Are you planning to take 7.2 baterries and use a register in series to drop down the voltage whatever your flash calls for?
I have a 45 something (I'm at work) and as I recall, it takes 6 batteries, which makes it 9 volts. It also has a different battery holder for rechargables.
Please know that using registers for this purpose works very poorly. The voltage drop across the register will be current dependent, so when the flash is charging at full, the voltage will drop more and when it's fully charged, voltage drop will be zero - meaning full battery voltage will be applied to the flash unit.
Also, whatever the voltage register drops will be dissipated as heat. It can heat up significantly.
I think you mean resistors, not registers.
Your right about the batteries. I forgot it took six AA. So three lithium ion batteries wold be 11.1v. Finding the right size is going to be tough.
Resistors would have to be replaced so that the capacitors and strobes are protected. Increasing the resistance right behind the strobes (my flash has two) would decrease the voltage back down to a safe level.
It's only a 2v increase so I'm not too worried about the capacitors.
Personally, I wouldn't worry about it. I would happily run a flash expecting 9 volts on 11.1 volts. A resistor won't work to drop the voltage as the voltage drop across it will vary with current. If you set it so it drops 2 volts at the start of the charging cycle, when it is fully charged it will hardly drop any voltage.
Three silicon diodes in series would do a better job dropping about 0.7 volts each.
I've pretty much given up on using rechargeable AAs except in a digital P&S. I generally use the Costco (US Warehouse store) brand for AA and AAA. I avoid other off brand batteries in anything where the battery stays in the devise and can leak. Though I have had a number of Ray-o-vac batteries (left in a flashlight) leak.
I do like the Pro-cells (made by Duracell). I used to get bunches of "used 1 time" Pro-cells free (they where used in recording and they would use a fresh battery in each session). I found that they had a very long shelf life and I never experienced any leaking.
Of course, for all I know, all the batteries come out of the same factory in China.
I've had a couple of small devises come with multiple generic alkaline button cells where 1 cell depletes much faster than the other cells (used concurrently) in that devise. I attribute that to inconsistent manufacturing quality; but I can't back that up with any actual testing.
"Far more critical than what we know or do not know is what we do not want to know." - Eric Hoffer