Battery Switch

[Big D]

Despite having been around boats my entire life, this seems like a rookie question but here goes...

Boat has 2 batteries and the traditional (off - 1 - 2 - both) battery switch.

I was told not to run the boat in the both position because it will damage the alternator?

I typically alternate between the 2 batteries but recent battery issues has me questioning if the info I was given was correct?

 

[STV_Keith]

It won't hurt the alternator. In multi-engine applications this could be true if both alternators were tied together, but not with a single engine/alternator applications.

 

[Shlbyntro]

2 batteries will not hurt the alternator. 3 is pushing a fine line, and 4+ is no go

 

[DarkHorseRacing]

You can run two batteries at the same time, that's super common. We had three batteries and I could see the alternator couldn't keep up (never got to 14), so we reduced it down to two (the third one died anyway). With one or two batteries, the alternator comes right up to 14.

Also, if you want you can look into a battery isolator device that will sense the voltage of both main and aux batteries and connects/separates them according to their charge, which allows the main cranking battery to be protected. Look for a Cole Hersee Smart Battery Isolator (Part # 48530), it good up to 200 amps.

 

[port austin pirate]

I was told you shouldnt run more then one at a time unless you have an isolator, but never really dug into it, you guys may be right, gonna run it by my brother just for the heck of it, he was a damn good journey man electrican know his electronics

 

[outboard_256]

I was told you shouldnt run more then one at a time unless you have an isolator, but never really dug into it, you guys may be right, gonna run it by my brother just for the heck of it, he was a damn good journey man electrican know his electronics

The problem is really if you run 2 batteries at the same time and they are at different voltages (one is half and one is full) then it can mess with the batteries until they equal out. How much of a problem is that for you, is a question you have to answer. They aren't going to blow up but it could shorten the total lifespan of the battery.

 

[Uncle Dave]

Despite having been around boats my entire life, this seems like a rookie question but here goes...

Boat has 2 batteries and the traditional (off - 1 - 2 - both) battery switch.

I was told not to run the boat in the both position because it will damage the alternator?

I typically alternate between the 2 batteries but recent battery issues has me questioning if the info I was given was correct?

Substitute the word "will" for "could" then add a bunch of conditions to it that reduce the probability into the low single digits and there is the correct answer.

In 5 decades of boating I nor any of my friends I boat with (most have 2 batteries and a perko) have blown an alternator. We've had voltage regulators go, but thats different.

There is a chance if you have two very low batteries that you could overheat an alternator thats putting out full power for an extended amount of time, but typically when you get there the boat won't start anyway. Pretty much the smallest of them put out way more than 2 batteries can take for very long especially wet lead acids.
They won't absorb a lot and will taper very quickly lightening the load on the alternator in short order.

There is danger is with a small alternator and 2 BIG AGM's (like odysseys or full river tppl type batteries ) - they will take massive current inrush and keep the alternator " pegged" for a log time if they are really down.

Keep the rig on a smart charger when sitting. Run to your destination on 2, switch to 1 to blast the stereo all day, switch back to two and go home.
Worked for Grandpa, Dad, me, and will for you.

 

[DaveH]

its not the alternator you risk damagin.....its the batteries that are the problem.

as has already been pointed out........if one battery is "lower" then the other.....say from prolonged stereo use on one battery......when you fire it up, the alternator cant tell that one battery is lower then the other and applies a high charge rate to BOTH batteries.....commonly overcharging other battery.

this is easy to see......if you have one battery where the case is swelling or bulging....it is being overcharged and damaged.

this is also an example of why replacing batteries singularly is a bad idea.