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Date:         Wed, 3 Nov 1999 07:33:48 -0800
Reply-To:     Davidson <wdavidson@THEGRID.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Davidson <wdavidson@THEGRID.NET>
Subject:      Re: Is battery isolator better than a relay?
Comments: To: CARY CHIANG <chiang1@USA.NET>
Content-Type: text/plain; charset="iso-8859-1"

Auxiliary Battery,,, Switch Choices (pros and cons):

Manual Switch: A manual switch is simple, and it is inexpensive. It simply connects or disconnects the positive terminals of the starter and auxiliary batteries. There are fewer things to go wrong with a manual switch. To charge the auxiliary battery, you simply use it to connect the starter battery and the auxiliary battery (close the circuit) after the engine is started and to disconnect them (open the circuit) before or just after the engine is shut off. The main problem with a manual switch is remembering. If you do not close the circuit after the engine is started, then you will not be charging the auxiliary battery. If you do not remember to open the circuit after the engine is stopped, your appliances can drain the starter battery to the point that it will not start the engine.

Relay: A relay is generally used as a remote switch that can be set up to work automatically. You don't have to remember to connect and disconnect the starter and auxiliary batteries. That is done by a pair of actuator wires that are 'live' only when the alternator operating. When these actuator wires are live they force the contacts inside the relay that carry the current between the two batteries to physically contact each other. This is probably the most common non-manual switching device in the automotive industry. Because of this I am inclined to believe it may be the most reliable and most readily available. And they tend to be the cheapest. The disadvantage of using a relay is that if it fails it could fail with the contacts touching. If this happens you may not know about it, and your appliances will drain both batteries leaving you unable to start the engine.

Solenoid: A solenoid is very similar to a relay. Generally they are used as a remote switch between the ignition and the starter, and they are designed to handle much larger currents than relays. It is my understanding that Solenoids have pretty much the same disadvantage as a relay. In addition, since the contacts are larger in order to handle larger currents (often over 160 amps compared to the typical automotive relay rated at 30 or 40 amps), the actuator current must be larger in order to move the contacts into place. There has been some concern expressed on the list that the actuator circuit was not designed for and could be damaged by this larger actuator current.

Isolator: These are the square or rectangular boxes with colorful fins. It is my vague understanding that isolators do not connect the two batteries. They are a solid state device that directs charging power from the alternator to the each of the batteries independently depending on whether each battery needs a charge or not. It is my opinion that the main disadvantage of the isolator for the Vanagon owner is that there have been many anecdotal reports that it is difficult to get the isolator to work correctly with the Vanagon. Even the tech support person at Sure Power said this to me. (I think this is because the Vanagon alternators put out less voltage than the typical American vehicle and there is also a voltage drop across the isolator, thus not leaving enough voltage to activate the solid state switch. I think.) That being as it may, Harald has reported that he prefers the isolator and it seems to work fine in his vanagon.

Another disadvantage is that this device is larger, making it harder to find a place for it. And it should not be installed in a hot environment (the fins are meant to dissipate internal heat). Also, according to Bob Livingston (RV Repair & Maintenance Manual, p. 1.20), other problems that can be caused by the isolator include: 1) a shorted diode can cause the battery(s) to overcharge or boil (I think this is because the isolator essentially replaces the stock voltage regulator.) 2) an open diode can cause one battery not to charge or both batteries to go dead, and 3) a defective diode can cause low battery voltage.

Battery Combiner: This device is offered at West Marine (a boating supply store), and the device has the West Marine name on it. It measures about 4 x 4 x 3 inches, and I believe this too is a solid state device. As I understand it from reading the box, the Battery Combiner combines the starter and auxiliary batteries while they are charging (above 13.1 volts) and isolates the two batteries when the are discharging (below 12.8 volts). Unlike the isolator, West Marine claims that there is no voltage drop across the Battery Combiner. There are two models, one rated at 50 amps and one rated at 150 amps. The disadvantage of the Battery Combiner is that it is relatively new and unknown. I haven't read or heard any independent review or anecdotal evidence regarding this product. It also concerns me that it takes 13.1 volts to activate the device. My experience in measuring the voltage output of the alternator at my battery is that it goes down very close to the 13.1 volts and is known to vary considerably.

-----Original Message----- From: CARY CHIANG <chiang1@USA.NET> To: vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM> Date: Wednesday, November 03, 1999 2:03 AM Subject: Is battery isolator better than a relay?

Listees: Regarding the posts yesterday about aux. battery charging via a relay, I was wondering what the benefits were of a battery isolator versus a simple relay? Is it that the isolator can alter charging current to one battery over another (or something like that) while a relay can only charge both batteries equally, or what? Don't both setups charge both batteries while the engine is running and disconnect (and prevent discharge from) the main battery when the engine is off?

Cary

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