Date:         Sat, 16 Aug 2008 21:46:51 -0400
Reply-To:     Edward Duntz <eduntz@HOTMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Edward Duntz <eduntz@HOTMAIL.COM>
Subject:      Relay polarity
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I've noticed a difference in the wiring diagram that comes with the Bosch WR1 relay that is available from the various online vendors, typically for "hot start" problems, and the wiring diagrams that I've seen on every other source for wiring them. This leads me to wonder if polarity is important, both on the "trigger" side and the "supply" side.
 
The Bosch WRI diagram shows terminal 85 as the trigger wire contact, and 86 as ground. All the other sources show the opposite. I'm thinking it really doesn't matter. Both give the current a path to flow which completes the circuit in the coil. 
 
The website http://www.the12volt.com/relays/relays.asp says that it doesn't matter which way the polarity is for 85 and 85 as long as there isn't a diode in the coil. How do I know if there is a diode in my relay (Bosch or other wise)? 
 
This leads me to wonder something else. Does it matter if terminals 30 and 87/87a are reversed? I would think that if 85/86 polarity doesn't matter, neither does 30 and 87/87a. If I reversed the wiring in a SPDT relay, instead of providing power to either the normally open or normally closed terminals, or things you want to power, you could use 87 as one power source (battery) and 87a as the other, and use 30 as your load. 
 
Here's the application I'm thinking of using for this: Instead of connecting a second battery to the relay in the normal fashion (30 to the starter battery, 87 to the load, the rest in the normal fashion to ground and trigger wire, I would use 87 (normally closed) to the starter battery, 87a (normally open) to the secondary battery. Terminal 30 would go to my load. When the alternator trigger wire was energised, the load would be taken from the starter battery. When the van was off, it would come from the secondary battery. 
 
Why would I want to do this, you ask? I could just wire it the normal way and have the secondary battary charge while the vehicle was running, and isolated when not. But I figure that if my secondary battery can supply my needs for my weekend of camping, I'm better off waiting until I get home and charge with a deep cycle battery charger than to use the alternator for charging. 
 
What do you think?
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