Rebuilding Mac Portable batteries with caps

From: joe <rigdonj_at_intellistar.net>
Date: Wed May 16 18:04:37 2001

   Whether or not you need the caps depends on what kind of power supply
you plan on running the Mac from. Generally, the lower the frequency of
the power supply, the more capacitance you'll need. In other words, a
linear supply that operates at 60 Hz will need a lot more capacitance than
a switcher suppy that operates at 24kHz. I realize that you'll use a "DC"
power supply but very few power supplies put out a pure DC and you need a
pretty pure source of DC to prevent glitches in the logic of the
Mac. That's one of the biggest reasons that switching power supplies were
developed originally. The high frequencies that they run at means that you
can filter out the hum and still have a power supply that's not the size of
a refrigerator! This is assuming that the load is steady. However a
computer isn't steady due to drives starting and stopping and the
like. Because of that, you'll need even MORE capitance! In any event, you
certainly won't be able to cram enough caps inside the Mac to adaquetely
filter it. If I were you and I wasn't concerned with portability, I would
run the Mac off of some kind of large battery, perhaps even a car battery
or a battery out of an old UPS. Of course, you'll have to find something
that is the right voltage. However most most computers that use lead acid
batteries run off of either 6 or 12 VDC so that makes it easy. FWIW I
have a couple of old Mac portables and they seem to be real power hogs.
(That's probably why they use lead acid batteries instead of NiCads.) My
small HP power supply won't run them. Even with the drives off, it will
only run them with a partially charged battery to take up some of the peak
load. It won't run at all when the batteries are dead. And forget it when
the drive kicks in!

    My 2 cents worth,

      Joe

At 02:56 PM 5/16/01 -0700, you wrote:
>I've been thinking about rebuilding some Mac Portable batteries, and
>instead of putting in the Cyclon or whatever leadacid cells I am planning
>on just using a good sized capacitor with maybe some kind of current
>limiting resistor. The goal isn't for portable use, but just to keep the DC
>smooth and provide the "kick" to get the hard drive spun up. Opinions?
>
>Marvin you still have those caps?
Received on Wed May 16 2001 - 18:04:37 BST