Will I harm my starter battery if charged at 14.8 volts?

[Chewbacka]

But what about when he is charging from the alternator that is now frigged to run at a higher voltage, I know a diode would fix that

A big diode like this might do what you want - http://www.ebay.co.uk/itm/DC-Power-Supply-M12-Thread-General-Rectifier-Diode-Stud-1000-1600V-ZP-100A-/291561707042?hash=item43e26e9622:m:mTIt-oKcsQQY5HXtYqhStJw

 

However make sure when you wire it in that when cranking the starter will not be powered by the leisure batteries, especially if the starter battery fails as the full current could flow through the diode, in which case you will need a bigger diode like this one - http://www.ebay.co.uk/itm/200V-2200V-200A-Rectifier-Diode-Stud-Version-ZP200A-for-AC-DC-Converters-/121938588735?hash=item1c641b343f:m:m7JYLze_oIIpOrHp86o2M3A

Edited February 12, 2017 by Chewbacka

[Sir Nibble]

A big diode like this might do what you want - http://www.ebay.co.uk/itm/DC-Power-Supply-M12-Thread-General-Rectifier-Diode-Stud-1000-1600V-ZP-100A-/291561707042?hash=item43e26e9622:m:mTIt-oKcsQQY5HXtYqhStJw

 

However make sure when you wire it in that when cranking the starter will not be powered by the leisure batteries, especially if the starter battery fails as the full current could flow through the diode, in which case you will need a bigger diode like this one - http://www.ebay.co.uk/itm/200V-2200V-200A-Rectifier-Diode-Stud-Version-ZP200A-for-AC-DC-Converters-/121938588735?hash=item1c641b343f:m:m7JYLze_oIIpOrHp86o2M3A

How do you do that? It's either the right way round with the same risk of being fed by the house battery if the relay fails sc, or it's the wrong way and the battery doesn't charge.