Nikon SB800 flash and 18650 li-iron battery

I try use 18650 li-ion battery vs cable. my question is 4.2v or 8.4v ? I know SB800 have 7.5v (5x1.5v) before.

The SB800 should nominally work on 4.8V, 6V, or 7.5V. With Li-ion I'd probably target 4.8V.

Honestly, though, if you're already going to an external pack anyway my inclination would be to use the high voltage input on the front of the SB800. The Nikon SD8a takes 6 AA sized cells(7.2V or 9V depending on whether you're using alkaline or rechargeable-I've used both in it) and contains and inverter/transformer that feeds the flash capacitor directly with ~330V. 8.4V fed into this would PROBABLY work fine.

If you really want to risk not blowing something up, though, first of all both Quantum and Lumedyne make their own external battery options that feed the high voltage input(Quantum uses the "Turbo" branding for HV packs, and there are about a dozen different ones). Lumedyne, which is a newer company than Quantum in this area, was smart enough to make their battery packs work with Quantum cables, although they sell their own also. Both use a DIN-type plug on the battery pack end. There are a few Quantum PNs I'm aware of that are SB800 compatible-the CKE, CKE2, and CCKE are all compatible. Lumedyne's set-up with the Megacycler(HV supply) is a bit clunkier than Quantum's, as the battery is a separate unit in the Lumedyne system. They offer 3 different sizes(small, medium, and large) in 3 different chemistries-NiCd, NiMH, and Li-ion(the latter is something Quantum still AFAIK won't do). Where this is relevant to you, though, is that Lumedyne actually sells empty battery boxes so that you can build your own battery packs if you're so inclined.

So, a Lumedyne Megacycler+an external battery box that you add your own cells into would probably be the best solution. I don't have a TON of experience with Lumedyne other than having a Megacycler and a single small NiCd pack(that needs a rebuild). The small Ni-Cd pack contains 9 Sub-C cells, or 10.8V. You MIGHT get away with 8.4V of Li-ion in one, or you might be able to go to 12.6V. Given that Lumedyne sells the empty boxes, I'd guess that they would probably be forthcoming with information on what the Megacycler can handle and how to configure it if you wanted to ask. I've not had a reason yet to interact with Lumedyne-I quite literally have had my Megacycler just a few days now and it just happened to be a chance purchase that came with a Metz 45 CL-4 Digital and I was mostly buying for a good price on the rest of the stuff.

Just as another off-the-wall thought that occured to me-the original Quantum Turbo(which I LOVE, BTW, even though they're heavy) is designed for a type PS-832 8V gel cell(lead acid) battery. Even though lead acid is often stated at 2V/cell, the "real world" accepted voltage for a fully charged one is 2.2V/cell(and gel cells often want to be charged at more like 2.3-2.4V/cell-I normally don't consider a 6V lead acid gel cell fully charged until it draws .01C connected to a 7.2V PSU). I say all of that to say that an original Quantum Turbo would probably work great on 8.4V, and a couple of 18650 cells should fit nicely in place of a PS-832 battery. You'd want to run an external charger for it since the charger is definitely designed for an 8V lead acid-it's been a little while since I've played with the charger in one, but IIRC it is constant current at around 250mA until batttery voltage hits 9.4V, will sit there a little while until current drops to ~30mA at 9.4V(this can take ~16 hours depending on just how dead the battery was when you started), and then drops to a float voltage of 9.2V for as long as you leave it plugged in content to be plugged in for literally days at a time supplying(I think Quantum says up to 72 hours). If you do this, you'd want to run a pigtail outside the case for an external Li-ion charger. The original Turbo with a dead battery(assume it's dead unless the seller has tested and/or recently replaced-find one that already has the battery removed to save on shipping) is dirt cheap. I've bought them in bulk for like $10. A PS-832 battery has a capacity of 3.2Ah(the batteries Quantum fitted when they first introduced it were 2.5Ah-technology has improved and the Powersonic PS832 is now their service replacement-I sometimes fit those or other less expensive brands since the PowerSonics are now $40 each and I can buy equivalent in bulk for ~$18-20 each). I'm happy to test one with a bench PSU if you want to go this route but I bet it would work fine on 8.4V without any issue other than the built-in power gauge maybe being inaccurate and artificially reading low. The PS-832 is a snug fit in the case, but definitely fits. Without measuring but just visualizing in my head, 2x 18650s should fit end-to-end in the space of it, and you could probably stack at least sets of 18650s and run in parallel if not 3 or 4 in the available space-that would give you a LOT of power on tap-potentially 2-3x what the lead acid offers.

The nominal voltage of a single Li-ion cell is 3.7 volts on load. 4.2 volts is given as the maximum charging voltage and would only be seen for a short time as a terminal voltage on a freshly and maximally charged cell off load. 

Two series connected Li-ion cells with a nominal terminal voltage of 7.4 volts should be fine to feed into an SB-800. However, you could drop the voltage by putting a 10 amp rated silicon rectifier diode in series with the cells - just to be utterly safe. 

The diode will drop about 1 volt at the ~ 8 amp draw of the speedlight during initial charging. Giving a voltage at the speedlight of between 6.4 and 7.8 volts, with the latter being an absolute maximum with freshly-charged Li-ion cells at no load. 

Internet told that SB-800 is faster to cycle with fresh nimh:s than alkaline batteries, one used to get nimh:s like grocery stores, why not to stick with that? I have tried and get like three passable exposures without batterypack, then pause for recycle