Hi everyone,

I'm currently designing the electrical schematic for my boat and had a question regarding the Dual Battery Isolator I'm planning to use:

Battery Isolator Link

From what I understand, the starter battery is recharged by the engine’s alternator, and then the auxiliary battery is charged via the starter battery through the dual battery isolator. Am I right in saying that?

If so, then the current passing through the isolator could get pretty high, depending on the demand from the auxiliary equipment. I’m also wondering about how the voltage threshold works — does the relay monitor the starter battery only, or does it take both battery voltages into account?

I've put together a schematic focused on the equipment located at the rear of the boat — happy to share it if anyone’s open to giving some feedback or suggestions.

Thanks in advance for any insights!

Best,

 

From what I understand, the starter battery is recharged by the engine’s alternator, and then the auxiliary battery is charged via the starter battery through the dual battery isolator. Am I right in saying that?

Not from what I read.

• Voltage Sensitive Relay allows 2 batteries to be charged from your engine alternator at the same time.
• When your engine is turned on and the main battery is fully charged, the relay will start charging the Auxiliary battery at the same time.

Looks small to be rated at 140amps.
Video and image shows Loads on the aux. battery connected to the negative side ???

 

Not from what I read.

• Voltage Sensitive Relay allows 2 batteries to be charged from your engine alternator at the same time.
• When your engine is turned on and the main battery is fully charged, the relay will start charging the Auxiliary battery at the same time.

Looks small to be rated at 140amps.
Video and image shows Loads on the aux. battery connected to the negative side ???

The negative side of the battery connected to the isolator is only there to power the internal electronics of the VSR.


Also, isn’t that what I said?


The main battery reaches the voltage threshold first, which then triggers charging of the auxiliary battery. But if the auxiliary battery is running a lot of equipment, it could be drawing a significant amount of current just to stay charged.

 

I thought you meant the aux battery was being charged by the main battery.
It says it can charge both batteries at the same time but I would think only if they need charging.
It also says the aux battery is connected or disconnected depending on the voltage of the main.
To me that says the main battery is powering everything until it requires charging.

 

What I mean is: the main battery is being charged by the alternator, so by extension, if the auxiliary battery is being charged through the main battery, then it’s effectively being charged by the alternator too.


And you're right — there’s a voltage threshold that determines when the relay opens or closes the circuit. So when the alternator is charging and the main battery voltage rises (typically around 13.8V), it goes above that threshold. That causes the relay to close the circuit, allowing the auxiliary battery to be recharged through the main battery — and ultimately from the alternator.

The relay will open the circuit if the main battery voltage drops below the threshold. This ensures that the main battery doesn't get discharged to the point where it can no longer start the engine.

 

This whole idea depends on whether or not you're boating in Salt-Water or Fresh-Water,
and whether or not You are capable of walking back to Shore.

Toys and Gimmicks are fine to play with,
until they fail,
and you're ~30-miles off-shore,
with a dead Battery.

Salt-Water EATS EVERYTHING ELECTRONIC.

The best solution has been around for at least ~80-years,
it's called a Marine-Grade-Manual-Battery-Switch, or Dual-Battery-Switch.
It will keep You from walking back Home.

3-or-4-Batteries is even better insurance.

If You want even more insurance, get a Dual-Rectifier-Alternator, or a second-Alternator.

For additional insurance, add one, or several, Solar-Powered-Battery-Maintenance-Charger(s)
( they must be protected from Salt, Dirt, Leaves, and various other Trash, they are NOT "Maintenance-Free" !!! )
Solar-Power can keep your Radios working when everything else has failed.

Certified at age-8 for the US-Coast-Guard-Auxiliary, and I was the best in the Class.

If it can break, sooner or later it will.
.
.
.

 

Hello sir,

I do agree with you, but I'm fishing in the Ontario lake, so no salt water there.