Thanks, Grey told me the max voltage is 5 volts into the connector on the side, I responded with another question regarding how much is the maximum voltage to the battery spring inside the handle but no response yet. I believe they could be different, the connector on the side is isolated from the hosts common ground (positive ground), this means there is a blocking diode inside which will drop some voltage, perhaps the reason the external conn. can take 5 volts while the battery spring may be limited to a lower voltage. Don't know yet. If I can get an extension tube, I can build a special second battery with a couple of diodes inside it so the second battery only adds a half volt, that will allow me to keep the laser at 4.7 volts when the batteries are fresh, but I need to confirm this voltage is OK.
Edit: Update, Grey wrote back about the maximum voltage to the battery input spring is 4.2 volts but with a external power supply I applied 4.5 volts and it worked without a problem and produced 25% more power than at 4.0 volts, where my batteries normally like to sink to after a short amount of time. Perhaps the extra three tenths of a volt is abusing the input circuitry, but I have doubts that much more would cause a problem, given normal design parameters for drivers, they wouldn't be designed to just take 4.2 volts and no more or they fry once you pass the line, so I don't think 300mv more, or three tens of a volt more should cause a problem, for that matter, that much more shouldn't even increase the 532nm output power through a properly matched constant current regulator, but it does.
Because of the increase of power output with just a few tenths more voltage above 4.2 V, I believe the driver in the PLE Pro lasers are not well matched to the battery voltage being used. However, for all I know this was done on purpose to allow the laser to put out more power with fresh batteries, but the longer the laser is used, due to dropping battery voltage, the lower voltage reduces the heat stress on the diode and crystals inside which if operated too long at 4.2 VDC, could cause problems. If this is why the laser behaves this way, I would be very conservative in regard to the length of time the laser is operated if your input voltage is at or above 4.2 VDC due to running on an external DC power supply which is directly wired to the battery spring, or due to some kind of modification to the battery tube to increase the total voltage beyond 4.2 VDC. All of this said, I'm going to build a second half dummy battery and use an extension tube to hold it. The "half dummy" battery will be made from a half length 4.2 V battery with dropping diodes inside to reduce the voltage in that one battery down to a half volt, that in series with a normal battery will produce 4.7 VDC from freshly charged batteries, exceeding the manufacturers recommendation, but I think I will be safe if I don't let the laser play too long. Even then, the voltage will drop down to 4.2 volts when the batteries are nearly discharged.
The half dummy battery will need to be easily removable from the dropping diodes, so will have to be a normal half length battery which drops inside the dummy tube which contains the voltage dropping diodes. The half sized battery must have the same current ratings as the normal battery or you could have problems, perhaps an explosion, so all of this needs to be studied very well before playing around with this kind of mod. Can I get a battery with the same ratings as the primary battery, except for maH capacity? If I can't there better be a lot of extra head room as far as the current capacity of that smaller battery than what is being drawn, the PLE Pro laser I have draws about 2.5 amps from fresh batteries.
Edit again: Grey reported back regarding the voltage the PLE Pro must be run at for full output; 4.2 VDC. This is nearly impossible to obtain with 18650 or 26650 batteries, after charging all of mine sink down to 4.15 or so. Giving that these batteries will only stay at 4.15 VDC for a few moments when pulling 2.6 amps, the current this laser pulls on fresh batteries, you will never reach full power output capability that way. The above work around using two batteries, one of them having voltage dropping diodes in series with it to drop all but half a volt is the only way to get the unit at full output capacity and to do this, modding the tube will need to be done as well as a custom half dummy battery being made with the diodes inside.
Edit Jan 04, 2015: After much testing, I've found my Jetlasers PLE Pro likes to run at 4.5 VDC to the battery spring and at that voltage, with about 2.7 amps of current draw, will put out at and above 1 watt, up to 1.1 watt peak.
when under load. I wish they made these units to allow two batteries in series, that would solve that problem, but the driver inside most likely won't take double the voltage, as it is designed now. I'm not inferring I'm going to try that, no, no... 
