Have Herbie's batteries failed before their time? Well according to my research they have done almost OK. Various sources quote various cycle lives for domestic (leisure) wet cell batteries, but something around 200 t0 250 charge/discharge cycles seems to be the norm. Ours were bought in November 2007 and have probably now done over 200.
What makes batteries last less long is a) discharging them below 50% - something we have been careful never to do, and b) not getting them fully 100% charged each time - something that inevitably happens when you are relying on the alternator.
Tony Brooks's course notes imply that under these conditions batteries will lose about 20% of their capacity each year, so what started off as a 330amp hr battery bank is probably now down to only just over half that. Then if we still aim not to discharge more than 50%, we have to half the number again. So in we reality the we would expect only have roughly 80 amp hours per day available.
I think ours have rather less but not all that much less, so they have done a bit worse than you might expect but not outrageously so.
Anyway it seems that they are near the end of their life and so we need some new ones. Should I get cheap and cheerful (as last time) and accept they will only last two and a half years, or pay more and get other types with a claimed longer life? I've read too many stories of people being disappointed with expensive batteries to go that way, but I have found some that are only a bit dearer than cheapos that might be significantly better.
Numax, the maker of our current (excuse the pun!) batteries still sells them at about £80, but they have a new type CXV 113 amp hr battery which has the guarantee extended to 3 years and an "expected service life of 500+cycles" (pinch of salt required here I suspect) rather than 250 or so. These batteries can be got for about £95 each, so the extra £15 per battery seems to be worth the punt.
So I think three of them are next on my shopping list.