Electronic – ‘Stiffening’ capacitor/s for DIY flash battery pack

capacitorcharging

I'm planning to build a DIY battery pack for one of my camera flashes that involves basically wiring a bigger external battery to the battery leads in the flash. Admittedly I don't know much about electronics but keep my feet just oh-so wet in it.

The main goal for doing this is actually to improve recycle times of the flash with the added benefit of longevity from the battery.

Question #1 is: Whether or not there would be any benefit to adding a capacitor or two to provide a limited rapid-charge for a quicker, subsequent re-charge after the flash goes off the first time.

My rationale is: often the flow of taking photos involves a flash or two; or three; change spots/framing/talk to subject (which by then the capacitor/s might be recharged to supply the benefit again)

Question #2 is: Is there a formal word/phrase for what this process (a capacitor used for rapid discharge in tandem with a battery) is so I can do further research for academic purposes.

More details:
The battery supply is 6V – there's a variance on how much the flash is willing to draw based on the supply (4AA's vs. the bigger battery mentioned in my links); I don't know what, or if there are limitations on what kind of current the flash can draw (hence if it can benefit from caps). There's an external AC adapter that supplies 200V@4mA for the flash.

Best Answer

I doubt that there would be much benefit to adding a capacitor across your battery. Consider the following:

  • The charging of the flash can be seen as a process of moving energy from the battery to the high-voltage capacitor in the flash.
  • Based on some quick searches, the capacitor in the Vivitar 285HV is 1000 uF. Assuming it's charged to 200V, it stores 20 J of energy
  • A capacitor to hold 20 J at 6V would be 1.1 Farads, and that's just enough energy for a single charge of the HV cap. Assuming 50% conversion efficiency, and that we want 3 shots that's a 6 F capacitor. The least expensive capacitor I found with some margin over the 6 V working voltage and with that kind of capacitance was over $100, and 8 inches long.
  • Just to supply 20 J directly from a 6V battery in 1 second with 50% efficiency would mean a current draw of about 6.7 A. That's a lot of current, but it's something that a large battery can do.
  • The charging circuit in the flash will limit how quickly we can charge no matter how much power is available at the input. An easy way to see if the charging circuit is the limiting factor rather than the input power is to measure the voltage across the battery while the flash is charging. If the voltage remains close to the open circuit voltage of the battery, then it's the charging circuit that's limiting, not the battery.
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