Based on the RAW, it would only require a single round to recharge the batteries using available power:
They can be recharged in any round by excess Power not being used by other systems. These Power points may then be used in any subsequent round as if they were being produced by the power plant; simply add any amount of Power stored within the batteries (they need not be completely drained) to the Power the ship has available that round.
So in the case of the X-boat, there is no excess power to recharge them. Presumably the recharge is provided by the tender at the X-boat's next stop. It seems like this might be a great technology to use for very predictable routes, such as a free trader plying a main. If you're simply going from one planet's 100-diameter limit to another, employing your maneuver drive to work your way in and back out for trade purposes, why invest in extra power? Gotta play around with this some more.
A little experiment:
Based on the Garu
-class far trader (the TL-11 far trader used in "Lords of Thunder," the MegaTraveller Journal #4 far trader), we can realize some pretty significant savings by employing budget drives and adding high-efficiency batteries:
- Budget maneuver drive, energy inefficient. Cost savings MCr1.
- Budget jump drive, energy inefficient. Cost savings MCr5.625.
- Reduced power plant to 75Mw (standard is 90). Cost and space savings MCr1 and 2 tons.
- High efficiency batteries added. Cost is only MCr0.1 taking 1 ton of space for 40Mw of power.
Total savings: MCr7.525
The ship uses the high-efficiency batteries to power the jump drive which requires 52Mw. It has a normal-sized fuel tank, so the power from the power plant and batteries can combine to initiate a jump. The power plant uses 40Mw for basic ship systems and life support, leaving 35Mw for the jump drive, a shortfall of 17Mw. The extra power is drawn from the batteries, leaving 23Mw of Power in the batteries after the ship comes out of jump. The ship's power plant does not have enough Power to run the weapons, but given a fully functional power plant running basic ship systems, maneuver drive, and sensors (total: 67Mw), leaving 8Mw for the weapons (a 5Mw shortfall), the batteries can power the weapons for four rounds of combat if only partially charged after a jump. This can be rectified by not adding a budget maneuver drive, but it could be argued that a ship class like this one has no business being involved in five or more rounds of combat anyway.