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Old Jul 21, 2006, 12:05 PM   #1
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I have aunusual application, and I'm curious about calculating run time on a device. No, it's not a camera; it's a small depth/fishfinder that I'm installing on a kayak.

I plan to make a couple of power packs out of 1.2V, 2300mAh NiMH AA batteries slid into 1/2" PVC pipe (contacts on each end, spring on one end, contact wires coming out of sealed holes). The purpose of the PVC is to make the unit as waterproof as possible, as well as making it easy to attach to theinside of my kayak.

Operating voltage for the depthfinder is 10 to 17 volts. So, for each power pack, I'm planning to use 10 AA batteries, 12V total. I found a smartcharger that will charge 10 at a time, so I should be set in the overnight-recharging department.

My question is this: How long can I expect to get out of apower pack? According to the specsin the depthfinder's manual, current drain is 170 mAh in standard use and 240 mAh when using the backlighting at night. Won't be doing much night fishing, but one could perhaps use 200 mAh as a rough estimate to be on the safe side.

Are the mAh units cumulative the same way the voltage is? With 10 batteries at 2300 mAheach, does that add up to 23000 mAh? That just seems way too high to me. (115 hours of use at a current drain of 200 mAh?) Or do you simply divide 2300 by 200, which comes out at 11.5 (hours?). The last figure seems more reasonable, but I'm still not convinced it's right.

I'm a little lost here, given my ignoranceof electronics and current. Any help would be appreciated.
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Old Jul 21, 2006, 4:09 PM   #2
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Parallel connection always increases only capacity while serial connection always increases only voltage.
And discharge current is such small that you should definitely expect to get full stated capacity out of batteries. (even more if capacity is stated for high discharge current)
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Old Jul 21, 2006, 4:33 PM   #3
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Hello Guyon,

As E.T has pointed out you increase voltage when the cells are in series and increase capacity when they are in a parallel set up.

Your 10 cells in series will give you a 2300 mAh 12 volt pack. When I do these types of estimates I take 90% of the rated capacity, to allow for a "less than perfect world," and go for the maximum rated current draw.

In your case we would have 2070 mAh at a 240 mA drain which works out to a little over8.5 hours.

Keep in mind that the battery capacity is determined with a discharge down to 0.9 volts. Your device may utilize a low voltage cut off higher than that, which will result in less usable capacity.

I would figure 8 hours and then do some testing to see how it works out.

Tom
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Old Jul 21, 2006, 4:38 PM   #4
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E.T wrote:
Quote:
Parallel connection always increases only capacity while serial connection always increases only voltage.
And discharge current is such small that you should definitely expect to get full stated capacity out of batteries. (even more if capacity is stated for high discharge current)
Uhmm... could you do that again, in English? :G

(see note above about my electronics knowledge gap)

I'm going to take a crack at decoding here and assume that what I'm doing (lining up the batteries end to end) is called serial connection. Good so far?

So... my voltage is cumulative (1.2V x 10 = 12V), but my capacity is not (stays at 2300 mAh). Right?

Does this mean that my 11.5 hour estimate (at a 200 mAh drain)is correct?

Thanks!
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Old Jul 21, 2006, 4:42 PM   #5
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SilverFoxCPF wrote:
Quote:
Hello Guyon,

As E.T has pointed out you increase voltage when the cells are in series and increase capacity when they are in a parallel set up.

Your 10 cells in series will give you a 2300 mAh 12 volt pack. When I do these types of estimates I take 90% of the rated capacity, to allow for a "less than perfect world," and go for the maximum rated current draw.

In your case we would have 2070 mAh at a 240 mA drain which works out to a little over8.5 hours.

Keep in mind that the battery capacity is determined with a discharge down to 0.9 volts. Your device may utilize a low voltage cut off higher than that, which will result in less usable capacity.

I would figure 8 hours and then do some testing to see how it works out.

Tom
Tom, you posted while I was replying.

What do you know? I was right!

Thanks for the estimate rules of thumb. 8 hours on the water is a looong time, and one power pack should be plenty. Still, I picked up 24 Lenmar batteries at a good price, so I'm going to rig up two power packs. If the first goes out, I'll have a spare.

Thanks.
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