propper use of dual batteries

Dicor -

All of your points make sense to me - except number 2. Why never start engine with battery switch set to "Both" [except in an emergency]? Isn't starting your engine on Both, and then leaving the switch there while running, the best way to have both batteries be at full charge when you get to yur destination, stop and go on the hook?

What is the downside to starting on Both? Too many amps going to the starter motor?
 
raybers-
If you start and run on both selected you will be charging both batteries. If you have one battery weaker than the other the alternator will continue to attempt to charge the weaker battery which results in overcharging the normal battery which will reduce it's life. As far as using both to start your motor how many times have you started your car and said "I wish I had a couple more batteries under the hood'? If you need 2 batteries to start your boat you should get that fixed before you pull away from the dock.
 
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Something else to consider regarding running on 'Both'.
I was returning to the marina last year after Riverfest, and had the switch set to 'Both' after using Battery 2 all day.
Because the river was new to me, I was using the spotlight often to make sure I didn't hit any floating hazards, e.g., driftwood, etc.

There were plenty of patrol boats out, and they were constantly urging everybody to 'slow down', even though most boats were not producing a discernable wake, nor were we passing the other boats.
Running at 'reduced' no-wake speed (3 mph?) did not produce enough juice to power the spotlight & other accessories (running lights, GPS, etc.) and recharge the batteries.
As a result, I was depleting both batteries during the 45 minute trip!

When the wife pushed the button on the Electroflush, it was the straw that broke the camel's back.
We lost power, engine shutdown, dead in the water with no way to restart!

I quickly dropped anchor, and waited for another boat (there were plenty out returning upriver).
As luck would have it, I got a tow from a boat that was also returning to the same marina, 100 yards upriver.

Since then I've always run on one battery at night, usually the 'House' battery, if I've depleted it, and I'm far more judicious with my use of the spotlight!
 
agree with dicor
thats how i was instructed when i got my dancer
 
Dicor,

It is a little hard to check since I have sold that boat already.

When it happened I went back to the marina and the house mechanic was out on the piers. I hollered over and told him the problem. He asked if I was Bat #1 or Bat #2 or Both. I replied Bat#2. He told me use Bat#1. I did and everything worked. Go Figure...
 
This will be the first summer I have shore power to charge my batts. I always ran on both to charge both with my alternator. While on the hook we only use the house battery(2) but I then start on both and run on both. With shore power I can be more selective on just using one or the other. I monitor my voltage with my Garmin and the alternator puts out 13.6 volts....when not running the engine I get 12.6 volts and my onboard battery charger puts out 12.6 volts. I thought it should be more like 13 volts. I'll have to check my Sears battery charger at home. I run a starting batt on #1 and a deep cycle batt on #2, Mike.
 
Should be more like 13.5 Mike.
Dave
 
If you have one battery weaker than the other the alternator will continue to attempt to charge the weaker battery which results in overcharging the normal battery which will reduce it's life.

Is this necessarily true? My understanding is that alternators are designed to hit a specific target voltage, typically between 13.5V and 14.5V, and stay there. The voltage regulator that is part of the alternator circuit regulates this output by controlling the field current on the alternator rotor.

A battery connected to the output of the regulator will accept current so long as there is a voltage differential to induce current flow, and once the differential is diminished (i.e. the battery is charged), no more current will flow and the battery will stop charging. There is no circuitry to detect the battery voltage level and subsequently control the voltage presented to the battery (or batteries) since it happens automatically as part of Ohm's law.

With two batteries in parallel, one battery will accept charge more readily than the other and will draw the most current (a function of internal impedance due to temperature, contruction, etc., and charge state (voltage differential)). Regardless of the charge state of either battery, the alternator voltage continues to be pegged at the design point. Once one battery is fully charged it will stop drawing current due to the balanced voltage differential and the other will continue until it reaches a similar state. At this point, the alternator continues to generate its designed output (and you monitor this on your dash).

Do I have this wrong or am I over-simplifying something?
 
It is critical that you do not change the position of your battery switch when the engine is running, because if you cycle through off inadvertantly, you will fry your alternator. Otherwise, running with the switch on batt 1 or 2 does no harm.

I'm new to boats of this size. First time with dual battery switch. Had an alarm going off when I first had it out. I saw what appeared to be a low voltage indication on the dash. We switched (a couple times) between 1 and 2 multiple times. We never went to or through the OFF position. When we got back to the marina the boat shutoff, and we couldn't start the boat. No battery juice. Is it possible I fried the alternator and ran off the batteries until they were dead?
 

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