Mailinglist Archive: opensuse (1620 mails)

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Re: [opensuse] UPS : replacing battery with 12 volt Jelly Car Battery
On 11/25/2014 01:10 PM, James Knott wrote:
On 11/25/2014 12:51 PM, Anton Aylward wrote:
You can power a 19 volt laptop from 2 car batteries for many days
on end. All you need is a LM7809, which will set you back $1.95 plus
shipping.
Which begs the question ... If everything runs on 5V (the electronics)
and 12V (motors, burners), then why is 19V needed?

The computer has an internal power supply that converts the more or less
standard input voltage to what's required by the computer. This also
includes voltage regulation to ensure the equipment runs properly.
Otherwise, you'd need a battery that provides 5V and another for 12V and
then try to keep them both charged by the same amount.

It doesn't quite add up.

If there is a stabilized 12V supply then that can drive the part that
needs 12V and *also* drive a step-down to 5V.

The step down from 19V to 12v is already loosing 7V.


The only explanation I've found is that the 19V is used in charging of
the Lithium-Iron cells. That have a charging voltage of 4.2V and ganged
to 4 that makes 16.8V ... Give some headroom and get 19V.

But if you are running the laptop without an internal battery, that's
not an issue.

Heck, if the laptop is an 'always on' device such as re-purposed for a
firewall, for example, then take the battery out! Being permanently "on
charge' is a good way to kill a battery -- for a variety of reasons.

So you have the battery-less old laptop running of a car battery. Fine.

Now wait, why do you even need the 12V? Perhaps you have a disk drive
that is low voltage and you're not using it to burn DVDs. Won't a 6V
motorcycle battery do?

John raises the point about the screen, but hey, this under-the-desk,
re-purposed-as-a-firewall laptop is running 'headless'.

Does all that make sense?


Actually it has wifi, so why can't I access it though a VPN via my tablet?

Hmmmm, what voltage .... Well my tablet charged from the 5V 1A little
'box' that I plug into my power bar, just like my phone ...


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