Mugen makes an extended capacity battery for the N810; it bulges out the back so you're buying both the battery and a replacement back plate. Great idea, I thought!

I'm now in the process of returning the second unit to Mugen. Neither battery, although new, would take any charge at all. So it appears they're just fulfilling out of old stock, and sadly it appears the cells have aged to the point of inertness.

Based on my own experience, I'm afraid I have to recommend that you stay away from Mugen batteries for the N810.

Andy