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- Thread starter Solstice
- Start date

Help Support Candle Power Flashlight Forum

I might switch to rechargables eventually, but for my new current EDC (the .5 Watt single AA from advancedmart, unknown brand), a 3+ volt li ion would fry it and a NiMH would not be as bright and add to the weight that is pretty much non-existant with a li AA primary.

For the price of a 4 pack of AA lithiums you can only get about 2 123 cells in most stores.

Using the plots from Silverfox, at both 500ma and 1 amp, the AA Li setup wins by almost a 2 X margin if I am reading the plots correctly.

I may need to re- think my 123 addiction.

Double post - edited out.

There is one other method of comparison, which is "what is the simplist setup and most efficient using it ?". One of the most simplistic setups is 3 x AA or 2 x 123 with a resistor driving your LED.

Using the plots from Silverfox, at both 500ma and 1 amp, the AA Li setup wins by almost a 2 X margin if I am reading the plots correctly.

I may need to re- think my 123 addiction.

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I think if size is important (smaller medium to low output lights) the 123 cell has its virtue as not everyone wants to carry around a 3AA cell light all the time when they could use a low output single or even 2 cell 123 light. 2 cell AA lights using 1.7v lithiums usually end up having to struggle against the efficiency of a boost circuit in order to achieve decent runtimes and battery usuage.

There is one other method of comparison, which is "what is the simplist setup and most efficient using it ?". One of the most simplistic setups is 3 x AA or 2 x 123 with a resistor driving your LED.

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Actually, I don't think that is a fair comparison. You want to get the voltages as close as possible. So you really want:

1 x 123A vs. 2 x AA

- and -

2 x 123A vs. 4 x AA

-john

SO, the price is the same, but a CR123 has twice the voltage (3V) but less than half the mAh (1300) whereas the only 1.5 Volt Lithum AA has 2900 mAh.

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Ok, I'm out of my element here, but I didn't think that you can directly compare mAH at different voltages. I think you are not comparing apples to apples.

While not scientific, here is how I think about it:

123A = 1300mAH @ 3V ~= 2600mAH @ 1.5V.

So my guess is there is a little less energy in a 123A cell, but not the half that you suggest.

Of course, it would be great if someone who actually knew what they were talking about stepped in here!

-john

P = I*V

P = Power (Measured in W, watts)

I = Current (Measured in A, amps)

V = Voltage (Meansured in V, volts)

So... to find how much energy is stored in a cell, just take the mAh rating and multiply it by the voltage.

Quick example:

Take a 2500 mAh AA NiMH cell,

2500 mAh * 1.2 V = 3000 mWh

Hope that helps!

Dan

But for simplicity, let's just say they have the same number of Wh. The next question then is what is their internal resistance? If we had this piece of information, comparison would be a little simpler.

(Thanks to Dacali for providing us that very useful equation.)

We will set power draw at 1200mW. By doing this we see that since the Lithium AA has half the voltage of the CR123, it needs to provide twice as much current to maintain the 1200mW power output.

1.5V x 800mA = 1200mW

3.0V x 400mA = 1200mW

So even if both cells had the same internal resistance, the effect is felt more in the 1.5V cell because of the higher current passing through it. Unfortunately this comparison is not so simple because the data sheets don't seem to list the internal resistance of the cell. Besides which, internal resistance actually increases as a cell gets depleted.

What is available are tables showing this:

The lithium AA discharging 1500mW will run for just over 2hrs. 1500mW discharge translates to 1000mA current draw.

The CR123 discharging at 1000mA will run for 1hr.

The cutoff voltages are 2/3 of the starting voltage, i.e. 1.0V for the AA and 2.0V for the CR123.

Thus, interestingly enough, they both seem evenly matched in terms of discharge performance. Same current drain, but the cell with half the voltage runs for twice as long.

However, as stated earlier, for the AA to match the CR123 in power output (NOT current drain) it needs to deliver twice as much current. If we ignored internal resistance, we could just say that the AA could deliver 2000mA for 1hr. In terms of power delivery, this would be comparable to the CR123 delivering 1000mA for 1hr.

But internal resistance must be taken into account. And since the current going through the AA is twice that going through the CR123, the power loss due to internal resistance would be at least twice as much.

It is actually a little higher than two times because for each bit of power lost to internal resistance, a little more is needed to cover for it (if we are still attempting to extract the same amount of power from the cell). That little bit of extra power means the current will actually be a little bit higher too, which translates into a little bit more power lost to internal resistance, and so on. Also, with twice as much power lost to internal resistance, we can expect the AA to heat up a little bit more than the CR123 (afterall, the lost power must go somewhere). We know that elevated temperatures will increase the internal resistance of the cell. I'm sure you don't need me to tell you what that means.

So we have established that for the same rate of power extraction, the AA will actually prove slightly less efficient.

Next, the AA will probably get a second slug due to the efficiencies of boost/converter circuits. Most electronics experts will tell you that the larger the difference between input versus output voltage, the less efficient the converter tends to be. It always costs more to boost from 1.5V to 3.6V than it does to boost from 3.0V to 3.6V. It's just the way the universe works.

So given the same TYPE of boost circuit lighting the same LED, a CR123 will be just that little bit better as the power source, not to mention that it actually started out with a 50mAh advantage in capacity.

Hope that wasn't too long or confusing.

P = I^2 * R

So that means that if the current is twice as high, the actual effect is 4 times.

Taking the above example:

1.5V x 800mA = 1200mW

3.0V x 400mA = 1200mW

Assume internal resistance for both cells is 0.000005 ohms.

Then power lost due to internal resistance:

For AA cell:

P = 800mA ^2 * 0.0005 = 640000 * 0.000005 = 3.2mW

For CR123 cell:

P = 400mA ^2 * 0.0005 = 160000 * 0.000005 = 0.8mW

That is power subtracted from the final output, so to maintain 1200mW output, the cell actually needs to be producing more power, which means higher currents which means more losses, which means more power again to cover the losses...

OK, I've just been playing with Excel and the wonderful goalseeking tool.

Assume internal resistance is 0.000005 ohms.

For the lithium AA to achieve 1200mW output, the cell must actually produce about 1203.217mW (802.145mA).

For the CR123 to achieve 1200mW output, the cell must actually produce about 1200.801mW (400.267mA)

So this exercise shows that the AA actually needs to produce almost 3mW more than the CR123 to maintain a 1200mW output.

But don't forget, the extra power becomes heat in the cell which raises the temperature, which increases the internal resistance which causes more power loss in the cell... It just gets worse and worse. /ubbthreads/images/graemlins/smile.gif

Since both batteries can be bought for about $1/battery online, I would vote for the CR123 if you're going with only lithium batteries bought online.

At retail stores though, CR123 batteries are approximately twice as expensive as lithium AAs which greatly affects which one is cheaper.

Remember, the AA form factor will always have the advantage of having batteries which are easier to find.

Just pick whatever works best for you and stick with it.

Dan

1) The capacities listed on the spec sheets are typically at very low discharge rates, probably matched to make the specific cell look best. I expect real-world applications typically will not fare as well. AA spec sheet rates capacity with a discharge rate of 200mA. 123 is listed as "100 ohms at 21C".

2) We have been talking about the BS 123A and the BS AA Li, mostly based on the low price of the AA cell. Based on This_is_NASCAR's post on the BS AA Li, I wonder if this is a good considering the odd performance profile he is seeing. Certainly it makes me think twice about buying these cells.

3) If we consider the Energizer Li AA (L91), the pricing structure changes quite a bit. I think these cells bottom out at about $2 ea. If you compare that to a high end 123A cell like the Streamlight (picked for robust overall performance and what people consider "expensive"), we are only talking something like $1.58 per cell (qty 12 from Brightguy).

In general, I guess I'm saying that things don't even look close w/o the BS AA Li. And I'm not so sure we have enough experience with that cell to make a good determination about it.

-john

Regarding point one, I do agree with you. So let's look at what the capacity might be closer to for the current and power drain we are planning. For this, I'll like to refer you to the same data sheets from the Energizer website. For convenience, these are the links for the CR123 and for the Lithium AA.

If we only look at the description at the top right of the data sheet, we will get those fantastic values, which are for conditions nowhere near our actual. However, there are tables lower down on both data sheets that show data for higher discharge rates. The EL123 table is titled "Simulated Application Test". (This does make me wonder how was it simulated exactly. Was it actual discharge rate to simulate a device drawing that much current, or was it merely data extrapolated from lower discharge rates to indicate how long a higher current might be sustained?) Similarly for the L91 which has two tables of interest, one showing "Constant Current Discharge, Typical Service", the other showing "Constant Power Discharge".

If the EL123 actually does perform as the table shows, then it provided 1000mA for 1hr. That is 1000mAh.

Similarly, for the L91, it delivered 1000mA for 3hrs, that is 3000mAh. The constant power discharge table shows only 2hrs for 1500mW drain, most likely because constant power means that as voltage dropped, current drain would be increased.

So it seems that for 1A current drain, the EL123 is 500mAh or 1/3 off the mark, whereas the L91 seems to still be on the money. Even at 1900mA drain (which is as high as the table shows) the L91 is still pretty close to the mark, running for about 1.5hrs, which gives 2850mAh.

And yet, this is not exactly earth-shattering news, since we knew that the closer the discharge rate is to the capacity of the cell, the less power we would be able to extract. For the EL123, 1000mA is 2/3C. For the L91, 1000mA is 1/3C.

And yet, after all that, we also know that the EL123 only needs to provide half the current of the L91 to deliver the same power. Thus, it would only need to provide 500mA instead of 1000mA. That is 1/3C. If all the relationships hold true, then we can expect the EL123 to perform in a similar fashion to the L91, which means that it should deliver close to its rated capacity. (And 500mA vs 1000mA is closer to what we we looking at to provide 1200mW from either cell.)

It's a pity that the power and current discharge rates which are of interest to us are not explicitly stated. All we can go on is extrapolation and conjecture, which we know may actually be quite different from reality.

However, the calculations in my above post should hold quite true because we are looking at ratios and relationships in 2 cells of similar build and chemistry. Thus the internal resistance should start at a similar value and increase in similar proportion. The current in the L91 will be twice that of the EL123, so losses due to internal resistance should follow the ratios mentioned above.

At the end of the day though, it will probably be the boost circuit that determines the majority of power loss. And generally, it seems that boosting from a lower voltage source tends to be less efficient.

Of course, as you have pointed out too, the cells in question are actually from Batterystation, not Energizer. We don't have enough experience or performance data from this cell manufacturer and all we have said so far is mere speculation.

If it performs as well as the Energizer stuff, which it doesn't always seem to be, all is well. But if it doesn't, then all the above exposition is wasted. Then, enough hot air from me! /ubbthreads/images/graemlins/grin.gif