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brokeh
24th of June 2010 (Thu), 08:55
I was planning on using the BG-E7 with 6 x 2500mAh 1.2V NiMH rechargeable batteries in the BGM-E6 carrier. 6x2500mAh = 15000mAh. I thought it might increase my runtime versus using 2 LP-E6 = 2 x 1800mAh = 3600mAh. Anyone have any experience with this?

$20 for 10 rechargeable AA batteries seems like a better deal than 2 LP-E6 batteries. Basically pays for the battery grip, and (hopefully) gives an extended run-time.

Jon
24th of June 2010 (Thu), 10:12
No, 6 A cells won't give you 15000 mAH, at least in that usage. mAH is affected by the voltage; since you use the AA cells in series, you're only getting 2500 mAH from them at 7.2 V output. If you wired them in parallel, you could get 15000 mAH, but at 1.2 V, which wouldn't run the camera.

The formula is E=IR, where E is voltage, I is amperage, and R is resistance. The resistance of the batteries is constant within a battery and cumulative over the series when you have a series circuit. Likewise, voltage is additive in a series circuit. So as you increase the voltage 6-fold through a series circuit, you also increase the resistance 6-fold. So the amperage will remain constant.

brokeh
24th of June 2010 (Thu), 10:54
Aren't you confusing energy (mAh) with power (A)?

kokakaste2
24th of June 2010 (Thu), 11:12
As far as my knowledge of electronics goes, Jon sounds right.

http://ctaps.yu.edu.jo/physics/Courses/Phys102/Phys102_Web_Resources/images/How-batteries-work/battery_series_parallel.gif
Think this might explain difference. :)

ecub
24th of June 2010 (Thu), 11:22
I don't know anything about the science regarding the batteries, but one thing to consider is, NiMH batteries do not have the best low discharge rate. Plus charging usually takes several hours for a NiMH batteries, compared to Li-Ion batteries. You can use 1 hour quick chargers for NiMH batteries, but I understand those wear out a battery faster.

brokeh
24th of June 2010 (Thu), 11:53
As far as my knowledge of electronics goes, Jon sounds right.

http://ctaps.yu.edu.jo/physics/Courses/Phys102/Phys102_Web_Resources/images/How-batteries-work/battery_series_parallel.gif
Think this might explain difference. :)

He's right about the volts. And I think he may be right about the amps (electromag was never my strong suit). But I was talking about amp-hours, which is a measure of energy, and he was talking about amps, which is a measure of power. I'm pretty sure amp-hours are additive in this way; it's why you can run the 7D 2x longer with 2 batteries.

Regardless, all I really care about is the run-time of my 7D with 6 2500mAh NiMH AA cells in it. Does anyone have any experience?

brokeh
24th of June 2010 (Thu), 11:59
I hope I didn't come across as rude. It's just that I've seen a lot of threads here veer sharply off course in to a ditch, crying, bleeding. Just trying to stay on target! :)

brokeh
24th of June 2010 (Thu), 12:22
Pulled out my old physics books, and Jon is correct. Batteries in series only have the mAh of a single battery. My apologies.

kokakaste2
24th of June 2010 (Thu), 12:33
Could be could be, last time I done physics was few years ago. :D
As far as I understand it the battery packs give twice as long life because they are two, like in the link before they are linked as shown in part 1 (2x1800 = 3600). While 6 rechargeable batteries have to be linked as in part 2 to make 7.2V (so the 6x2500 equation does not apply)

edit - Im too slow at typing :(

Jon
24th of June 2010 (Thu), 12:53
Aren't you confusing energy (mAh) with power (A)?
All one Amp-hr is is the ability of your power source to produce a 1 amp current for 1 hr. If you can't get 15,000 mA out of the battery it's not going to be able to produce 15,000 mAH worth of current.

brokeh
24th of June 2010 (Thu), 13:53
All one Amp-hr is is the ability of your power source to produce a 1 amp current for 1 hr. If you can't get 15,000 mA out of the battery it's not going to be able to produce 15,000 mAH worth of current.

Yeah, stupid of me. With two batteries, you either double the volts or amps, but not both. Otherwise we could solve the world's energy crisis. Hook up two batteries and get four times the power. Anyway.

zebek
24th of June 2010 (Thu), 23:06
so number of shots approximately?

Jon
24th of June 2010 (Thu), 23:08
Not a lot, since the power curves of NiMH and Li Ion batteries are different. The AA capability is intended as an emergency measure, not a viable full-time alternative.