Is this voyage strong enough to keep in a 70lb dog once fully grown or should we invest in 9 volt?

We recommend to use these systems for pets that are 8lbs+ and at least 6 months of age. This is because proper training is required for success with the system. Often young pups may not understand the training and these systems can not be as effective. Each pet is different and may need different levels of static correction than others. This is the advantage of choosing a collar with different levels of correction to choose from, however with proper time and training either collar can be successful.