How far is the heat limit from the power limit on speedlights?

Question

Sort of questioning the premise of What to look for in a flash battery pack?: Can a speedlight work significantly faster than on its internal batteries without overheating?

The reason I ask is that I've been running my speedlights on Gen-2 Eneloops. Sure, if I want to run full-power it takes a couple of seconds to recharge. But I imagine that if I cycled at full-power much more quickly I'd overheat the flash after not too many shots. My solution has been to buy the highest-power speedlights I can and run them at no more than 1/8 power. At that level even on internal batteries they can keep up with my 12fps body, and I almost never overheat.

I suppose that for shorter or less-frequent bursts it could be useful to shoot full power with fast cycle times. So:

1. How much faster can speedlights run on the best power inputs?
2. How long can they run at maximum speed before overheating?

I imagine this would be something like a table showing full-power cycle speed for each power source along with the number sequential full-power shots at that speed before hitting thermal limit.)

Answer 1

Unfortunately this is very difficult to answer.

I imagine this would be something like a table showing full-power cycle speed for each power source along with the number sequential full-power shots at that speed before hitting thermal limit.)

There is no table that can be found or created to display this kind of information, on a general scale. This is because of the specification and construction for every flash is completely different. The materials, quality and circuit type of a flash can greatly affect and change the number of possible shots before overheat.

In regards to buying the highest power flash, the most powerful guide number flash that I'm aware of is GN60/GN58 (at ISO 100) which is held by the the flagship Canon/Nikon flashes.

You might find that something like a Metz flash or a Godox Wistro might have more power, however as a consequence you sacrifice size and portability.

Just because you have a higher power flash dosen't necessarily mean that you can shoot at certain power levels for longer. This is a direct consequence of how the flash is designed to dissipate heat and channel the flash's power.

I would suggest that you try renting/borrowing several different flashes from various manufacturers and then comparing their performance.

You could of course try a more creative DIY approach which would involve adding external heatsinks and increased airflow in order to reduce the heat of the flash.

At that level even on internal batteries they can keep up with my 12fps body, and I almost never overheat.

By using the internal batteries of the flash,you are limiting the possible recharge speed of the flash. This will slow the flash down and not allow it to keep up with your 12fps body.

 Can a speedlight work significantly faster than on its internal batteries without overheating?

Theoretically a flash can work indefinitely if the optimal power level is found. Aka you have found the balance between the flashes power and heat created.

A worthy note is that continuous lighting can be incredibly effective. Especially when using lights that utilise LEDs.

Answer 2

Test protocol: Start with fully-charged power supply. Start with "cold" flash – i.e., having rested at ambient room temperature. Don't put modifiers on or in front of flash that could increase heat retention.

• Pops to overheat is number of sequential full-power pops that will trip thermal cutoff.
• Cycle time is time elapsed from start of test until cutoff divided by number of pops fired during test.

Pop flash as fast as it recharges until it hits thermal cutoff.

LightSource       Guide #    Power Supply      Cycle Time   Pops to overheat
----------------------------------------------------------------------------
Sony FM-43          43       Gen2 Eneloop AA     0.875s     39
Neewer TT 660 II    58       Gen2 Eneloop AA     2.250s     60 [Note 1]

• Note 1: Stopped test when flash began to emit burning smell. Manual claims it has thermal protection but also warns against more than 10 sequential pops without allowing 15 minutes to cool.