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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?

Although NiMH are charged at 1.4 or so volts, they quickly drop to 1.2 under load. However, unlike alkalines, they stay around that 1.2 for a long time (this is called a "relatively flat discharge curve").

Alkalines by contrast have a much steeper discharge curve but do present a higher terminal voltage when new.

Either way, your flash seems to be absurdly sensitive to battery terminal voltage. No doubt the reason your flash "uses up" the alkalines so quickly is that they have dropped below the voltage the flash is happy with. There's likely still plenty of energy in them.

Conclusion: Maybe the flash is defective but I bet it's just a poorly designed unit. I have never had a flash that failed to work very well with 4xAA NiMH cells. This includes camera-maker-brand flashes "dedicated" to my camera, other-maker flashes also "dedicated" to my camera, and some "vintage" Vivitars that just do "auto-thyristor" mode with their own sensor.

Aside: I wouldn't trust Ikea brand batteries of any type. Nor any other store brand. Big chain stores pay the real battery manufacturers to wrap their store name on whatever batteries are cheapest this month. otoh I've had absurdly good results from not-nationally-advertised-brand alkalines, and far less leakage than from one of the best-known name brands. Your results may vary.

For NiMH cells, from my experience, I'd try Panasonic (Eneloop or not, your choice, Eneloops are low self-discharge but lower energy storage), Tenergy, or Powerex (Maha). Powerex/Maha also make some of the best NiMH chargers available.

I use them all of the time in several different hot shoe mount flashes. The only difference I've noticed is that when I put in a set of fresh alkalines, the battery level indicator shows 'full'. When I put in a set of freshly charged NiMH batteries, the indicator does well to show three out of four bars. However, a set of fully charged NiMHs that show three bars will last longer than a set of alkalines that show a full four bars.

The Neewer NW670C is listed as capable of using Alkaline or NiMH batteries. I would start by trying a different set of NiMH batteries from a different source. If you have the capability to test actual battery capacity, my hunch is that your 2000 mAh batteries aren't really 2000 mAh. There are a LOT of cheap NiMH batteries in circulation that have vastly overstated capacities printed on them.

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)

If anything, rechargables should be better than alkalines because they can deliver more peak power. But that is a problem too, because they could cause a fire. Cheap toys refuse to accept rechargables because it would be unsafe. How do they know ? The + cap of a rechargable is a bit wider than a regular battery, so with a bit of extra plastic around the + of the toy it won't make contact. That extra plastic is there for your safety.

I find NiMH batteries to be unreliable. If you haven't taken good care of them then they won't hold much of a charge and won't deliver much power. A good charger that can do repeated discharge-charge cycles can tell you about the remaining capacity of each cell. One bad cell in a set of 4 can make the whole pack useless. Always bring spares, and spares for your spares.

The older NiMH batteries have a high self discharge, you can't keep them in your bag for many months. The newer low-self-discharge ready-for-use types (Eneloop etc.) have a lower capacity. I have seen failures with both types, or maybe I just need to upgrade to a better charger.

Finally, the discharge curve of rechargable batteries is different from single-use batteries. They hold their voltage longer, but at the end it drops more abruptly. There is little or no warning that they are almost empty, it comes as a surprise. Again, bring spares, even keep a set of fresh alkalines in your bag.

This is one of the most ignorant answers I have ever written, but I will keep it posted in case it helps somehow.

In my opinion, it is not about the voltage, but the mAh.

I only use 2500mAh batteries (1.2V). I tried once 1800mAh(same brand) and they did not work.

According to my experience then, a 2000MAmp battery is too close to the ones did not work for me. :o(

I am sorry I can not give any technical explanation, just experience in my case.