I have a Neewer NW670C flash which uses 4 AA batteries. I've been using regular alkaline ones so far because I couldn't get myself to buy rechargable ones yet. However, the flash ate their capacity in no time.
Now I got rechargable ones. Ni-Mh 1.2V 2000mAh. They are fully charged at 1.41V each, yet my flash won't power on. Not one blink, not even the LCD, not a "lo-ba" sign, nothing.
I've tested the regular alkaline batteries which are still able to power the flash (they are on the empty side though): 1.28V per cell. That's less than what my charged Ni-Mh's offer.
With 4 cells combined I'm looking at ~5.12V alkaline (works) vs. ~5.64V Ni-Mh (doesn't work).
Can I not use rechargable batteries inside a flash? Do I need a special type?
Another thing that surprises me is how external flashes tend to not have a 6V DC input. Can anyone explain why that is so? I understand the front power input is for ~230V to directly power the flash (and still requires you to have the batteries to power the circuits etc. of the flash), so that's out.
It'd be so much easier to simply plug a 6V DC power-supply. Why aren't we given such option?
A 6VDC power supply would actually have to be quite massive (think laptop rather than phone power supply sized), big speedlites can draw several amperes when charging.
There are a few older flashes that, as per the documentation, deprecate the use of rechargeables (eg the Metz 45CT); this should not be an issue with more modern ones.
The most likely cause is contact problems - the rechargeables might be just a bit mechanically shorter than your alkalines, or their contacts might have oxidised (which will cause problems with a high amperage load) - try cleaning the contacts (on the battery and on the flash)