The regulators in the alternator can be a bit leaky but I've left mine standing for months and it still started - and that was the cheapest battery I could fit. Mine's a petrol with bugger all compression left, so it isn't difficult to turn over . You might need more oomph for a diesel. If the electrics are a bit flaky, especially at low revs, the alternator might need overhauling or replacing with a new, bigger output one.