Multi-cell Lights w/serial setup & parallel draw

[Benson]

A misconception or two seems prevalent about parallel packs. It's true that different cells may supply different currents, but this is good, not bad. If all cells are charged to the same voltage initially, but one cell is "weak" (has lower capacity), it will supply less current to keep the voltage on it the same. This means all cells will hit 1V (and have dumped their full capacity) at the same time. Put them all in the charger, and they'll be charged back up, with different currents, but to the same voltage. You can't drive a cell into reverse voltage without driving the whole pack reversed, which can only happen if you force current through with an external source (e.g., hook up the charger backwards).

Now if you connect those same cells in series, and discharge to 2V, the low capacity cell will drain first, but when it hits 1V, the other cell is still at 1.2V, so the light keeps drawing current from both cells. The weak cell might be at 0.85V, and the strong cell only at 1.15, when the pack hits 2V -- you've overdischarged the weak cell, weakening it further. Repeat.

There's two possibilities: you can either use a higher threshold to compensate for the weakest cell that might be used, in which case you get less usable capacity than the parallel pack, or you keep the threshold at 2V and ruin the weak cell, eventually driving it into reverse voltage when it gets weak enough. (Obviously, in a 2s pack, it's hard to actually reverse-charge a cell, because there's just not enough voltage in the one other cell to keep the pack above any reasonable threshold. But in 3s and up, this very bad possibility does exist.) Particularly with Lithium and Lithium-ion batteries, this is rather dangerous; NiMH could overheat and start a fire, but won't burn or explode under such abuse.

But TANSTAAFL; a parallel pack's not perfect, either. If the cells are mismatched in voltage when you put them in, the more highly charged cells will shunt high current into the discharged ones -- especially bad with non-rechargeable cells! And if one cell is weak, it doesn't get abused; but it doesn't take its fair share of the current, and if you designed for the maximum current the cells can deliver, the stronger cells will actually be overloaded. You need to back off from maximum current (limiting power) by a similar amount that you had to back off the minimum voltage (limiting energy) for the series pack

 

[2benlightened]

ok... I think I understand... there is not much of a total advantage overall in making a parallel pack... so why did fenix do so with the 8AA's? because they DIDN'T want a long narrow tube? Couldn't the pack have been configured in series still? There must be a reason they chose to make it in parallel... any guesses?

also, I wonder what a 8AAA version would be like? Not quite as bright or as good a run-time, but potentially better proportioned?

 

[HKJ]

ok... I think I understand... there is not much of a total advantage overall in making a parallel pack... so why did fenix do so with the 8AA's? because they DIDN'T want a long narrow tube? Couldn't the pack have been configured in series still? There must be a reason they chose to make it in parallel... any guesses?

also, I wonder what a 8AAA version would be like? Not quite as bright or as good a run-time, but potentially better proportioned?

The Fenix is 2 strings of 4 cells or 4s2p, that gives a voltage above the led voltage, i.e. the light can be drive with a buck converter.

Your need about 2.5 AAA cells to get the same energy as one AA cell, i.e. a 3AA would be about the same as 8AAA

 

[Marduke]

ok... I think I understand... there is not much of a total advantage overall in making a parallel pack... so why did fenix do so with the 8AA's? because they DIDN'T want a long narrow tube? Couldn't the pack have been configured in series still? There must be a reason they chose to make it in parallel... any guesses?

also, I wonder what a 8AAA version would be like? Not quite as bright or as good a run-time, but potentially better proportioned?

Well, 4xAA in series gets you plenty in the way of voltage to run the MC-E. However, that is a pretty high current draw for the cells (~2.8A), and you lower your total capacity at that high of draw. By putting another set of cells in parallel, you immediately half the current each group is being asked to deliver (1.4A). However, by halfing the current, you are MORE than doubling the runtime because each cell can provide a more "complete" discharge.

This happens no matter which method you add the cells, either series or parallel. However, the reason they did not put all 8 cells in series is two-fold.

1) You would always have to be using 8 cells. As manufactured, you retain the option of running on only 4 cells.

2) The circuit would have to buck the voltage from 9.6v instead of 4.8v. Bucking from the lower voltage is more efficient.

8xAAA would get you decreased runtime and decreased brightness without much benefit. The light would still be a large 2xC sized light. Being a smaller size, heatsinking would also become a greater chalange. Also, in general, the multi AAA format is a rather poor choice, and rightfully avoided by most well informed flashaholics.

 

[2benlightened]

ok, here's a question then... I read somewhere that the TK40, when running on only 4 cells, can't be run on high; in fact they only recommend running it on the low setting...

is that because you are now trying to draw 2.8A through the cells? or is it the way the circuitry is designed? or is it because of the parallel setup that the brightness is increased?... I guess the one thing I still don't quite get is:

eg. light draws 2.8A... 4 cells in series gives the req. voltage needed to exceed Vf, but 2.8A is being drawn through them... add another 4 cells parallel to the first 4 and you are sharing the required amperage across 2 sets of cells (1.4A ea.)... change setup to 8 cells in series and you are doubling the voltage but dropping the amperage draw to 1.4A??? why don't any of these happen?: a) brightness decreases... b) circuitry demands 2.8A from somewhere so all 8 cells are supplying 2.8A of current and voltage imput supply is limited.... c) I don't know what because I have a misconception of how this works...

maybe what I am misunderstanding is which aspect directly affects brightness... I thought it was the amperage supplied that controlled this because the voltage had to remain constant/was limited by the circuitry...

W=V*A , in other words V and A directly affect W and inversely affect each other. the wattage available from 8 cells will be pretty well the same whether arranged in parallel or in series.

W=brightness * runtime in a similar fashion (but the relationship btwn brightness and runtime is not linear)...

is it the amperage, or voltage, which affects the brightness in the case of a buck circuit? if it's amperage, would the 8 cells in series not still have to supply 2.8A to get the same brightness?

Marduke:
"By putting another set of cells in parallel, you immediately half the current each group is being asked to deliver (1.4A). However, by halfing the current, you are MORE than doubling the runtime because each cell can provide a more "complete" discharge.

This happens no matter which method you add the cells, either series or parallel."

...not questioning whether this is true, but why it's true... is it a)my misconception that it's the amperage which controls the brightness?.. I guess I get this idea from the fact that the P2D can put out 180 lumens using a 3v cell and the L2D can't (or shouldn't) put out the same because the current needed by the 1.5v cell would be double/too great... then again, the P2D/L2D head doesn't use the same type of circuit as the P3D or TK40, right? (boost vs. buck?) or b)misconception of how the buck circuit works... my 4-cell P3D sends the same current as the 2-cell and limits the voltage to the head?

HKJ:
"Serial/parallel does not really matter, it is easy to do some electronic adjustment of the voltage/current. What matters is how much power you can draw from the cells."

yeah, but isn't it important, for brightness, how much power you can quickly draw from the cells? and that you can drain the power faster from a set of cells in parallel than the same size set in series?


tell me what I am missing...

on the other hand, I'm sure you guys didn't join CPF to play professor, so if you get annoyed with the 20x20 questions, I'll understand... :)

 

[HKJ]

As long as you have a flashlight with a buck or boost converter the following is true:

Brightness/lumen output is controlled by power (watt) sent to the led, the led needs a specific voltage/current for a given brightness, but this is handled by the converter and is independent of battery voltage/current, only the power (watt) draw from the battery must be enough.

To draw a specific amount of power from a cell, the current draw will always be the same current (because the voltage is the constant for a cell)*. When drawing power from a battery (Multiple cells) it does not matter if the cells are in series or parallel or some combination of series/parallel, to draw 8 watt from a 8 cell battery, each cell must deliver 1 watt.

*This is not completely correct, the voltage will depend on age and current draw, but for the above statement is an ok assumption.


When working with leds, it is best to get battery voltages outside the leds working voltage, this gives the best and cheapest regulation circuit (buck/boost converter). A led needs between 2.6 and 3.7 volt, i.e. keep the battery voltage below 2.6 volt or above 3.7 volt is best.

 

[Marduke]

is it the amperage, or voltage, which affects the brightness in the case of a buck circuit? if it's amperage, would the 8 cells in series not still have to supply 2.8A to get the same brightness?

I picked this statement because I think it shows the root of the misconception. Think of it this way:

The LED is asking to be provided about 10 watts of power. If there were only 4 cells total, those 4 cells would have to deliver all 10 watts. That is about 2.5 watts per cell.

But if you added 4 more cells, they would share the load with the first 4 cells, so each cell is now only being asked to provide 1.25 watts of power. In theory, it does no matter how you add these cells. They can be in either series or parallel and it accomplished your goal. In reality, there are certain practicalities to which method of addition is more advantageous. In this case, adding the second set of 4 cells in parallel worked out better for the various reasons mentioned before.

Marduke:
"By putting another set of cells in parallel, you immediately half the current each group is being asked to deliver (1.4A). However, by halfing the current, you are MORE than doubling the runtime because each cell can provide a more "complete" discharge.

This happens no matter which method you add the cells, either series or parallel."

...not questioning whether this is true, but why it's true... is it a)my misconception that it's the amperage which controls the brightness?.. I guess I get this idea from the fact that the P2D can put out 180 lumens using a 3v cell and the L2D can't (or shouldn't) put out the same because the current needed by the 1.5v cell would be double/too great... then again, the P2D/L2D head doesn't use the same type of circuit as the P3D or TK40, right? (boost vs. buck?) or b)misconception of how the buck circuit works... my 4-cell P3D sends the same current as the 2-cell and limits the voltage to the head?

Holding the voltage constant to the LED, yes, brightness is controlled by changing the amperage usually. This is the preferred method.

And yes, your 4 cell P3D bucks the voltage to the same level as the 2 cell version, and provides the LED with the same amperage. That is why all you get with that setup is increased runtime and a really long light.


Off topic based off your above statement, the P2D and L2D are the same brightness (180 emitter lumens), because they are actually the same exact head being driven off similar voltages.

 

[2benlightened]

yes, I should have edited that... meant to compare the P2D and the L1D

 

[Marduke]

yes, I should have edited that... meant to compare the P2D and the L1D

The L1D is also the same head as the L2D/P2D. The only difference is the turbo mode is 120lms instead of 180.

But in that case, yes, if the L1D head wad programmed to give 180lms on turbo no matter the cell, the required draw would be WAY too high for 1xAA.