Mailinglist Archive: opensuse (1620 mails)

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Re: [opensuse] UPS : replacing battery with 12 volt Jelly Car Battery
On 11/25/2014 01:33 PM, Anton Aylward wrote:

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.

Computer power supplies typically have multiple outputs at different
voltages. It's not that hard with modern tech.

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

Yeah, that 7V was too tight. ;-)

Actually, you're losing very little. Unlike linear regulators,
switching supplies are very efficient. If your linear regulator was
passing 1A, then you'd lose 7V x 1A or 7 watts and the output current is
never greater than the input, in fact it's slightly less. On the other
hand, a switching power supply can provide greater current & lower
voltage or greater voltage & lower current (depending on how it's
configured) so that you lose very little power in the process. That is,
the output power is almost as much as the input. Switching supplies are
also very tolerant of input voltage variations. This is why the same
power supply can often be run off 120V or 240V.

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.

Some computers have optional battery packs, with a different number of
cells and terminal voltage.

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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