SP2 on a Laptop

I've got kind of a weird problem when I try installing XP SP2 on my laptop.
When it begins to install an error message appears stating that I cannot
continue without AC power connected. The trouble is that AC power is
connected, its just that the laptop doesn't recognise it. The reason for this
is that the AC power board got shorted out and stopped working. So I re-wired
the power connection, bypassing the AC board and going straight to the
battery terminals, which seems to work a treat. But what I didn't bank on was
the unique problem i've been presented with trying to install SP2. Does
anyone have any ideas what-so-ever? Is there a way of installing SP2 without
it needing AC power connected? Can XP be fooled into thinking AC power is
connected?