http://www.expansys-usa.com/p.aspx?i=162374

This is for the N90, but for $12.95, I added it to my order for an extra battery and it works fine without the phone and will keep the battery charged (and has a fully charged indicator).

However it has a USB passthrough and the right kind of connector to charge the n810. If you take it apart carefully you can move the charger plug so it will fit into the n810.