Do any of the camera manufacturers manufacture a power supply that lasts longer than the normal battery? The battery in my Canon 70D limits me to approximately 4.5 hours of operation before the battery runs out, and I am not able to get the number of time lapse frames that I need in that duration.
Canon (and every other manufacturer) make AC adapters for their cameras. If that's an option, your problem is solved - see Can my Canon Eos camera be used with continuous power? for a bit more detail.
If AC power isn't an option, are you using a grip? These contain a space for an extra battery so should double your available time.