I was looking for some opinions on my setup. I have a 2007 Hunter 36. Original setp was a house bank with 360 amps (2 batteries) and a engine start battery (90 amps I think). After a nite at anchor I always found myself running the engine half the day to recharge before our next day at anchor. So I decided to increase the house bank size since a bigger alternator was not possible due to the engine box size. So I left the boat at my local repair shop for the winter to do the upgrade. I wanted to double the house size, and upgrade the shore power to DC charger, currently a 20 amp charger.
Here's what they did: they installed two more batteries so now I have 720 amps. They separated the banks with a selector switch so that only one bank is in service at a time. They rewired the alternator so it only charges the engine start bank. To charge the house bank I have to be plugged into shore power and can ony charge one bank at a time with the original 20 amp charger. They were concerned with the amp pull with the bigger bank and damage to the alternator trying to charge that big bank?
First problem I see with this is that the alternator is charging the starting bank—it should be charging the house bank, which carries most of the loads and generally requires charging far more than the starting bank does. Starting an engine requires relatively little energy as things go.
Second, you need a larger shore power battery charger. A good rule of thumb is to size the charger to about 20% of the bank's capacity to get efficient charging. In this case, you're looking at a 100-120 Amp charger or so. A 720 Amp-hour battery bank is a fairly big battery bank for a boat the size of yours IMHO.
What I would have recommended is the following, based on what you requested:
: 720 amp-hours with all charging sources attached to it.
: single starting battery connected to the house bank via an echo charger
: Upgrade the alternator on engine to largest amperage unit, say 90 amp or so, that will fit in stock space, and add external three-stage smart regulator
: Get an Iota three-stage smart battery charger, say 90 amps or a bit larger.
: Get and install a Blue Sea Dual Circuit Plus battery switch.
Passive recharging capability
: Possibly add some solar or wind generation capability to allow passive battery recharging when anchored out or on a mooring. This can extend your time between recharging, as well as reduce wear and tear on the engine.
This would give you fairly rapid recharging under power or at a slip with shorepower. This would give you a relatively long run time without needing to run the engine or return to a marina to re-charge the batteries. It would allow you to keep the starting battery topped up without user intervention. It would allow you to combine the battery banks in an emergency, while normally isolating and protecting the house electronics from the engine starting voltage surges and dropouts.
BTW, with respect to your current yard, they're full of S***. The 20-amp battery charger is really undersized for your original house battery bank, much less the 720 amp-hour battery bank you currently have. Also, most alternators are relatively similar in physical size even if the output rating is different.