Re: Dual batteries and switches.
It is safe to start and run on either #1 or #2. It is safe to start on both ONLY if you cannot start on #1 or #2 alone. On many battery switches it is safe to switch from "Both" to either #1 or #2 to run. IT is ALMOST NEVER good to run on BOTH ( see caveat below).
Determining if it is safe to start on BOTH and then switch from #1 or #2....
If the #1 or #2 battery position is immediately adjacent to BOTH, then it is likely safe to switch from BOTH to #1 or #2.
To verify this:
Disconnect both batteries from the switch.
Put switch in BOTH position
Using an Analog (not digital) multimeter set on OHMS x1. Put one lead on the terminal post for #1 and the other lead on the terminal post that goes to the engine.
Meter should indicate Zero ohms.
Move switch as slowly as possible from BOTH to #1
IF the needle on the meter drops, even momentarily to zero, then it is not safe to switch from both to #1 or #2 while engine is running..
Repeat test with terminal post #2 and the engine lead.
When is it safe to run on BOTH...
IF both batteries are bought at the same time and are from the same manufacturing lot
AND IF you ALWAYS run on BOTH!
Otherwise....
When the alternator charges a battery, the charge current is controlled by the regulator in the alternator.
When you are in BOTH, the alternator controls the charge from the alternator to the battery pair, but the current flow
between battery #1 and #2 is totally uncontrolled and dangerously high currents can result.
Additionally, as batteries age (especially if not bought at the same time and are "identical") their internal chemistry and state change. This can cause batteries charged/run as BOTH to have one battery be "parasitic" to the other. This eventually shortens the life of both batteries.
In an emergency start situation,where neither battery has enough "juice" to turn the engine over and you need to set switch to BOTH, it is unlikely that high currents can flow long enough to damage either battery, especially if this is just done to start an engine, and then you switch to #1 or #2.
Battery types and "operational considerations"
The following is how I run my boat and have done so for decades... You can do... whatever.
Battery #1.... an oversize "start" duty battery.
Battery #2.... a dual cycle battery. ( start and deep cycle)
Engine normally runs from #1 as does my 500 gph "rainwater" bilge pump ( My boat does not self bail...spill a drink on the deck and it has to get pumped out)
All my electronics run from battery #2 as does my "HOLY S$$T" 2000 GPH bilge pump.
Both batteries have caged external float switches. ( I bought my first boat in 1964... I've never found a pump with an RELIABLE internal switch and have been using caged float ones without incident since Hurricane Irene) BTW... caged ones need cleaning from time to time (every couple of years...sooner it you have a "dirty bilge")...Don't ask!!!!
The switch for #1 sits as low in the bilge as possible. The switch for #2 sits an inch higher.
#1 operates all the time in auto. #2 has a manual/off/auto switch, normally set (and taped) to auto.
How I use the switch...
If there has been a lot of rain, I start and run on #2. Mid voyage, I switch to #1.
No rain, or just used the boat the day before...
Start and run on #1. Mid voyage, switch to #2.
According to a study I read last year, if you start normally, it takes about 20 min of run time to restore the charge you used to start the engine. Soooo... running down to the boat and running the engine for 10 mins to "recharge the battery" is a waste of gas.
If I were to spring for the money for an onboard AC charger, I would not buy a 2 battery one, I'd save money and get a single battery high quality marine grade charger , and use it to maintain battery #2. If you think about it, that is all you HAVE to do.
Winter time. If you are in a freezing climate, store them inside. Recharge on an "intelligent" charger, preferably one with a "desufinate" cycle. You do not need a dual battery charger, alternating them works just fine.
The batteries in my boat were just replaced last month ( they still worked OK). These batteries were 2 years old when Hurricane Sandy gave them a real workout. Most folks who use the BOTH position get about 3 seasons out of a battery ( base on my discussions with customers at the "big blue box" marine store where I work part time.) .
