analogWrite () sets the pin as an OUTPUT, so that's not an issue (although its poor coding style).
We don't know which motors you are using, nor the current ratings of that battery - can you provide links to the
data for the motors and more information (such as manufacturer's datasheet) for the battery?
[ edit: a lattern battery is not a motor battery... Motors may require a lot of current (but how much
do yours need?) also which motor shield? ]