While out for a Solent day sail a few days ago, later in the day I tried to start the diesel engine using the Domestic battery. This usually works fine if there has not been too much drain on the battery. But, no joy. There was enough power for the instruments but not enough power to even make one turn on the engine. So I switched to the engine start battery and everything was fine. I left the battery on the solar panel for a few days to recover. Today the investigation revealed: Battery water level all OK, specific gravity of each cell OK for full charge, Battery voltage indicates full charge, green indicator says battery OK, age of battery is 4 years 5 months. Battery type was a budget marine leisure type, 105 Amp hr, typically sold in Chandleries for £100, wet cells. A search on line say these batteries typically can last 200 discharge cycles. I sail at least 50 days a year so I now suspect the battery is end of life. I take the battery to Ecobat in Fareham, they test it with a special battery tester and tell me it is end of life, limited capacity capability remaining. I walk out with a new 125 Amp hr AGM battery and a rather a lot of pounds lighter in my wallet. This new battery weighs 31Kgs! The old one was 23kgs. The yacht now has a noticeable slight lean to starboard. Note to self to move the dinghy and outboard to the port side of yacht, or better still leave it at home.