Date: Fri, 13 Aug 1999 10:53:37 -0700
Reply-To: Davidson <wdavidson@THEGRID.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Davidson <wdavidson@THEGRID.NET>
Subject: Auxiliary Battery Switches
Content-Type: text/plain; charset="iso-8859-1"
Volks,
the following is an excerpt from my March 10, 1999 post. In light of the
current discussion regarding auxiliary battery charging systems, I thought
some people might find it interesting. There is no perfect system... they
all have there strengths and weaknesses... you have to decide for yourself.
And really, more important perhaps than the choice of a switching system is
upgrading that 1.5mm wire between the starter battery and the fridge
relay...it is way inadequate in the Westfalias (see March 10
post..."Auxiliary Battery Madness (looong)" for a detailed discussion of
this).
Good Luck,
Bill
90 Westy Syncro
"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"