For a replacement battery, here you go
, the site sells alot of Archos stuff.
As for charging techniques, you'll get alot of different opinions on what the best method is to prolong the battery's life.
I usually charge it completely, and use it till it dies, and then repeat. I've always used this method for my electronics, since there is supposed to be a finite number of charge cycles that rechargeable batteries can go through. However, since most Archos players use Lithium ion polymer batteries, I'm not sure if it would be different.