I have used a 10v, 0.8ma, + centre pin regulated charger successfully.
From the (Archos) battery FAQ:-
http://www.rockbox.org/wiki/BatteryFAQ"Q10: Can I use a different charger?
A10: The short answer is: Using a different charger will void your warranty and can be dangerous to your unit, so we can not recommend it.
On the other hand, people have successfully used different chargers with similar specifications as the stock charger. The charger must have the same kind of connector with same polarity (center positive+). The charger should supply at least 7-8 volts, and if it provides more than 10V, it probably should be regulated to 10V (do not directly plug it in your 12V car battery or it's charger!). The universal travel charger sold by Archos is specified at 12V, 1.2A so that's probably the maximum rating you should use. A good circuit for powering from a car battery or other source with a higher voltage would be a 600 mA fuse and a 10V regulator like a 7810 of your favourite manufacturer in series.
Inside the Jukebox there is a regulator which limits the charging current applied to the batteries. The higher the input voltage, the hotter this IC gets. Don't use an input voltage above 12V. Using lower charging voltage < 10V will slow down the charging. But the rockbox charging algorithm should work then, too. ...."