It would probably be more effective to use lithium and have a charge threshold where it just stops and acts as a sensor and mesh communications node. In that kind of mode, the battery life would be nearly indefinite. If the front line moves well past, they are recovered. If not, they are a mined explosive or ready to make a last stand.

Even at $3k-$10k robot dogs are dirt cheap next to training a human for 3-6 months.