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Old 06-19-2021, 12:29 PM   #1
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Battery isolator question

This is actually about my boat, which I suppose can be called a Recreational Vehicle.

I'm installing a second battery for non-essential accessories like the radios.
The ignition, starter, gauges, lights, and other stuff for navigation will remain on the main battery. I've also purchased a cheap little battery isolator made in China. The idea is to have both batteries charging when underway but, when the engine is not running, to be able to exhaust the 2nd battery without drawing current from the engine battery.

My understanding is that the accessories will draw from both batteries until the main battery falls below 12.8 volts, when it will disconnect. When the voltage on either battery exceeds 13.3 volts, it will re-connect.

My question is about where to connect a battery charger when needed. It seems to me that it should be connected to the engine battery. After that voltage exceeds 12.8v, the both batteries can continue to charge together. Isn't that what happens when the alternator is running?

But, the instructions for the isolator say to connect the charger to the 2nd battery. I'm not sure why. Maybe it will work equally well in either configuration, but I think I prefer having main battery topped off first.

Who is right? Me or the people in China?
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Old 06-19-2021, 01:09 PM   #2
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Since this is my first encounter with battery isolators, another question has just occurred to me.

This model of isolator has a by-pass feature. If you connect the yellow wire to ground, it behaves like a closed bidirectional switch. Why would anyone need to do that? Perhaps if both batteries were nearly depleted and you needed both combined to get the engine started. But, shouldn't that happen automatically? I mean, shouldn't current always be able to flow from Battery2 to Battery1? Maybe not when both are below 12.8.

My question is about what size cable I need to connect from the batteries to the isolator. The device is rated for 170 amps in intermittent duty, so I guess the cables should be able to handle the same? It would only matter when trying to start the engine with two batteries in "bridged" mode.

I would ask these questions on Amazon except that the people who sell them know near nothing about how the device operates.
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Old 06-19-2021, 03:19 PM   #3
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Use short 2 gauge cables and keep it close to both batteries. Don't forget to ground it, so the electronics work.

At rest, it will be open, so if the batteries are low, you will need to use the bypass wire to connect them together fro boost starting.

If its bi-directional, it won't matter which battery you charge, it will connect them if 13.2 volts is going into one.
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Old 06-19-2021, 06:47 PM   #4
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Quote:
Originally Posted by twinboat View Post
If its bi-directional, it won't matter which battery you charge, it will connect them if 13.2 volts is going into one.
It just seemed strange that the instructions showed a charger connected to the "house" battery. To me, that is backward.

But, you are right. If the bypass switch is activated, both batteries will charge together.

I'm going to put the charger on the starter battery if I ever need to. If I don't flip the bypass switch, it should charge the primary first, then both together after the threshold voltage is reached.

Thanks for the input on the wire gauge. The run will be less than two feet, so #2 AWG should be enough to turn the starter.
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