Powerback for The DYI Photographer with electronics knoledge

Discussion in 'Lighting Equipment' started by rdm, Jun 20, 2014.

  1. rdm

    rdm

    I have a Metz 48 Af-1. I use it allot , so far I just carry 8 extra batteries, but that has become annoying. I know they do not make a powerpack module for this flash but i was thinking I could at-least make a cheep powerback.
    I have an engineering degree and I am very good with tools and repairs. I hardly ever modify beyond what the factory designed things for. Although I know Metz never intended this flash to be used with a powerpack, we engineers usualy build things with higher than specified tolerances. It might be able to take the extra power.
    Well this Flash problem go me thinking. I know I can make a power cable that fits into the battery compartment (using dowels with screw ends to make the power connection) and Maybe I can use a holder that takes 4 C sided batteries. I know the voltage would be the same as 4 AA batteries but the Ampes would be more.
    My thinking is 1 of 3 things could happen:
    1. I would just have longer battery life and the flash would heat up faster requiring me to turn it off and rest it for a few minutes every so often.
    2. I end up with a slightly faster recycle time and Would have longer battery life and need a rest every 10 minutes.
    3. The flash fires once (or a few times) then everything inside blows.
    OK, Place your bets...
     
  2. None of the above.
    1. The flash circuitry will almost certainly have regulation to prevent it taking more current than it can handle.
    2. The recycle time will be much the same - see (1)
    3. The flash won't know the difference between battery capacities - only their voltage counts. And again see point (1)
    I'm really not too sure what you're trying to achieve here Dan. There won't be much weight saving, since battery capacity and weight are fairly well linked. i.e. 4 C sized cells are likely to weigh nearly as much as eight AAs, and won't offer twice the mAH capacity.
    Also, the capacity of a cell in milliamp-hours doesn't necessarily relate to the amount of current it can deliver - that depends on the internal resistance of the cell, its type and construction. For example the old NiCd AA cells only had a capacity of around 500mAH, but their internal resistance was often lower than modern hybrid NiMH cells, and as a consequence they gave a faster recycle time with older and less well-regulated flash units.
    Proper high-voltage external power packs give a faster recycle time because they have their own inverter circuit built in that can deliver 360 volts or thereabouts to a flash designed to take such an input. Not because they simply contain more or higher capacity cells.
    So, would carrying 8 extra AAs be any more annoying than trying to knock up a reliable battery insert, cable and C cell battery holder + drilling a hole in the flash for the cable + the awkwardness of constantly carrying the external power pack? Plus again the fact that it'll take longer to recharge the C cells than it will to charge AAs.
    Alternatively you could buy a nice Nissin Di866 that takes extra battery inserts ready loaded for a quick change of cells. It takes about 10 seconds to swap the battery inserts over. The cells also go into the insert all the same way round - nice touch. And are you using high capacity hybrid type AAs? These last a lot longer between charges than standard NiMH cells due to their low self-discharge characteristics.
    Edit: "...we engineers usualy build things with higher than specified tolerances." Yeah, and then the production engineers downgrade everything to pare costs to the bone. You're lucky these days if a single component is overrated above the bare minimum needed to just work.
     
  3. As an EE with some experience in designing battery powered circuits, and specifically, having done something similar, I can tell you what will happen:

    - You'll get more shots from a set of batteries.
    - You *might* see an improvement in recycle time.
    - You'll need to let it rest, so you don't overheat it

    The C cells have more ampere-hours (energy) in them than the AAs. The cheap inverters in consumer-grade flashes draw a considerable amount (3 - 4 amps was what I measured on a Sunpak 433) of current as they charge the flash capacitor. The C cells (or a small 3Ah 6V SLA gel-cell, like I used) can deliver that amount of current with less of a terminal voltage drop. You'll see better recharge performance and more flashes per charge. However, you need to be careful, because many flashes are designed, assuming that the repetition rate will be low and the number of flashes in a burst will be limited. If this is not the case, the inverter may overheat. Bad Things will then happen...up to and including the release of the "magic smoke" which powers all things electronic.

    So, go forth, experiment, but be aware of the hazards. Check Digikey.com for 6V sealed lead-acid cells. The 7.2V maximum fully charged voltage shouldn't be an issue. I used a soft pouch with a shoulder strap (designed for a larger point and shoot) to hold it. A fuse in the positive lead, close to the battery, is strongly advised. If you paid close to list for your flash ($100+), you might want to experiment with something less expensive...like the aforementioned Sunpaks. You can get a several of them used off eBay for the price of a new flash, and there won't be as many tears if something goes wrong. You can also experiment with more efficient inverters (linear.com has photoflash control ICs), low voltage trigger circuits and TTL. These flashes are quite simple inside (and extremely cheaply built) so they're fun to hack with. Just watch the capacitor...400V can give you a dangerous shock.
     
  4. AFAIK, hybrid type (so-called "ready to use") C-size cells aren't available in mAH capacities any higher than those of AA-size cells; despite costing a lot more. IMO the value of having cells with a low self-discharge far outweighs that of pure capacity. But that does depend on how often you use them. YMMV, but if you go a week or more between use of the flash, then having hybrid type cells would seem to be a must to me.
    So if you need low self-discharge, then there seems little point in building a C-size battery holder, since the mAH capacity will be much the same. It appears to me that often when you buy a C cell, and especially a suspiciously lightweight D cell, all you're getting is an AA in an oversized wrapper.
     
  5. If you want to do this for fun, go ahead. But Quantum already sells rechargeable battery packs that are normal voltage (in addition to their high-voltage Turbos), and cords that can connect into the AA-contacts of a flash. If they don't make a cord that specifically fits your Metz, you could still do that half of the project yourself. New Quantum batteries are expensive, but you can find used and re-celled ones cheap.

    I wired up exactly what you're talking about several years ago with a cord and AA contacts from a rechargeable battery that had died (I forget the brand) and a C-battery holder from Radio Shack and used it with a Vivitar. Battery life was longer but I don't recall the recycling being any faster. No problems with overheating or anything like that. No reason it couldn't be done with D batteries for that matter.

    There was somebody about 20 years ago who sold a battery called the Underdog that was basically what you are describing but with rechargeable cells, and it fit neatly under a camera in the tripod socket. Some of the Quantums have a similar design but cost more. If you have the technical skill to make something like that and sell it for less than the big companies, you might have a profitable little sideline in your future.
     
  6. My 2 ct: a) A battery holder taking C cells is unlikely to get you anywhere. It has the same tiny metal springs as a AAA holder i.e. contact between the cells will be lousy and not allow high current. Get SubC cells for RC model stuff instead and a dedicated charger for those. it will safe you lots of hassle or do at least mod some powertool's welded together battery to power your flash.
    b) I am tempted to guess that a Metz designed for 4 AAs is rated for 6V so if you are strong enough a huge Dryfit Pb battery might work for you too or you could try using a 5 Sub C cell pack. - I believe I was told the 45'S NC pack held an extra cell compared to the AA holder. - I never had one so I don't know for sure.
    I ran Metz 60CT4 and 45s on strong ham radio PSUs adjusted to something in the 7V range they survived it and recharged fairly quick. I did surely over 1k pops with flashes powered up for hours but at less than full discharge and not at a machine gunning pace
    I believe the inner resistance of disposable AAs is slowing your recharge times down for sure. Metz claims their flashes shouldn't be fired at full power as fast as they recharges for a long time to prevent overheating them, but I guess this is all about the flash tube's heat in the head and not about the capacitor charging circuit?
    Bottom line no need to worry about anything besides contact issues in your battery case with your original plan. - Good luck.
    Disclaimer: I have no electrical degree at all. I have a bit of basic experience with RC cars and similar and read a book or 2 about automotive wiring. All I know for sure: With 6V systems you can loose a significant chunk of voltage with a few connectors and switches between battery and consumer... maybe you'll gain confidence to use the 5th cell once you got a Voltage reading at the end of your adapter cable under load.
     
  7. If I was doing this, I would make it with five cells and run it at 7.5 volts.

    The extra cell size on its own will give you more capacity but won't speed up charging time. A small increase in voltage will.
     
  8. ".. I would make it with five cells and run it at 7.5 volts." - Except that NiCd or NiMH cells only have a maximum voltage of 1.35 volts each. So the most you'll get with 5 rechargeable cells is around 6.75 volts - about what you'd get from a fresh set of 4 Alkalines. Even then I doubt that the slight extra voltage will buy you a faster recycle time. Modern flash circuits are far better regulated than the old 45CT-x and 60CT-x simple inverters. They have to be to accept Alkaline or NiCds or NiMH cells interchangeably.
    Unfortunately, the miniaturisation of modern flashes also means that they're more susceptible to overheating, and most decent ones have an overheat detector that simply switches off the inverter until things cool down, or they "throttle back" the recycle time to prevent overheating damage. Either way, they're not built to deliver endless maximum power pops at short recycle times. The only current flash that I know of that promises that is Nissin's MG8000, but it ain't cheap!
    There was a lot to be said for the big old unbreakable Mecablitz hammerheads, and that's why I've still got mine. All seven of them!
    BTW, yes, the Metz rechargeable packs did have an extra cell in them; precisely to make up for the lower voltage of NiCd cells over disposables.
     
  9. If you have electrical knowledge and can devise a power pack where the batteries are connected in Parallel rather than in Series then it might work. If I remember correctly batteries that are connected in Parallel should give you a longer life without increasing the amps, of course all the batteries would have to be the same voltage...
     
  10. If you use Alkaline C cells vs the alkaline AA cells what you would get it about 3 times the number of shots and about same recycling time. There is not danger. The mAH rating for the C are about 3 times as the AA. The internal resistance are the same.
    But if you compare NiMH then the story is different as most C size NiMH are actually AA cells in a larger cell and thus there is no gain. You can possibly get true C cell NiMH but they are usually available in sub C size. I didn't check the internal resistance which affect the recycling time and heating problem.
     
  11. Guys, I really don't think the internal resistance of cells makes very much difference to the recycling time of a modern flashgun. Anecdotes about flashes designed decades ago are a bit meaningless WRT this Metz 48 AF-1.
    If you stick an ammeter across the average rechargeable AA you can draw well over 10 amps from it - in fact it'll probably blow the internal fuse of your ammeter. Not so much so with Alkalines, they do self-limit as their energy runs down. But you've only got to time the recycling of a modern flash using various types of cell to see that as long as the state of the cells is good, then there's very little difference in timing between cell types or total voltage. All that happens is that cells with a higher mAH capacity can keep the recycle time down to its minimum for longer. Until the flash overheats that is.
     
  12. Is the Nikon SB900 considered a modern flash? Recycle speed is different for different battery types with the same nominal voltage. It also recycles faster with the lower voltage and less internal resistance NiMH than the higher voltage and higher internal resistance Alkaline-Manganese.
    I'd try running the flash of a Li-Ion battery pack. For instance something like this: http://www.batteryspace.com/Polymer-Li-Ion-Battery-7.4V-4000-mAh-29.6Wh-4.2A-rate-with-PCB.aspx
    And disassemble the flash and attach something like a powerpole connector directly to the flash battery terminals to avoid having to use battery wood dowels.
    The advantage of using a battery pack like that is that Li-Ion is lighter than NiMH for the same capacity. And less changing of the batteries which is nice when it's used off camera. Discharging batteries also produces heat which normally goes inside the flash (which is hot enough already) so it's better for cooling. Li-Ion is also very low discharge so they can be charged in advance.
    There are regulated 6V Li-Ion battery packs as well if someone worries about the 7.4V compared to 4 cells of alkalines (6V). I wouldn't worry about it too much since modern flash units have power regulation anyway and when testing I have fed hotshoe flashes both older and new even up to 9V without any signs of problems.
    Alkaline could actually show up to 1.65V per cell so the circuit in all 4xAA cell flash units must at least be able to handle 1.65Vx4 = 6.6V . The non rechargeble lithium AAs (LiFeS2) is also 1.5V but can be up to 1.8V so the circuit must be able to handle that as well in a modern flash. That's 1.8x4=7.2V
    Come to think of it , another option that would be a safer bet, is to use to the newer lithium iron phosphate battery packs, LiFePO4. They are lower voltage (3.2V per cell) so they will be 6.4V nominal voltage. http://www.batteryspace.com/lifepo4-battery-6v-5ah-30wh-10a-rate---replace-sla-with-5-times-longer-life-and-lighter-weight---un38-3-passed.aspx
    The LiFePO4 technology also have a really long life with many recharge and discharge cycles. Not as high energy density as Li-Ion but still pretty good.
    Also when it comes to overheating it's usually the flash tube that gets so hot that the front plastic melts and the tube can get damaged. It's probably possible to overheat the electronics as well (DC-DC converter that feeds high voltage to the flash capacitor) but that seems a lot less common.
     

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