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From: Ron Hardin on 1 Apr 2010 20:00
If you power a few laptops with a UPS, their batteries can do you in.

If the power goes off and the UPS runs down completely, then the laptops
begin using their own batteries.

So far so good.

When the power comes back on, the laptops continue runing except go on
AC power, and in addition start charging their batteries up again.

That additional battery charging power can be enough to shut down
the UPS even though it's on AC power now, if it's over the limit for
the UPS.

Lesson: find out how much the laptop draws with a dead battery when
adding up the power needs. It's more than it draws running with a
charged battery by a good bit.
--
rhhardin(a)mindspring.com

On the internet, nobody knows you're a jerk.
From: RnR on 1 Apr 2010 21:53
On Thu, 01 Apr 2010 19:00:27 -0500, Ron Hardin
<rhhardin(a)mindspring.com> wrote:

>If you power a few laptops with a UPS, their batteries can do you in.
>
>If the power goes off and the UPS runs down completely, then the laptops
>begin using their own batteries.
>
>So far so good.
>
>When the power comes back on, the laptops continue runing except go on
>AC power, and in addition start charging their batteries up again.
>
>That additional battery charging power can be enough to shut down
>the UPS even though it's on AC power now, if it's over the limit for
>the UPS.
>
>Lesson: find out how much the laptop draws with a dead battery when
>adding up the power needs. It's more than it draws running with a
>charged battery by a good bit.


Based on what you say, I would think that if you size the UPS battery
to be equal or greater than the laptop battery, you're okay assuming
the UPS will only run this laptop.
From: William R. Walsh on 2 Apr 2010 09:40
Hi!

> That additional battery charging power can be enough to shut
> down the UPS even though it's on AC power now, if it's over the
> limit for the UPS.

I find that surprising. I haven't seen very many overloaded UPS units,
but when I have, the only result was a lit "overload" indicator and
occasional beeping on some units. If the unit went to battery, then it
would usually just shut down.

Yes, laptops do pull a lot more power from their power adapters when
they are charging batteries. The temperature of the power adapter will
prove this as well--compare temperatures of the power adapter when it
is only powering the system to when it has been charging a battery for
a while.

William
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