Here's a little hint: i ran out of battery on a trip and i used a Nokia BL-C5 (i think) battery. Works swimmingly
. You just have to use a little piece of paper on the lower end to make the contacts stick together. I'm not sure which model i used so i don't really recommend using one witout checking that both the battery and x20 contacts are in the same order and placement, must be 3.7V and not higher than double of the original battery mAh. Original means 700mAh.