Anybody who has tried to find their way out of a frozen forest after dark can tell you that batteries don't last long in the cold. With any battery, you are talking about a chemical reaction that has to occur between two elements, and that reaction rate is dependent upon temperature. Any battery manufacturer's performance curves can show this discharge vs. temperature characteristic.

Regards, DPW [Spent years making heaters out of op-amps.]

 

With rechargeables, as long as they are charged at roughly room temperature, they will be fully charged, and undergo the cooling/warming the same way...

If you charge a battery while it is in the freezer, and there is no temperature control to keep the battery from getting to room temperature, the charger will cut off before it is fully charged. When the battery warms up, it will take the full charge.

This is exactly the kind of information that is difficult to get from manufacturers curves, publications and books. It is also the type of behavior I need a model to reproduce.

The first statement is one I expected to be true, but the second one is an example of a question I was unsure about. I'm also not sure that the existing models I've seen would make this prediction. I will find out for sure and report back.

Thank You!

 

Anybody who has tried to find their way out of a frozen forest after dark can tell you that batteries don't last long in the cold. With any battery, you are talking about a chemical reaction that has to occur between two elements, and that reaction rate is dependent upon temperature. Any battery manufacturer's performance curves can show this discharge vs. temperature characteristic.

Regards, DPW [Spent years making heaters out of op-amps.]

Yes, this is obvious to me. My uncertainties pertain to issues such as whether the dead flashlight in the cold will work just fine a week later after it's been sitting in the house at room temperature.

The answer to this is probably obvious to many people including "frozen forest explorers", but not to me. Have you had any experience with a dead flashlight working after it has warmed up?

 

If you charge a battery while it is in the freezer, and there is no temperature control to keep the battery from getting to room temperature, the charger will cut off before it is fully charged. When the battery warms up, it will take the full charge. This is assuming you have a smart charger that uses ΔV/ΔT cutoff.

That could simply be a function of the smarts in the charger. The reactions go slower in the battery, so a full charge will take longer. The same total charge should be needed to fully charge the battery. The only variable affected by the cold should be the chemical reaction rate.

It would be nice to make a milliwatt-hour meter to prove/disprove the temperature effect.

On the other hand, being able to get the same amount of charge out of a cold battery as when it is warm is not really useful if the voltage tails off to a useless level.

 

The same total charge should be needed to fully charge the battery. ...

... On the other hand, being able to get the same amount of charge out of a cold battery as when it is warm is not really useful if the voltage tails off to a useless level.

I find these comments interesting and perhaps relevant to resolving my confusions. Does this explain why I'm hearing that "total capacity" is not a function of temperature, but "usable capacity" is a function of temperature?

Does this mean that a battery drained of "usable capacity" in the cold will still have "usable capacity" when it is warmed up?

 

If the increase in internal reaction makes the voltage go back up to a usable level, then yes.

A 222 bulb and a battery should be able to provide a test rig. Place the bulb in a tube with something like one of those TAOS light to voltage converters for an accurate and reproducible measure of the light intensity. Run a series of tests, using AAA batteries out of the same package. They should be very close in capacity and charge. That should also eliminate uncertainty in recharging the cells, due to variables not easily controlled - temperature being the greatest.

Then it gets tedious, checking to see how long it takes the light output to fall to some level when cold and when warm. And, of course. letting the battery fall to that level in the freezer, and see how much it recovers after warming up.

 

Yes, this is obvious to me. My uncertainties pertain to issues such as whether the dead flashlight in the cold will work just fine a week later after it's been sitting in the house at room temperature.

The answer to this is probably obvious to many people including "frozen forest explorers", but not to me. Have you had any experience with a dead flashlight working after it has warmed up?

Yes indeed if you warm the flashlight batteries in the forest they will yield greater output.

If you read accounts by photographers and arctic/ himalayan explorers or surveyors you will see the stories of warming the batteries in their underwear for this reason.

 

Yes indeed if you warm the flashlight batteries in the forest they will yield greater output.

If you read accounts by photographers and arctic/ himalayan explorers or surveyors you will see the stories of warming the batteries in their underwear for this reason.

Actually, my question is slightly different than that. What I'm asking is if a battery that has been drained when it is cold will still have useful life if it is then warmed up afterward.

I'm being told that the charge capacity is independent of temperature, yet it is also clear that a cold battery can not deliver as much charge as a warm one. What happens to the difference? It seems to me that the warmed battery should have the remaining unused charge still available. If not, ... why not?

 

Actually, my question is slightly different than that. What I'm asking is if a battery that has been drained when it is cold will still have useful life if it is then warmed up afterward.

I'm being told that the charge capacity is independent of temperature, yet it is also clear that a cold battery can not deliver as much charge as a warm one. What happens to the difference? It seems to me that the warmed battery should have the remaining unused charge still available. If not, ... why not?

In response, the battery that was in the freezer, which gave the terrible output while cold has now warmed up.

Internal resistance now measures 0.75Ω, Terminal Voltage is 9.11V. Short Circuit current for 1/2 second Started at 5.8 Amps, dropping to 5.4 Amps, average at 5.6 Amps, not a steep curve.

A battery that has a full charge at room temp, is frozen, then warmed back up, will have a full charge. This is the same battery that had a bit of life drained out of it last night when the internal resistance was 1.4Ω, and the maximum current it could supply into a short circuit was 2 Amps. The life is a bit diminished from the previous charge dump, but the result does show that temperature drops while in storage do not effect the battery once it is allowed to warm up. The same is not true of higher temp spikes, which cause the battery to self-drain faster.

 

A battery that has a full charge at room temp, is frozen, then warmed back up, will have a full charge.

Once again, this is not the question that I'm asking.

I fully understand that a battery charged at room temperature can be cooled and then heated back up again, and will still have the same charge.

The questions I'm asking are related to the following scenario.

Step 1: Fully charge a battery at room temperature 25 deg C.

Step 2: Cool the battery to freezing 0 deg C

Step 3: Fully discharge the useful service from the battery while maintaining the battery at 0 deg c

Step 4: Heat the battery back to room temperature

Now, I ask the questions.

1. How much useful charge is now in the battery?

2. Is the battery still dead, or is there a useful residual charge available?

3. If useful charge is still available and I now fully discharge the remaining charge, is the total used charge, (i.e. that while cold plus that while warm), the same as if I had fully discharged the battery at room temperature?

4. If it isn't the same, what happened to the unused charge?

 

Ok, I'll try to find a way to put the Cadex into the freezer without hurting it and answer your questions.

It will be with a 9V battery though, as there are 6 cells and so measurements are easier.

Give me a bit to figure out the setup, but I now understand what you would like to see.

I just realized it is going to be below freezing outside here in a week or so, no need for the fridge, I could put the whole setup in the car. I just need to remember again....

 

Ok, I'll try to find a way to put the Cadex into the freezer without hurting it and answer your questions.

It will be with a 9V battery though, as there are 6 cells and so measurements are easier.

Give me a bit to figure out the setup, but I now understand what you would like to see.

I just realized it is going to be below freezing outside here in a week or so, no need for the fridge, I could put the whole setup in the car. I just need to remember again....

That is very kind of you! I'll owe you a few beers or batteries, - your choice.

 

I know little about chemistry but here’s my theory...

The discharge current also flows through the internal resistance [IR]. With a higher IR (lower temperature), the terminal voltage for a given load will be lower than at a higher temperature for this reason alone. (Torch gets dimmer in cold)

Any energy dissipated in the IR is lost. So for a given load, more energy is wasted at lower temperatures. This obviously effects the apparent capacity. The effect is smaller for smaller loads because less energy is lost to IR.

For no load, no energy is lost in the IR, so after cooling, while a spontaneous measurement will show a lower terminal voltage, higher IR and apparently lower capacity, once the temperature is nominal again, the capacity will be the same as before the cooling.

EG:

Say I have a torch running on two AA’s (3v unloaded) 0.5R internal resistance with a filament bulb of 6R nominal. That’s a current of about 462mA. So that’s 231mV across the IR, wasting about 108mW in the IR and using about 1.3W in the bulb. 8.3% waste.

Now cool the batteries so the IR increases to say 1.5R. (Ignoring for the moment any variations in the filament resistance) That’s 400mA, with 600mV across the IR, wasting 240mW in the IR and using 960mW in the bulb. 20% waste and the bulb is dimmer.

I’m sure real world figures will be different, but the theory is the same. At lower temperatures, more of the total available charge gets wasted. The situation gets even worse if you are controlling the load to maintain a fixed current across the temperature range. And I guess that’s why RC car racers like to use hot batteries. It allows higher spontaneous currents and less waste (higher apparent capacity).

Hey. It’s just a theory.

 

I find these comments interesting and perhaps relevant to resolving my confusions. Does this explain why I'm hearing that "total capacity" is not a function of temperature, but "usable capacity" is a function of temperature?

Does this mean that a battery drained of "usable capacity" in the cold will still have "usable capacity" when it is warmed up?

You Got it, that is exactly how a lead acid battery works. Different batteries have different characteristics, I know that flooded Ni-Cad batteries are effected very little by temperature. You need to have a model for each chemistry.

6 volt lead acid battery
( these are hypothetical draws and times)
60 amp draw at 30 degrees will hit 1.5 volts per cell in 2 1/2 hours
60 amp draw at 80 degrees will hit 1.5 volts per cell in 3 3/4 hours

If you reduce the current draw the terminal voltage will rise and you can continue to discharge the battery until the cell drops to 1.5 volts again.

Or you can warm the battery to 80 degrees and continue to discharge the battery a 60 amps for the remaining 1 1/4 hours.

The energy is in the battery you just can't get it out at the current draw you want, you have to change the temperature or the draw to get it out, just like a slurpy.

 

The energy is in the battery you just can't get it out at the current draw you want, you have to change the temperature or the draw to get it out, just like a slurpy.

Thanks. Things are slowly starting to make sense. Very cool!

 

The following extract from Sears may help. Start reading where I have overlaid the white arrow.

Steve I have also placed another extract in this thread to answer an earlier query of yours.

http://forum.allaboutcircuits.com/showthread.php?p=189538#post189538

see post #44

 

The following extract from Sears may help. Start reading where I have overlaid the white arrow.

Excellent. This seems consistanct with the previous comments that the loss of voltage (due to lower open circuit voltage and higher internal resistance) limit the low temperature capacity. However, the energy is still there and when the battery heats up again, the voltage is now available and the energy can be extracted.

In reading about other chemisties, I've found that there also exists a greater discharge rate at low temperatures, similar to the greater rate of discharge at high currents. This would represent energy loss that can not be recovered once the battery is heated again. It's still unclear to me how significant this effect is, but it may be that both ideas need to be part of an accurate cell model.

I'm still working hard to establish a good cell model, but I'm getting closer and it's clear that many research papers have discrepencies between each other. Ultimately, I will need to do experiments. When I do, I'll post some results.

 

I've found that there also exists a greater discharge rate at low temperatures, similar to the greater rate of discharge at high currents.

The Nernst Equation controls the temperature dependance of the half cells.

http://en.wikipedia.org/wiki/Nernst_equation

However Sears also suggested the 'ionic mobilities reduce with low temperature' which suggests another effect in the solution itself.

 

You might find this self explanatory extract from the Radiospares sheet 6294 giving temperature characteristics of their dryfit sealed lead acid battery useful.

 

You might find this self explanatory extract from the Radiospares sheet 6294 giving temperature characteristics of their dryfit sealed lead acid battery useful.

Yes, very useful indeed! The existence of the "cross-hatched" area was one of my concerns and a question I couldn't answer till now.

Thank You.