9 years on battery banks is as good as you could expect, particularly if you don’t have a means of keeping them charged when at anchor. It’s time to replace them.
Quite frankly, I don’t see the need for a jumper pack. I’d direct your efforts towards optimizing the design of a new battery configuration and paying attention to battery health.
There were comments earlier like 12.3 volts is essentially a dead battery. It’s not necessarily, as voltages will drop somewhat depending on the size of the real-time loads.
My thoughts.
1) Your starter bank should be fully isolated from the house bank. Ensure this is so. This bank should be sized sufficient to start both engines. I see no need for a separate battery for each engine. Nor do I see the need for an 8D. I have 2 Group 24s in parallel for my Lehman 135. More than sufficient. A Group 31 would also be fine.
2) seriously consider a solar bank in the absence of a generator. Properly sized, it may even get you up to float charge after travel.
3) optimize the size your new house bank based on understanding the daily total amp hours used. Ideally your house bank should be twice the size of 2 days amp hours of usage, if you would want to be at anchor for 2 days.
4) check all connections, replace older cabling, and review whether the current cabling is optimized for charging and discharge.
5) review CMS’s site. Really good stuff here:
https://marinehowto.com
6) bring in an electrician to review what you intend to do, or actually do the work.
7) I am always reviewing the Magnum panel throughout the day. Over the years with the same bank, I have a pretty good idea how it is aging. I now have 7 summers of cruising on it, with pretty extensive usage. The voltages are beginning to get lower before mornings charging and are consuming more and more water. I will replace the batteries before next years cruising season.
Jim
