Originally Posted by Badsanta
Just a thought, if one bank is to low to start a motor use the other bank. If you combine them the more charged bank is both trying to start and charge the discharged bank at the same time. If you have a bad battery in the mix it will act as a load and discharge your good battery. Figure out why it wont give enough omph and fix that.
This is one of the reasons I dislike 1/2/both switches. Many also do not understand its proper use and leave it on "Both" all the time. As posted a dead bank should never be paralleled with a good bank.
On my boat I have 2 separate on/off switches - one labeled start and one labeled house. I have always thought it makes no sense to have the engine circuit live when not planning to start it as happens with a 1/2/both switch.
A third switch can be installed to allow starting with the house bank or powering house loads with the start battery. It need not be with the others to avoid confusion as it's use will be very rare. A dedicated start battery that is not used for anything else rarely has an issue.
For fusing (not breakers) I regularly use 2/0 for all battery wiring
and only step down in size for the panel feed. This makes for easy fuse sizing - 250 amp commonly which will not cause nuisance blows when starting even a 75 hp diesel
the switches are left totally out of it. The alternator
output goes directly to the house bank - always the most in need. This is fused at the battery with an oversize fuse. This is the last place you want a nuisance blow. All other charge sources also go directly to the house bank with proper fusing. The start battery is charged with either an ACR or a battery to battery charger
like an Echo Charge or Duo Charge. This makes charging of both banks fully automatic, needing no owner input.
This is a simple system that is reliable and easy to use. When on the boat the house switch is on and the start switch isn't until the engine is required.