As a frequent user of PND devices I have to live with one of the disadvantages with PNDs: which is you always have to have the power line connected - hanging down infront of the windshield and always getting in the way when shifting gear.
So lets have a look at the different reasons for a poor cable-free experience and possible remedies:
Yes, believe it or not very often enough the reason is a bad choice of hardware, i.e. battery.
Due to the price decline in the PND area manufacturers are caught in the vicious circle of bringing cheaper devices to the market - and sometimes getting the device to market, cheap and fast, is more important than a rich user experience.
The bad thing about this is that often enough the experience is blamed on the underlaying software technology or the guys who put the software together.
From a consumer point of view: There is nothing you can do about it other than throw it away and buy a new one.
If it's not the hardware then it obviously has to be the software which in some cases gives the consumer a possibility to improve the experience, depending on which part of the software is consuming the power.
Many PNDs nowadays have additional hands-free functionality via Bluetooth. Now it goes without saying that Bluetooth itself has a high consumption, however on some devices the bluetooth hardware is still active when the device is on stand-by. In this case you can check the documentation or forums of the device and in most cases you will find a means of turning BT off when the device is on stand-by.
Power consumption, of course, is also affected thru power management. Depending on the device this can be something which a consumer can directly influence, i.e. by changing display settings such as brightness. Most devices support switching between day and night view which is beneficial for power consumption.
Sometimes, however, the internal power management is simply faulty and it depends on the manufacturer if they will supply a respective firmware update which solves the problem.
So long and thanks for all the fish!