Originally Posted by codprawn
An 85AH would be fine. If 1 will start my diesel Discovery on a cold morning it will cope with a 90hp outboard easily.
Yes that is correct. A standard type 072 diesel battery as found in most older Discoverys or Defenders is only about 70AH but produces 600 amps which if it will turn a 2.5L diesel easily, will certainly not have a problem with a petrol engine of anything up to about 4.0L. My 115hp is 1.8 litres I think, and the current battery is not an 072 but it is about the same size and has no problem starting even after weeks without being used. I do top it up occasionally with the trickle charger though.
Whether you want the extra capacity on a boat as a get-you-home safety net if the alternator packs up, now that is another matter. I think I'll be fitting a 110AH Td5 Discovery battery when mine needs changing, just for that reason. I would imagine that is why a bigger one is recommended?