Sorry for the delay jcpowless. At this time, it is not possible to purchase extra batteries for the mobile hotspot (They are not sold by the device manufacturer either), we may consider offering this in the future.
For now, you may consider purchasing a vehicle power adapter.
Thank you for the feedback.
There is an alternative solution (not requiring access to a car or electricity)
many sources are availaible for battery operated USB power.
Basically plug this device into the USB port of the HotSpot and it runs off of that battery while charging your internal battery.
The bonus is that with more and more devices using USB ports for charging this device becomes a support for all of them.