Originally Posted by hellosailor
A 30-year old battery switch may not be making proper contact anymore, the contacts do wear and sometimes shift. I'd be tempted to replace it, or to install a battery combiner switch (West/Yadina or Hellroaring) which starts you on the starting battery, then combines both to charge/run once the alternator has come up to full voltage in a minute or two. When you shut the engine, it disconnects them again, so your starting battery is just isolated for starting use while you are on the house bank.
Be sure to use a high quality switch with more than enough of an amp hour rating. Best ones are Blue Sea and Cole Hersey. Most quality models can be opened to clean/inspect the contacts. A "Zap-Stop" on the alternator will protect against transient spikes, including switching off. There are battery switches which have an alternator field cutoff, so the alternator won;t be zapped.
The combiner option is one I have on my boat, but with an override, so I can switch it off manually. Sometimes you want to.
Personally, I recharge the engine start batteries to about 95%, then the house batteries to a similar level before switching to "Both". When in the "Both" position, the batteries will flow juice from one to another. Gnerally not too big a deal when the engine is running, but can be. If the "house batts" are flat, you switch it on "Both" there can be a current overload as both the alternator and engine batts try to recharge the house batts. Also, if the engine should die, or be shut off at that point. it's possible the engine batt will have been drawn down so the engine can't be restarted.