Battery run time question (1 cell vs 2 cells)

Batteries in parallel will double capacity and batteries in series will double the voltage, that being said I've read where some flashlights you can use a battery tube that has 1 battery or a longer tube with 2 batteries. Does the flashlight run any longer with the 2 batteries in series? Some lights may be able to use the higher voltage to increase brightness, but I've read where run times are reported to be longer with 2 batteries in series. If 2 cells only doubles voltage and not capacity why would a light run any longer with 2 cells in series?

Well, it's best to look at it from an energy perspective.

There is a certain amount of energy stored in a battery cell.

Two cells store twice as much energy.

The flashlight converts energy from the battery into light.

Assuming equal efficiency, a two cell light will shine twice as long as a single cell light.

But if it shines brighter, the two cell light will use energy faster and you won't get twice the run time.

Most of my LED lights have multiple brightness settings (but clicking the on button).

So in the case of the two cell light, just select a reduced output setting that gives you about the same brightness as the single cell light and you will get about double the run time.

As the voltage increases, the current draw decreases. In other words, a flashlight with an input voltage of, say, 4v to 9v, will draw less current at 9v than it will at 6v. So, a hypothetical flashlight running on 3 x CR123's could run longer than the same flashlight on 2 xCR123's, even though the addition of the third cell does not increase the battery capacity.

OK, Now I do remember also reading that about the lower amp draw as the voltage increases a while back. I recently converted a 3D incan maglite to an led, XML I think, it will run on one li-ion or 2 cells (looks like same brightness) I'm using it with 2 cells at 8.4v fully charged with a dummy spacer I made out of PVC and factory spring and a glass ar coated lens.
Big improvement, and run time seems to be pretty good too. I make sure not to run the cells down to low so one of will not back feeding the other if they are not perfectly even.

In an optimal situation going from 1 to 2 cells should double your runtime but there are other factors involved other than having twice as much available power available that can at times make for even more than double runtime such as battery type and light design.
If your light is designed with a 1 cell battery that has a boost circuit that can take 1 or 2 cells the additional cell doubles voltage input and would reduce current draw from the battery source. Depending on the design and quality of components the reduction in current needed can decrease power loss in the circuitry due to small wiring and marginal component sizes that would result in circuitry heating up instead of that power going directly towards the LED itself.
If high quality components and large wire were used the power loss on 1 cell could be more minimal thus the difference between 1 and 2 cell runtimes would be more closer to double than beyond double.

In lights that don't have to boost voltage adding a second cell in parallel should double runtime but it could more than double runtime if the load on a single battery is heavy enough that the internal battery resistance itself causes noticeable power loss due to heating of the battery itself a second battery in parallel can halve that loss. If the light has a buck circuit then you can either double runtime or more depending also upon the design and quality of components. A buck circuit vs more direct drive can be less efficient if their is little loss in the wiring and battery internal resistance but at higher current levels these can account for losses and power that a buck circuit can reduce.

In other words your higher quality well designed lights that don't push the batteries to their limits should about double runtime, lights that push the batteries hard can more than double runtime when an additional battery is added along with lower quality lights that experience power loss due to having marginal components unable to handle higher current draws that accompany 1 cell vs 2.

So is a flashlight or any electronic device that uses batteries in series and not parallel using the capacity from both batteries?
If yes than it sounds like batteries in "series" does increase capacity as well as voltage.

(forwarded voltage)x(current draw)=Wattage, Power or energy used. VxA=P
Each alkaline cell has voltage of 1.2 volt under 0.9 amp and this is maximum current draw they can provide with acceptable life or run time.
Let's compare four different incand flashlights:

Flashlight#1 - single alkaline D cell with 1.2 volt bulb and current of 0.9 Amps (1.2x0.9=1.08 Watt bulb)

Flashlight#2 - Two alkaline D cell in series with 2.4 volt bulb and current of 0.9 Amps (2.4x0.9=2.16 Watt bulb)

Flashlight#3 - Two alkaline D cell in parallel with 1.2 volt bulb and current of 0.9 Amps (1.2x0.9=1.08 Watt bulb)

Flashlight#4 - Two alkaline D cell in parallel with 1.2 volt bulb and current of 1.8 Amps (1.2x1.8=2.16 Watt bulb)

As you see, flashlight #2 & #4 are brighter than flashlight #1 & #3 because total power or wattage or energy used are higher. In this case both flashlights #1 & #3 will have same run time because current draw of both flashlights are same (0.9A)

Flashlight #2 & flashlight #4 have same brightness or energy used because total power or wattage or energy used are equal. In this case both flashlights #2 & #4 will have same run time because total energy of both flashlights are same.

LED Monkey said:

Batteries in parallel will double capacity and batteries in series will double the voltage, that being said I've read where some flashlights you can use a battery tube that has 1 battery or a longer tube with 2 batteries. Does the flashlight run any longer with the 2 batteries in series?

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2 cells in parallel will run longer compare to single cell if current draw is same.

LED Monkey said:

Some lights may be able to use the higher voltage to increase brightness

Click to expand...

This is true if current draw remain same.

LED Monkey said:

I've read where run times are reported to be longer with 2 batteries in series.

Click to expand...

This is "NOT" true if current draw remain same.

LED Monkey said:

If 2 cells only doubles voltage and not capacity why would a light run any longer with 2 cells in series?

Click to expand...

This is "NOT" true if current draw remain same.
This will be true if current draw is half or power remain equal.

LED Monkey said:

So is a flashlight or any electronic device that uses batteries in series and not parallel using the capacity from both batteries?

Click to expand...

It works like this. If you have two 3.7V batteries, each with 3000mAh capacity and you connect them in series, the result has 7.4V and 3000mAh capacity. If you connect them in parallel, the result has 3.7V and 6000mAh capacity. The total energy they can deliver is the same, but the ones in parallel can deliver more current, while the ones in series provide a higher voltage.

LED Monkey said:

If yes than it sounds like batteries in "series" does increase capacity as well as voltage.

Click to expand...

Not really, see above. Power equals voltage times current. For a given fixed power load, if you provide twice the voltage, the current demand will be roughly 1/2, so in a sense it sort of seems like more capacity, but really it is just a lower current demand. If the load is not a fixed power load (generally a resistor of some sort if not fixed), then if the voltage goes up, often the current goes up also, and you definitely don't have more capacity.

Not sure if that helps or confuses, but that's my attempt at an explanation.

In a somewhat-related note, I recall doing a set of calculations a rather long time ago - using data posted on CPF - for a Malkoff M60 module with 2xCR123 vs 3xCR123 (in series).

The lower current draw of the higher-voltage 3-cell configuration caused the CR123's to deliver energy more efficiently to the regulated module; i.e. the available watt-hours of each CR123 was approx. 10% greater at the lower current.

An additional 10% for free, can't complain about that. This was a time of considerably lower currents & while CR123's were still as popular as LiIons, but comparable calculations could be done with modern configs as well.

Best regards,

Although I have picked up a lot of information about batteries over the last 3 years or so, this was still sounding somewhat confusing. Thanks for the responses.
I guess the main thing to the "batteries in series do not increase capacity" is the higher voltage decreases current draw needed and will increase run time.
Interesting subject and still a lot more to learn. I wasn't sure at fist if the 2 batteries in series i was using in the modded 3D Maglite was adding run time over 1 battery, while 1 battery was providing it's max brightness anyway. And the Adventure Sport Flashlights dropin is giving the old light a 2nd life too.

Modernflame said:

As the voltage increases, the current draw decreases. In other words, a flashlight with an input voltage of, say, 4v to 9v, will draw less current at 9v than it will at 6v. So, a hypothetical flashlight running on 3 x CR123's could run longer than the same flashlight on 2 xCR123's, even though the addition of the third cell does not increase the battery capacity.

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that is precisely the case and data supports this.
IOW, if a Surefire with a Malkoff will run for 60 minutes on 2 cells, it will run not for 90 minutes on 3 cells but more like 100 or 105 minutes. You get 10-15 "free" minutes. All with the same lumens or brighter.

The runtime decrease / increase is not proportional. e.g. if 2 cells means 1 hour, 1 cell is not equal to 30 minutes or 3 cells are not 90 minutes.

3 cell configuration are better than 2 and 2 are vastly better than 1. When it comes to primaries but pretty much the same holds for Li-ion, too.

When running a Malkoff on only 1 primary, I knew you would deal with reduced lumens and increased runtime but I didn't realize just bad of a decrease. The increased runitme is not worth the drastically lower lumens. Now I am only talking about under-voltaged modules not designed to run on 3V which is what one primary offers.

I've upgraded my Hound Dog to run on 3x18650 for increased efficiency.

Kestrel said:

In a somewhat-related note, I recall doing a set of calculations a rather long time ago - using data posted on CPF - for a Malkoff M60 module with 2xCR123 vs 3xCR123 (in series).

The lower current draw of the higher-voltage 3-cell configuration caused the CR123's to deliver energy more efficiently to the regulated module; i.e. the available watt-hours of each CR123 was approx. 10% greater at the lower current.

An additional 10% for free, can't complain about that. This was a time of considerably lower currents & while CR123's were still as popular as LiIons, but comparable calculations could be done with modern configs as well.

Best regards,

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That is also correct.

gotta love higher voltage. The higher the voltage, the less hard it has to work.

I am not a fan of 3 Amps hitting my battery. 1 to 1.5Amps is about right. But, to get close to 1,000 lumens you have to hit 2.5-3 Amps.