Volks,
Several of you have sent good suggestions how to wire the isolation relay. In each case, it’s been recommended to have the relay disconnect the two batteries any time the system is not charging, then re-connect them when it is charging (using the K pilot-light lead from the VR to close the relay). As described in the factory’s shop bulletin, the relay is actually supposed to open the circuit only when the potential between battery #1 and ground drops below 10.0 volts, but keep them connected in parallel so long as both battery #1 and battery #2 hold 10 volts or better. In other words, the relay is “triggered” by having a (pos+) feed to both coil terminals, one from each battery. If one battery begins to run down, it creates a difference in potential (voltage) between the two batteries, opening the relay so #2 gets cut loose in order to stop it from running down #1. Otherwise, any plain old SPST continuous-duty relay (like a headlight or foglight relay, as somebody suggested) could be used, intended to open any time the system is not charging. The circuit from GermanSupply does that, I believe. That’ll work, but it’s less elegant. This IS a German system, after all! Eleganz über alles! If anybody can come up with the correct German system diagram, let me know. Bert From: vintagvw@googlegroups.com [mailto:vintagvw@googlegroups.com] On Behalf Of Bert Knupp Sent: Friday, March 15, 2013 10:11 AM To: Vintage VW Air-Cooled Discussion Group Subject: [vintagvw] Battery isolation relay Volks, I need some help. Various VW models over the years have used dual-battery systems: campers, sound trucks, fire engines, and police cars to name a few. The two batteries are connected via an isolation relay (in German it’s a “Batterie-Trennrelais”). The idea is to permit both batteries to charge from the generator or alternator, but to “uncouple” the #2 equipment battery when the voltage drops below 11 volts, making sure that the #1 or starting battery doesn’t get pulled down by the equipment. For example, in the police cars, it allowed the car to sit working an accident with the blue light, flashers and radio running but the engine off. If the available voltage dropped below 11 volts, the starting battery would disconnect so the car could start when done. So I’m recreating the Copbug’s two-battery system. I’ve mounted the #2 battery under the left rear seat and found a 75-amp Bosch isolation relay on-line. The problem: the four terminals on the new Bosch relay don’t match the four terminals on the VW factory bulletin for police-car wiring. I’m usually pretty good at logicking-through circuits, but I’m stumped here. The factory bulletin shows an isolation relay with four terminals: 51, 61, 86 and 87. 61 comes from the 61 terminal on the voltage regulator. Skinny wire. 51 comes from the B+ terminal on the voltage regulator. Fat wire. 86 goes out to the #1 (starting) battery (+). Fat wire. 87 goes out to the #2 (equipment) battery (+). Fat wire. The new isolation relay comes with four terminals also: 85-, 86+, 30 and 87. The 30-to-87 circuit seems to be the switch that opens and closes. The 85-to-85 circuit seems to be the coil activation. The 87 and 51 terminals are high-amp screw terminals. The 85 and 86 terminals are low-amp Faston slip-on tabs. But I can’t figure what’s what. Can anybody help me? How do I hook up the new relay to do the job? I’ve written to Bosch, but I won’t hold my breath. The last time I asked them for help, the reply came 5 months later – and they said they didn’t have information on the old equipment. Aargh! Bert Knupp in Music City USA -- Visit the VintagVW archives at http://mail-archive.com/vintagvw@googlegroups.com --- You received this message because you are subscribed to the Google Groups "VintagVW - Air Cooled Volkswagen Discussion List" group. To unsubscribe from this group and stop receiving emails from it, send an email to vintagvw+unsubscr...@googlegroups.com. To post to this group, send email to vintagvw@googlegroups.com. Visit this group at http://groups.google.com/group/vintagvw?hl=en. For more options, visit https://groups.google.com/groups/opt_out. -- Visit the VintagVW archives at http://mail-archive.com/vintagvw@googlegroups.com --- You received this message because you are subscribed to the Google Groups "VintagVW - Air Cooled Volkswagen Discussion List" group. To unsubscribe from this group and stop receiving emails from it, send an email to vintagvw+unsubscr...@googlegroups.com. To post to this group, send email to vintagvw@googlegroups.com. Visit this group at http://groups.google.com/group/vintagvw?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
