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When I bought my boat the guy who was showing her said that he had been told to set both battery switches to “Both”. Does this mean they are connected in parallel? Does this mean that both batteries are used to start each engine and both alternators are charging both batteries simultaneously?

Should I be using the “1” and “2” positions instead?

On both they are parallel.

I don't know how your system is set up but the majority of 1, 2, both batteries is the battery you are using is being charged. So on both, both are being charged.

Unless you have the altenator going out into an isolator that splits the charge to both batteries regardless of switch position, the battery or batteries being used see the charge.

If you only have one battery per engine, then normal that engine charges that battery. On both one altenator will usually back down to the other.

Single battery per engine is refered to as paralleling. Many older boats actually have a battery parallel switch on the dash.

I may be the least mechanically-experienced person on the Board, so take my comments with a grain of salt. I personally set my switch to either #1 or #2 and alternate my battery use; for example I'll use #1 starting out in the morning and then around midday I'll switch to # 2. The next trip I'll reverse the order; this way I'll know if each battery is holding a charge. My neighbor says he always runs on "Both" to keep both batteries charged; so I asked him how he would know if one of the batteries went completely dead (since the good battery would be carrying the full load). He said he hadn't thought of that and didn't know. By alternating batteries I'll detect when one is going south when it turns over slower while starting, and I'll know to change it out.

Peter,
This is going to be one of those everybody is right and everybody is wrong things.

I say this as there are so many wiring configurations possible one hard answer cannot fit.

I personally do not like both batteries tied together for charging and starting as Tommy said a weak battery can pull a good one down and you loose having a backup if one battery dies.

That said, an isolator can stop a bad battery from draining a good battery....depending on how they are configured. Tons of ways to do this.



My opinion is worth every bit you are paying for.


I like 1 starting battery dedicated to each motor.

I like one side to have a deep cycle house battery that gets charged off one of the motors when running and with house charger. House battery is what I use for electronics, inverter, auto pilot, radio...


I have a switch in case one of the batteries dies I can use the "other" battery to start motors.

Once started each battery bank is again separated...I replace bad battery.


What it boils down to is how they are hooked up.

Both can have lots of variables.




When I was single screw, it was two batteries, both got fed from an isolator, so both stayed chargers BUT like Tommy said...every few trips I'd flip and run off the other battery.


BOTH was IF both batteries happened to run down at the same time...Both batteries would be tied together to start motor. Otherwise it was always 1 or 2

I have two battery switches both of which have positions 1, 2, Both, and off. Is that a normal configuration? If you face the switches, the one on the right seems feeds the starboard engine and most of the stuff that runs of my breaker panel. The other one feeds the port engine and the remaining circuits.

Should I be running with one switch set to 1 and the other to 2?

While we are on the subject, I had an interesting incident yesterday. When I went to do my daily check on the boat late in the afternoon, I discovered a considerable amount of water in the bilge and the bilge pump was not running. I turned the port engine battery switch to "Both" so that I could run the pump with the manual switch, but before I could hit the switch, the bilge pump kicked in.

When I came back this morning to check it out in daylight, I found that the nut on the positive terminal of my #2 battery was loose. Even when I tightened it, that battery did not have enough of a charge to run the pump until I ran the engines enough to charge it a bit. Apparently, setting the port battery switch to "Both" had the effect of connecting the feed to the bilge pump to the other battery.

Peter,
The batteries and switches can be configured in a variety of ways so I'd rather not say anything...other than look at the cables going from the batteries to the switches to understand how they are configured.

On a two battery switch, you can expect to see two lower lugs marked 1 and 2 and a 3rd lug for your feed.

The switch works by disconnecting the 3rd feed lug from both 1 and 2 when you select OFF
Selecting 1 connects the feed to terminal 1
Selecting 2 connects the feed to terminal 2
And both joins terminal 1 and 2 to the feed.

Whether battery 1 of on the port or starboard is a choice made by the person who installed. It's best to look and see rather than assume.

As to the bilge pump...stepping on the boat and walking around may have tripped the automatic switch...unless as you said a bad connection and moving around on the boat.
I prefer to have automatic pumps wired before the switch so you cannot by mistake shut juice off to the pump. I also prefer to wire a second automatic pump to a 2nd battery in case battery dies...or pump doesn't shut for wahtever reason and kills the battery...you have a backup.


IF you have one bad battery and they are both connected together directly or through the switches...you'll have 2 dead batteries. That is the down side of running on "Both" if they are tied together that way.

I have always put both on "Both", for better or worse. It has worked that way since 1966.