Date: Mon, 19 Feb 2007 11:14:54 -0800
Reply-To: Michael Elliott <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Michael Elliott <camping.elliott@GMAIL.COM>
Subject: Re: your favorite 12 V accessory
In-Reply-To: <vanagon%2007021913234829@GERRY.VANAGON.COM>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey Troy, you are absolutely right that once the relay or other common
connection closes the battery with the lower voltage will draw down the
higher voltage battery until they reach a balance. In my setup -- a
standard setup used by many -- the aux battery is separated from the
starter battery while the engine is off. When the engine is running and
alternator is putting out enough voltage to extinguish the "ALT" light
on the dash the two batteries are bridged together. Under this
condition, connecting the two batteries doesn't draw either down,
because the alternator will deliver sufficient current to pull them both
up, unless the aux battery is so messed up that the alternator can't do
it. The starter battery probably wouldn't be a tough load because
without it the engine wouldn't start. I have a manual switch to bridge
the two together if for some reason the aux battery had a good charge
but the starter battery had been run low by me boneheadedly leaving on
the headlights or something. After a brief period of time I reckon
enough coulombs will have found their way into the starter battery
through the bridge to give me a second chance to start that engine and
make the beer run.
I do have a voltmeter with a switch mounted on the stove front so I can
check how the batteries are doing. It's not a real accurate one for
determining charge condition, but I would easily notice whether the
starter battery was getting unhappy. The aux battery has a Xantrex
battery monitor on it http://tinyurl.com/jf9ps which does a right nice
job of acting like a "fuel gauge" for it.
--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR
Troy Delnicki typed:
>> Hi Troy,
>>
>> The aux battery will not draw on the starter relay if your Vanagon is
>> wired so that the two battery automatically disconnect from each other
>> when charging current is not available (relay or other method). On the
>> other hand, if the batteries are always connected together, their two
>> voltages will be identical unless the wire connecting them is a very
>> poor wire. In that case, a switch will be unnecessary.
>>
>
> Michael:
>
> While it is true that the batteries would be isolated from one another via
> a relay or solenoid, the batteries tend to equalize once the relay is
> closed or power is going to both batteries. If one battery will not take a
> charge because of dead cells or other reasons, it will draw power from the
> good battery. The only true way for this to not happen is to get one of
> the more expenses battery isolators, which are far more sophisticated
> devices than the average relay or solenoid that most of us are usiing.
>
> The term isolator is probably incorrectly used most of the time when it
> comes to the relays we are using. Relays are nothing more than switches,
> which are activated by a trigger wire (the blue one behind the driver seat
> in this case) that then close the circuit. Again, once the circuit is
> closed the batteries will act as one. The bad battery will then take the
> good one with it in short order.
>
> Troy
>
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