I use one of these....http://www.portablepowersupplies.co.uk/usbliionbatterypack.htm
Certainly not cheap (I think I paid about £24 a year ago) but it charges my 3500, my iPod and my phone if I need it to as its a general purpose USB device when out and about. Great for transatlantic flights too. Large USB output, mini USB to charge the battery (same cable as the 3500) and comes with a mains adapter.
The battery pack takes about 6 hours to fully charge from empty. Certainly keeps the 3500 running when its getting low but I have not tested its power on time extension capacity with the 3500. I think I have only put it on the 3500 in anger 2 or 3 times.
There's a higher capacity version on the website, I have no idea how good that is.
Of course there are other devices around, and I am not necessarily advocating this one.
(No, I'm not connected to them other than as a happy customer, and the current model is a bit different to the one I have.)
Of course, the usual rider, your mileage may differ.