The battery chargers don't cost much. There is one that only goes for $30 and can draw power off a car battery so it is portable. For something really nice that is going to show how much mAh your battery has and stuff like that expect to pay $80. But you really only need something that will charge your battery and not having you to rely on estimation.
If you can't get the money right away just ask your parents, these chargers really do save trouble and money. You only need to buy it once.