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?
Update (2019-02-17): Thanks to everyone for the great answers. My local drug store recently got some new rechargable batteries which caught my eye instantly. They were really cheap ($5 for 4pcs) and what was on the box looked promising: 2.400mAh and labeled "for use with high current applications". Even quick charge supported! They are regular NiMh 1.2V ones.
Thinking I can't go much wrong with that price, I bought two packs - Ran tests on them with my battery tester - They actually have 2480mAh. Tried them in the flashes: Voilá. I've now had a guessed 300 flashes on 1/16th power since the last charge, still no low battery warning.