How really does the dual battery setup work?

[CarZin]

We are having an issue where the thermal breaker for the accessory battery is throwing itself from what appears to be the accessory battery not getting charged. As I have had it explained to me, when the battery gets low, the accesories are causing the battery to send more AMPs, and causing the thermal breaker to heat up and throw. This is a real problem at night when you have to wait for the thermal breaker to cool off and have no Nav lights.

When I had a mechanic look at the boat, he said the accessory battery was very low, but his tester stated the battery was still good.

Since we almost NEVER have the boat running accessories without the engine running, I am at a loss of why the accessory battery isnt getting charged much.

We have the switch set to 1. With this configuration, the alternator will never charge both at once, correct? It will only charge the starting battery, then charge the accessory battery when the voltage on the starting battery is high enough. Is this correct? If not, how does this work?

How would you go about troubleshooting the accessory battery not getting charged?

[Okie Boarder]

If you just have a switch and it is set to one it will only charge the main battery, unless you also have it tied into a isolator or seperator. To get a charge to the secondary battery, start your engine with the switch on 1 and switch it to both so it will charge both batteries.

[spankster]

This is what my 2010 manual has regarding a dual battery setup...

Dual Battery Option Hookup
The dual battery option is designed to give you extra battery
power to run accessories, but at the same time give you a safety
feature not commonly found in a dual battery setup. The Supra
Dual battery option features a Voltage Sensitive Relay, which
always monitors the engine starting battery, so that your engine
will always restart.
We recommend that the selector switch always be set to “1”. In
the “1” position, the Voltage Sensitive Relay monitors the engine
starting battery, which will ensure that you have a fully charged
battery to start your boat. The “House” or “Accessory” battery
may become drained during a high discharge cycle since it does
not get charged until the starting battery is fully charged.
In position “1” when the sensed battery (engine starting battery)
reaches 13.7 volts, the relay closes and parallels both batteries.
When this happens the LED light on Voltage Sensitive Relay
comes on.
In position “2” the sensed battery is the accessory battery. We
do not recommend using this position since it can cause the
starting battery to drain, which would not allow the engine
battery to have enough power to restart your boat.
In position “both”, the batteries are paralleled and in a high
discharge cycle, both batteries can become drained, which
would not allow the engine battery to have enough power to
restart your boat.
NOTE: It is recommended that you fully recharge your
batteries using a battery charger periodically or after a
session of high discharge (lots of accessories running for
an extended period).

[CarZin]

If you are running the boat at all times, and want to make sure that battery 2 gets a charge, what are the disadvantages of running on 1+2? Is this too much wear and tear on the alternator to always be charging both batteries?

I'm an electrical dummy, so try to explain this to me. Since a battery is suppose to output 12 volts, at what point can it output more than 13.7?

[mikebu]

Disclaimer I am not a Electrical Engineer but I did build alot of Heath kits in my 9th grade Electronics class...

As a battery gets closer to fully charged it consumes less of the charging current. The relay measures this and when it senses that the battery is fully charged it parallels the battery.

[mikebu]

If you are running the boat at all times, and want to make sure that battery 2 gets a charge, what are the disadvantages of running on 1+2? Is this too much wear and tear on the alternator to always be charging both batteries?

I'm an electrical dummy, so try to explain this to me. Since a battery is suppose to output 12 volts, at what point can it output more than 13.7?

What boat do you have? Do you only have a Perko switch or do you have Voltage Sensitive Relay which gives priority to charging the Starting battery before it charges the Accessory battery. If you only have a Perko switch you might never be charging the Accessory battery in Position 1.

It might be possible that your starting battery never gets to full charge preventing the Accessory battery from charging assuming you have the Voltage Sensitive Relay.

[CarZin]

I have the VSR. I 'suspect' the accessory battery is just dead. The mehanic tested it twice. One time his tester told him the battery was dead. The second time the tester told him the battery was just low. The batteries were replaced one year ago. I dont know if they are dual purpose or not.

I am going to head out in two weeks to the lake (working this week) and will be paying close attention to the light on the switch that is supposed to come on when it opens the batteries up to parallel charging.

Suspect I just need to get a new battery. But it explains all the silly electrical issues I have had.

[mikebu]

One thing I do every spring is take the batteries out of the boat to charge them and then switch the batteries so last years accessory battery becomes this years starting battery.

[docdrs]

A fully charged battery will put out roughly 12.7volts after it has sat for 15 mins post charging. A battery that is putting out only 12 volts is roughly 45% charged...not good. Your alternator is capable of between approx 13.8 - 15 volts......the lower the battery voltage the greater the alternator output. The worst thing you can do is leave a battery in a low state of charge for an extended period of time. Get a digital multimeter and you will know the state of your batteries.......

here is some battery info

http://www.wholesalesolar.com/pdf.folder/Download%20folder/battery_maint.pdf

[beast 496]

IF you have the Blue Sea voltage sensitive relay, as most new boats are now running. Check to see if the 18 gauge ground wire leading from the relay is hooked up to the battery ground. If this very small ground wire is not attached, the relay will not work and you will not get any charge to the secondary battery. Al