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rwy36 07-20-2012 11:22 AM

Typical isolating diode location?
Hello all,

I'm thinking about changing my electrical system to remove the battery isolating diodes (and their voltage drop) and use a 'voltage sensing relay' to charge both batteries instead.

My boat is an O'Day 302. Does anyone have an idea where the battery isolation diodes are physically located?



Waltthesalt 07-20-2012 07:26 PM

Re: Typical isolating diode location?
They should be on the charging output to batteries. Generally the charging line goes to the main battery bank and the diode is in the line to the secondary bank. Blue Sea calls theirs a Dual Sensing Automatic Charging Relay, BEP calla theirs aDual Sense Voltage Sensitive Relay (VSR).

rwy36 07-21-2012 08:04 AM

Re: Typical isolating diode location?
Thanks Walt.

In the O'Day 302 case, I think the charging line (from the alternator) goes up to the master disconnect in the electrical panel, and the diode(s) are downstream of that. The manual says both batteries will charge even if the master is in the '1' or '2' position ('both' not required), so that indicates to me that they have to be past that point.

I may just run the alternator direct to the batteries with a 'engine disconnect' switch and fused link added for safety. This would be another way to remove the undesired voltage drop I'm observing to the batteries when charging.

Take care,


Stu Jackson 07-21-2012 08:19 AM

Re: Typical isolating diode location?
Here's a topic about isolators. It's from the C36ia Forum, so you have to sign up to read it, but it's free. Has two wiring diagrams showing just what you are asking about.

Another Wiring Thread - Catalina 36 International Association Forums

Waltthesalt 07-22-2012 11:53 PM

Re: Typical isolating diode location?
I agree with your take that the charging cable doesn't need to go thru the isolation switch. In my experience not going thru a switch is the norm. I think you should go ahead with your plan to install a VSRs nonetheless. Although most alternator outputs are not fused, the ABYC standard I recall is to fuse anything within a certain distance of the battery terminal. You never know when you may short out that connection.

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