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.