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Date:         Wed, 3 Nov 1999 12:49:05 -0800
Reply-To:     Mark Drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mark Drillock <drillock@EARTHLINK.NET>
Subject:      Re: Is battery isolator better than a relay?
Comments: To: Mark McCulley <transporter99@HOTMAIL.COM>
Content-Type: text/plain; charset=us-ascii

Thank you Mark for getting this info from the manufacturer. This 3 diode approach sounds much safer than the system that senses only one of the batteries. This extra diode would provide a corrected voltage reference to the voltage regulator that does not rely on the existence of a good current path to the sensed battery. This means that the alternator would be much less likely to output an excessive voltage level. Otherwise, if something broke the current path to the sensed battery (like a failure of the power diode in that leg of the isolator), the alternator could run wide open (+17volts ?)and overcharge the other battery. Any electrical devices connected to the charging voltage at that time would be liable to fail, such as ECUs and ICUs, radios, even light bulbs, if given enough time. If all of the current production of isolators uses this 3 diode method, I would feel much better about using one. I still prefer the relay method as it does not require any alteration to the original battery wiring.

Mark Drillock

Mark McCulley wrote: > > Mark, > > I posed the following questions to tech support at SurePower: > > Q: I am using one of your remote sensing isolotors in a vehicle with a Bosch > alternator and a dual battery setup. It's my understanding that this is set > up such that the voltage regulator senses the voltage at the main starting > battery. > > SurePower A: Actually in a group 3 isolator system the regulator sensing > terminal is a diode downstream from the output of the alternator. This is > only a reference point for the regulator to look downstream of the diodes so > that the batteries will get the 13.8 to 14.2 volts for proper charging. > > Q: When you have a situation where the main battery is fully charged > and the aux battery is fully discharged, it seems that you won't get much > charging current to the aux battery (because the regulator is sensing a > fully charged battery). Is this true? > > SurePower A: No , that is not true. Current will flow to each battery based > on what the individual battery needs. If the isolator is wired into the > circuit properly and the sensing wire circuitry sees true battery voltage > (in some cases sensing will be monitoring the main engine battery ) both the > main and aux. battery will get the charging current necessary provided the > charging voltage is at 13.8 to 14.2 volts. > > Now these answers don't make everything perfectly clear do they? My guess is > that the isolator contains 3 diodes. The third diode simply provides a > voltage drop so that the regulator will compensate for the voltage drop > across the main power diodes. So the regulator sense voltage will be the > alternator voltage minus 0.7 (or whatever the forward voltage drop across > the diode is). > > SurePower is sending me a book on how their isolators operate. Maybe that > will help clear some of this up. > > MM > > >From: Mark Drillock <drillock@earthlink.net> > >Reply-To: drillock@earthlink.net > >To: Mark McCulley <transporter99@HOTMAIL.COM> > >CC: vanagon@GERRY.VANAGON.COM > >Subject: Re: Is battery isolator better than a relay? > >Date: Wed, 03 Nov 1999 07:53:37 -0800 > > > >The main problem with most cheaper battery isolators is that they > >require the simple alternator mod but then only sense the voltage of ONE > >of the batteries. This means that the alternator will be told to keep > >that one correctly charged but the other one can be under charged or > >dangerously over charged. The relay (solenoid) method is preferred for > >this reason IMHO and doesn't require any alternator modifications. The > >more expensive Marine type isolators sense both batteries separately and > >charge them appropriately. This is a critical distinction and isolators > >without this dual sensing should not be used IMHO. > > > >Mark Drillock > > > > > >Mark McCulley wrote: > > > > > > >I have heard that some folks have located the remote sensing connection > >in > > > >the alternator regulator and added a wire to remote sense. > > > > > > > > > > The SurePower isolator is designed to be connected to remote sense for > >the > > > voltage regulator. This requires a modification to the regulator, but > >it's > > > not very hard to do. > > > > > > >The good news is that the relay is not as bad as it first sounds. Lead > > > >acid batteries have charging characteristics that allow them to charge > >in > > > >parallel and share the charging current fairly well. And, connecting a > > > >discharged battery across a charged one isn't all that dramatic either. > > > > Again, they quickly adjust to one another and as long as you drive > >long > > > >enough to charge the low battery there should be no problem. > > > > > > I've used both approaches and the isolator definately does a better job > >of > > > keeping both batteries fully charged. > > > > > > ______________________________________________________ > > > Get Your Private, Free Email at http://www.hotmail.com > > ______________________________________________________ > Get Your Private, Free Email at http://www.hotmail.com


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