you find that one 12v battery in the series is a lot lower voltage than the other then you have identified the actual dead battery. This test for the house batteries will test in exactly the same way as you use them. ie time to discharge at a known current.ie the inverter and imersion heater. So this is a direct indication of their usefulness to do