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.

So, can I just put the BP4L battery in this device? If so this seems the best way to keep a spare battery charged

Yes - you need either the power or usb connected to maintain the battery, thought it would waste energy. I usually renew them about once a month, or if something happens where my main battery gets discharged, I leave it in the cradle and use the second battery until the first is recharged and I'm at a good point to swap