The main reason is that the charging circuitry in the UPS is not
designed for the load a discharged car battery presents to it.  There
are a whole slew of other small reasons (discharge depth, trickle charge
voltage/current, etc), but when you get down to it:
1) The battery supplies 10-14vdc
2) The UPS consumes 10-14vdc
End of story.  The only real unknown is the charging style and
capability of the UPS.  Chances are it'll be fine, but I would design
the cage that held the battery very carefully if I thought there was
even a remote chance that it'd start charging when I was away or
asleep.  If he really wants to build his own UPS type system, have him
look at some tripp-lite UPS units that do not charge the battery, and do
not get in the way of battery charging.  He can use any charger and any
battery within liberal specs and the UPS will autoswitch to the battery
when the power goes out.

-Adam

Ed Browne wrote:

>Can anyone think of a reason not to attach a car battery in place of the
>sealed battery in a battery backup system?  I've got a few of these
>carcasses laying about because it's always been cheaper to buy a new one,
>but a cheap car battery could breathe new life into them.  Clearly, I don't
>want to turn it over and spill acid, but think of the backup time available!
>
>Thanks,
>Ed
>
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