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Date:         Wed, 19 Aug 2009 13:38:24 -0700
Reply-To:     Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject:      Re: [WetWesties] auxiliary battery wiring, et al
Comments: To: vballestrem <ballestrem@gmail.com>
In-Reply-To:  <c4e7c5f90908191222m3dcd69ecx2adcd99f34e05e86@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Any loads you hang on the aux battery will increase the current going through the wire. Look at the label for your laptop's power supply, you'll find that it outputs around 19VDC at around 3 or 4 amps. That's maybe 80 watts. At 12VDC it will pull around 7 amps. This will place an additional demand on the wiring. If your aux battery is really low, it will be sucking down current real hard already. If it's topped off, then only the laptop's current will be passing through the wire.

Adding increased current requirement will necessitate greater current handling ability for the wiring and relay.

Size accordingly.

I don't fuse the line. I have a 100A relay and have a DC-in/DC-out battery charger between the relay and the house battery. The charger self-limits current in the event that the house battery is super discharged. It also will draw a lot of current when the house battery is low, as needed, to bring it up to full charge quickly.

-- Mike "Rocket J Squirrel" Elliott 84 Westfalia: Mellow Yellow ("The Electrical Banana") 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano Bend, OR KG6RCR

On 8/19/2009 12:22 PM neil N wrote:

> On Wed, Aug 19, 2009 at 8:39 AM, Brandon<brandon314@gmail.com> wrote: > ..... > >> Some people use fuses, some people use circuit breakers (more expensive >> once...but resettable). It is mainly there to protect your wiring and your >> relay. Beefier relay, beefier fuse could be used for less likelihood of >> opening when the camping battery is REAL dead. You may find, with a fuse, >> that it just blows the fuse if the aux battery is real dead (pulling more >> than 30A). > > > This is something I wasn't really clear on before installing my aux > battery. Thankfully though, I did install a fuse between charging > system and aux battery: > > starter battery --> 30A fuse --> 30A relay --> aux battery + post > > I now understand why that fuse is important. > > As for possible "REAL dead" scenario, if this helps other electrical > newbs, I now "look at" the starter or aux battery as an "appliance" > with *variable* electrical needs. > > The other aspect of the variable needs MAY BE the other appliances > being run while the aux battery is being charged by the alternator. > > Like if one is charging a laptop via a circuit connected to aux > battery, and aux battery really low, would this increase the demands > on the wire between aux battery and starter battery? > > In reading about wire ampicity, I found this: > > http://beta.circuitwizard.bluesea.com/ > > Likely more involved than most of us need, but if you float your mouse > over each field (i.e. "load current") you get a brief description of > each parameter. I found this useful. > > My near dead 45AH battery will be replaced with a larger capacity > battery. As it stands I think I should put in a lower rated fuse than > relay rating. Regardless, I might be blowing fuses if new battery run > down too far. Better that than hot wires though! > > Neil. > >


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