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Old Aug 23, 2002, 9:42 PM   #1
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Default Cheap & simple power for D7

After investigating several power sourced for my D7 I read some threads on another message board that lead me to a simple solution, a 7.2v NiMH RC car battery from Radio Shack. One of the users tested current flow and various voltages and found hey vary in opposite directions which indicates a switching regulator in the power supply. When you provide more voltage the PS draws less current, the opposite of a plain resistance load. The nice thing about a switching power supply is that it's very efficient , not wasting overvoltage power as dissipated heat. This is why one user got such long life using a 6xAA external pack.

I stopped by my local nurd central, AKA Radio Shack and found all the items I needed to make a plug and play external pack. I'll worry about niceties like a carrying case later.

The battery is not cheaper than AAs on a cost per watt/hour basis but close. I think the main advantage will be not fumbling with loose batteries in the middle of a shoot and risking losing a critical shot. The pack is 3000 mAH @7.2v = 21.6 watt/hours or a smigen less than three sets of 1600mah AAs. After I've had it through some charge/discharge cycles I'll setup some objective tests.

If anyone wants to give it a try here's a list of the RS part numbers and what I paid locally.

7.2v batt $34.99 23-431
5 hour charger $10.99 23-333
6' F to F Adapt a plug cord $3.29 273-1739
Adapt a plug B $2.19 273-1709
Adapt a plug lead $1.99 273-1742
7.2v pack repair connector $1.99

No reason you couldn't build your own battery pack. a set of Maha 7500mah NiMH D cells would be the equivalent of 7 sets of AAs.
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Old Aug 24, 2002, 11:23 AM   #2
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Default Performance test

A new NiMH battery should be cycled a few times to get peak performance. Still I took the new battery and plugged it into the 5 hour charger for about five and a half hours. The charger is simple with a switch for NiCad and NiMH and an indicator light for charging each type. After charging it was around 8.4-8.5v.

I plugged it into the camera and started napping pics. I decided to maximize drain by setting it to autofocus, full manual flash, LCD on with 2 second review. I plugged in the remote and started snapping pics while I was watching TV. The grip got warm under sustained use but it does that with AAs. Sometimes I'd let it cool down and pick up a few minutes later. 556 fine quality JPGs later the microdrive was full. Battery reading about 7.6v.

For grins I decided to dump the whole microdrive to disk using the USB port on the cam. Windows said it would take well over an hour but I went to bed and left it chugging away.

This morning I fiddled with the camera some more and formatted the microdrive, still using the RC pack. I set it to interval and let it take pictures by itself, same mode as last night. Battery is just now dropping below 7.2v. Fluke 12B DVM reads 7.19 with no load and drops to 7.07 during the high drain. I'm going to leave it like that until it gets to 1.1v per cell.

This is working out even better than I hoped. With normal battery conservation I'm sure I could fill the microdrive three times or more. I'll use my stash of AAs for the NiMH converted battery racks in my Metz flash and keep two sets aside for when I don't want the external pack on the Minolta.
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Old Aug 24, 2002, 9:11 PM   #3
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Default Good Idea!

Thanks Padeye

I like this one, compact battery with dedicated quick charger at reasonable cost. Even in case of problem , just unplug and continue on the AA's, double security.

I saw all the parts in Radio-Shack site, but I can't find dimensions and weight of the battery pack , could you post them please ? will it go in a cell phone case ? ( which is perfect with the belt clip )

Will you add also a led voltage monitor as well ?

Please post some photos ok?

(PS: For the Metz , I always have an alkaline rack ready , even the NiMH and my other NiCad could run a long way.)
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Old Aug 24, 2002, 9:49 PM   #4
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The cells are sub-C arranged in two parallel columns of three... I'm in the middle of moving so of course I can't fine a ruler. ....Okay, found something. The pack is about 5-1/4"x1-7/8"x1" plus the pigtail coming out of the end which is 3-1/2" long Might fit a cell phone holder but good luck finding one. My postage scale is at my fiancee's house so I'll get the weight to you later.

BTW if you use the 9.6v pack repair leads. 23-445, to make your connector you'll find the plugs are exactly the same as 7.2v but it uses thinner gauge wire which is easier to solder to the thin wire of the adaptaplug pigtail. Also if you use a coiled cord use the ten foot F to F cord, 273-1739. The coils are tight and have a lot of memory so the six foor coil cord is too short to be useful.

I have to get moved in less than a week so this project is going on hold. I may add a voltage meter but I'll have to make one with a custom scale to make it useful as voltage doesn't decrease in a linear fashion like lead-acid batteries.

Other possibilities pop to mind Like my Metz flash or panasonic camcorder.

One thing to note, the adaptaplug ends don't have the retention groove. I may rig a little clip so it desn't fall out so easy. Maybe something that attaches to the tripod socket and wraps around the plug.
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Old Aug 25, 2002, 2:04 AM   #5
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Thanks for all the infos, it will be very tight in a cell case, but do-able. and happened that I thought about coiled cord too.
Happy moving !
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Old Aug 25, 2002, 6:51 AM   #6
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That's really cool Padeye, and off the shelf from RadioShack too!

You've also confirmed the 550+ frames with the microdrive on the new upgrade. It depends on what one shoots, but I've see 580-600 terminal counts sometimes with the original v1.22 version.

BTW a leather cell phone holder works very well for the R/C battery pack...
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Old Aug 25, 2002, 9:21 AM   #7
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Okay, I just want to get this absolutely straight, before I do something irrevocable.

You are taking the 7.2V from the RS battery pack straight into the camera, even though the camera operates on 4.8V internal AA's and says 6V external supply.

Nothing to reduce the voltage to 6V?

Just want to make sure I haven't missed something here.
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Old Aug 25, 2002, 10:35 AM   #8
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You heard right. Actually the pack has around 8.5 volts when freshly charged. Minolta's own external pack has 7.2v Li-ion camcorder type batteries.

I don't have a D7 scematic but this works because likely because of a switching power supply. This is a very small, light and efficient type of voltage regulator. It switches the input on and off rapidly so that the resulting pulses can be averaged out with the help of a capacitor to give the voltage you want. It draws a constant power from the pack, one of the reason the 7.2v packs last so much longer, the current flow is far less than from the 4.8v from the AAs. There are limits of course but so far it seems we're well within them.

NHL, I got about 660 frames, nearly all taken with a full power flash plus downloading the entire MD to computer via the USB cable.

Say, I don't have my camcorder stuff handy just now. Can anyone check the voltage of a freshly charged Li-ion battery for me?
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Old Aug 25, 2002, 10:46 AM   #9
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If you wanted to stay within lower voltage limits you could easily build a 5 cell pack with a nominal voltage of 6v. Fully charged it would be a shade over 7v and discharged at 1.1v per cell would probably still make the camera work. With the 3000mah cells radio shack sells you'd lose 17% of the power in the RC pack but you could get Maha 3500mah C cells and have about the same capacity or a little more.
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Old Aug 25, 2002, 11:18 AM   #10
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I haven't checked it lately, but the 7.2V Canon BP-915 Li-Ion measured about 8.4V and that's what its charger output anyway...

[Edited on 8-25-2002 by NHL]

Both 7.2V Canon BP-915 and BP-914 Li-Ion put out 8.40V when fully charged! My guess is the Minolta's ones would be about the same...

[Edited on 8-25-2002 by NHL]
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