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Aaron D wrote:
Li-Ion and Li-Po are low internal resistance, thus allowing huge current discharges. I would think this would be ideal for a flash...
Then again, maybe that large discharge is more than the flash can handle?
Possibly no need for charging a capacitor in future flashes. LiIon and LiPo, being able to discharge such large currents, could possibly mean no need to wait for a flash to "charge" between flashes.
My electronics knowledge is a little rusty and outdated. Maybe someone more knowledgable can chime in on this.
The lithium-metal primary (single-use) cells are not the same as Li-Ion or Lithium-Polymer type batteries. In the latter two, under normal operation, the lithium exists only in ion form. It only becomes its dangerous metal form if there is a fault or the cell is mis-used (high temperature, overdischarge, overcharge, etc.). So these batteries, especially if designed for it, can endure high-rate discharge safely. Typically, as long as the temperature stays below 50-60C.
Lithium-metal cells, while they have a low internal resistance, can also become extremely dangerous at high temperatures. So, without a temperature monitor of some type, it's dangerous to use them in applications where the battery could experience continuous and prolonged high-rate discharge.
I know from my own experience, even when using eneloops, which are low internal resistance cells -- if I fire the flash constantly and at high power for a period of minutes, they can be too hot to touch when I have to change them out. So it's feasible the same type of thing could happen with the lithium-metal cells, and these would be much more dangerous when overheated (too hot to touch is getting into the range of 70C or more).
Also the lithium-primary cells, while they may start at 1.5V, the voltage may actually rise as the cell becomes discharged (to the point of about halfway or two thirds discharged) and as the cell rises in temperature. Though from this datasheet it looks like the new cells start as high as 1.8V for low current loads.
http://data.energizer.com/PDFs/l91.pdf
Also of interest, on Page 2 of the datasheet the internal resistance profile is close to 0.12 ohms, but rises significantly, above 0.4 ohms, between the rate of discharge of 2400mAH and 3200mAH. So this is another reason not to use it in your speedlights, where the rate of discharge can exceed 10A. For comparison, eneloop internal resistance is around 0.025 ohms.
And the other reason for the capacitor in a flash tube is because of the high voltage required to fire the flash (hundreds of volts for a xenon flash tube). A circuit generates high voltage (typically a flyback/inverter-transormer circuit) that charges the capacitor, this is why especially some of the older flashes would have a high-pitched noise as they charged.
Edited on Feb 27, 2015 at 04:16 PM · View previous versions
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