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Old Jun 11, 2007, 12:06 PM   #1
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Over in the "Pentax / Samsung dSLR" forum...

http://forums.steves-digicams.com/fo...mp;forum_id=80......

penolta wrote: ..........Batteries arranged in series are also drawn down differently depending on their position, so changing their position on reinsertion can also make a difference (try reversing the sequence of the batteries in your flashlight, radio, or tv remote control the next time they run down, and you will see this pheomenon for yourself).............


This is rather a surprise to me. Would any experts care to comment? I'll have my say later.

Alan T
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Old Jun 11, 2007, 1:43 PM   #2
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It's been too many years since I took basic electricity but I do remember that all elements in a series circuit had the same current flow regardless of the position. I can see some cells not having the same capacity and failing sooner but I don't understand why the position in the series would matter. Maybe I slept through that topic?
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Old Jun 11, 2007, 1:52 PM   #3
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Bob Nichol wrote:
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I don't understand why the position in the series would matter.
I don't either, but I've been doing it for years with flashlights and remote controls until I can get the batteriesreplaced. It won't last long, but it can keep you from tripping over something in the dark. Try it before you say it can't happen.
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Old Jun 11, 2007, 1:59 PM   #4
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Battery capacity ratings and internal impedance are averages; every battery has individual variations. So even if the device drains all batteries equally the batteries won't all reach zero simulaneously. There will be aweakest battery that goes first.

With NiMH batteriees, when this happens, the other batteries (which are still alive) can drive the dead one into reverse polarity. This kills the battery dead - it cannot be recharged anymore. Intelligent chargers will sense this and refuse to recharge the battery. It can only happen when the batteries are connected in series, but that is how batteries are usually connected in most devices. I believe this effect is exclusive to NiMHs - it does not happen NiCD or Li-Ion. So if you use NiMH batteries it's good to drain them low but not to run them absolutely to zero.

Most devices have a voltage sense and shut themselves off when the voltage drops below a threshold. This should protect NiMH batteries. Also, the few devices I've poked around in have a DC-DC converter that supplies a constant voltage to the device even as the battery voltage slowly drops.

Capacity also depends on current draw, so the rated capacity is only effective for certain applications - high current draw usually reduces the capacity. NiMH and NiCD rechargeables usually perform better under high current draw than alkalines, so in some applications they actually last longer than alkalines.

As everyone knows, NiMHs lose their charge rapidly when sitting on the shelf. Much faster than alkalines. But there are new NiMH formulations designed for long shelf life. Often called "low discharge", they typically have lower capacity - in the 2000 to 2500 mAH range compared to 2500 to 3000 for a normal NiMH AA - but they only lose 1-2% per month compared to normal NiMH which lose something like 30% per month. These batteries usually have better performance at high current draw too.
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Old Jun 11, 2007, 3:39 PM   #5
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Well, I'd like to know by what means a cell is supposed to knowwhat's upstream or downstream of it in a series; whether it's got a weak cell or a strong cell on either end.Folk who believe this myth presumably believe it's done by telepathy.

Having been a research electrochemist, practising from 1971 to 1999, I can assure everyone that's it's a myth, perhaps stemming from the fact that you can't try it out on any set of cells without giving them a rest. However, I don't expect to be able to convince the faithful, as over many issues.

I suggest the faithful do some experiments, as they'd learn a lot about electricity. You'd have to do the experiment a few times, though, as it it wouldn't be easy to match the cells precisely at the start if you use lots, andfrom one run to another if you use the same set all the time.

mrc01, what you say is correct, but I can't see how it addresses the question.

Have fun!

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Old Jun 11, 2007, 3:58 PM   #6
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My point is that batteries in series are not drawn down differently - I believe uneven drawdown is just an illusion. It's true that whena device shuts down due to low battery, some of the batteries may appear dead while others still have life remaining. But the reason for this is NOT because the batteries are drawn down differently. It's because even supposedly identical batteries have different capacities.
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Old Jun 11, 2007, 4:44 PM   #7
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Hello Alan,

Cells in series are drawn down at the same rate, however if the cells are not balanced at the beginning, they will not all reach empty at the same time.

The low voltage cut off is supposed to minimize any problems with slight differences in cells, but if the imbalance is great enough, it will still occur.

In a balanced battery, once the low voltage cut off has been reached, the device will shut off and the battery will slowly recover.

In an unbalanced battery, the low voltage cut off can signal the device to shut down, but as the shut down is initiated, the voltage can quickly rebound interrupting the shut down. This pulsing can raise havoc with the electronics and cause some interesting results.

With flashlights we have found that the equal draw rule of series cells is bent a little. Often we find that the cell closest to the lamp empties first. Please note that we are beyond the normal low voltage cut off for secondary cells and are in over discharge territory now. This is an area where primary cells often find themselves, but the results are similar for both secondary and primary cells.

The theory is that the heat from the lamp warms up the cell that is closest to the lamp making it more "vibrant." This cell hold higher voltage under load and gives more power during the discharge. This results in it emptying before the other cells.

As far as TV remotes and radios go, the voltage of a cell can be raised simply by holding it in your hand.

Tom
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Old Jun 11, 2007, 5:01 PM   #8
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Hello Mrc,

Reverse charging can happen in all chemistries, not just NiMh.

Tom
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Old Jun 11, 2007, 5:53 PM   #9
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Interesting you would say that. I learned about this the hard way with NiMH cells - killed one and then learned about this effect afterwards trying to figure out what happened.

But I've been running multicell NiCDs in series to zero numerous times over the years and haven't killed one yet. For example NiCD batteries driving high current electric motors that go all the way to zero with no voltage protection. I can't imagine I am that lucky, so perhaps NiCDs are less sensitive to this effect.
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Old Jun 11, 2007, 5:58 PM   #10
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deleted duplicate ( no I did not press Send twice!! )
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