How many batteries for best power?

摘要

Q our batteries keep running flat. The lights have been very dim at night, we've given up running the fridge and, recently, there hasn't even been enough charge to run the water pump. The batteries are very old - way past their use-by date, but we have only three (including one for engine starting) and i wonder whether we are underpowered. My friend's boat, which is bigger but has the same electric draw, has five batteries. Having said that, the three i have exactly fit the battery box.