lithium button cell cr1620 cold performance?

[crizyal]

I am just curious to know how much cold weather will affect the performance of these batteries. Temps ranging from 32F - 50F degrees. Can any one tell me what percentage of loss I might see? I am trying to figure out what cold weather runtime might be in a red dot optic in the fall. Thank you!

 

[Battery Guy]

Most lithium button cells are designed for very low discharge rates, equivalent to C/100 (discharge current equivalent to cell capacity divided by 100) or slower in ambient conditions. Whether or not you will see a decrease in performance will depend upon the discharge current your device needs, and how hard you are pushing the cell. If the load is very small, there will be little or no difference between ambient temperature and 32F. At higher loads, the difference could be huge.

Take a look at the graph in this spec sheet from Renata that shows the cell capacity as a function of resistive load at various temperatures. If you know the resistance of your red dot optic device, you can use this plot easily. If you don't know the resistance, then you need to know how long a 1620 cell lasts in your device at room temperature. For example, if your battery typically lasts 60 hours or longer, then your device is probably drawing less than 1 mA (> 30 kohm resistance), and operating at 32F (0C) will probably not affect performance. However, if your battery typically lasts less than 60 hours at room temperature, then you are probably going to see a noticeable decrease in runtime at lower temps.

If possible, put your device in the refrigerator and see how it performs. This is probably the easiest way to figure out if you can use it in the field at lower temps.

Cheers,
BG

 

[crizyal]

When I spoke with the manufacturer, I was told that the optic would run for 12 hours @ 77 degrees. They also told me that it would run longer at colder temps. I asked for clarification because that didn't sound right to me. He said "you know, when you want to preserve a battery you put it in the fridge". At that point I realized I couldn't trust what I was hearing. So if a device takes 12 hours to deplete a CR1620 at 77F degrees how long at 40F or 30F degrees. Thank you for the help as this is clearly over my head. I will try the optic running in the fridge test if I decide to order it.

 

[Battery Guy]

If you take a look at that Renata spec sheet you will notice that all of the discharge curve times are in the hundreds or thousands of hours. A 12 hour discharge on this cell is extremely fast...too fast actually. The manufacturer chose the wrong cell for this application.

Look at the Renata spec sheet and you will notice that the minimum resistance on the temperature curve is 3kohms, which is equivalent to 1mA (3V/3kohms), which is the maximum recommended discharge current. At a 1 mA discharge current, the cell is expected to have about about 50 mAh capacity, and so should last about 50 hours. If you extend the 23 degC curve to 1 kohm, the cell capacity is down to 40 mAh and the discharge current is about 3 mA, so the discharge time is going to be in the 12 hour range, which is how long the manufacturer said it will last at 77 degF. So I would estimate that your optic device has a load in the 1 kohm or 3 mA range.

Now extend the 0 degC (32 degF) curve to 1 kohm. The estimated rated capacity is only about 10 mAh. So at a 3 mA discharge (1 kohm load), you will only get about 3 hours of runtime at best.

So, my best guess is that you will only get about 3 hours of runtime at most if the battery is at 32 degF. Very likely you will get much less than that.

Cheers,
BG