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Re: Nomadic Notebook Power

From: Adam Wozniak <>
Date: Thu, 17 Dec 1998 10:27:41 -0800

 > Using a second car battery would be an option though. I assume I could 
 > wire it in serial with the live battery to allow it to charge whilst
 > driving?

In parallel.  In serial would double the voltage.  Your car will not
like it, especially not if you have a newer vehicle with electronic
ignition, or other smarts.

Here in the United States they sell something called a "battery isolator"
for RVs.  It's a large, beefy looking three terminal device.

You disconnect the wire from the alternator to the battery, then
connect one terminal to the alternator, one terminal to the original
battery, and the third terminal to the new (second) battery.

Now you can do anything you want to the second battery, without running
the power down on the first.  It's designed to run various "convenience"
items in the RV (lights, TV, electric shaver, whatever) without impacting
the function of the primary battery.  i.e. the kids can watch TV all night
if they want; you'll still be able to start the RV in the morning.
When the vehicle is running, both batteries get charged.

For your second battery, I would recommend getting a "Marine" or other
"Deep Cycle" battery.  Normal car batteries do best when they get hit
once (starting the car), and get charged back up rather quickly.  Normal
car batteries spend most of their life at almost full charge.  Putting
a normal constant load on such a battery will reduce its life dramatically.
"Marine" or "Deep Cycle" batteries are designed to perform better in
scenarios where the battery will get a lot of use between charges.

--Adam

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