Brace yourselves, summer is coming! But not just yet, as we investigated the lithium battery behavior in cold.
In Part 1 we talk about the issue in general and in Part 2 we look at behavior of Samsung Galaxy S7 and iPhone 6s .
The cold performance of smartphone batteries is a hot topic every winter. It is annoying how some devices just don’t work in cold environment at all, while the others seem to have zero problems with cold. Why is that so? After all, every smartphone uses a lithium battery as a power source and it is a fact that lithium batteries just do not like cold.
The main problem in cold weather is the higher impedance of the battery, due to battery chemistry. Once the battery temperature starts to go below 0degC, the impedance will already be quite high and rising at increasing rate. The battery manufacturer can improve the battery cold performance a bit by fine-tuning the chemistry, but that usually means giving up on something else like battery cycle life or battery capacity.
High impedance leads to high voltage drops (remember Ohm’s law from Physics class…?), and once the battery voltage drops below certain level, the device will shut down. If the current consumption stays very low, the device may survive even in extremely cold environment. But if you try to take a picture with a flash, you’ll draw a huge current from battery and the consequent voltage drop is also …huge. Some devices tackle this by limiting features in cold environment to avoid high current spikes. Apple takes that approach to the extreme, by refusing to operate at all in minus degrees.
Still, some devices seem to operate in super cold temperatures just fine. The battery capacity doesn’t even flinch and everything just works. In these cases, you have to consider several things:
- What is the real battery temperature (not the environment temperature) ?
- What is the current consumption ?
- Does the battery monitoring even care about temperature ?
- Does the device care about low battery voltage ?
The first two items are easy to understand ; If the device is actively used, the battery heats up even in cold environment. And if the current consumption is quite low (like during a voice call), the device can survive the increased impedance. But the last two items are trickier.
The battery monitoring algorithm is a very complex creature, and every device seem to have their own twist on how to do it. Some monitoring algorithms don’t put a lot of emphasis on battery temperature, or react to it very slowly. Others may react to low temperature almost instantly, seen as a quite dramatic capacity loss. Both approaches are correct in a way – the same overall capacity is in the battery regardless of temperature, but the available capacity is lower when the impedance increases. Which capacity would you prefer to know…?
The final item is about the device cut-off voltage. There is always a hardware based cutoff voltage – if the battery voltage goes below this level, the device shuts down immediately without any warning. There may also be a software based cutoff voltage – the device monitors the battery voltage and detects, when a certain configurable voltage level (higher than the hardware based cutoff, obviously) is reached and initiates a graceful shutdown process. Hardware based cutoff is something you want to avoid, because it is bad user experience and also may corrupt data if the shutdown occurs in the middle of writing data. If you have big balls, you can allow the battery voltage to get very close to hardware cutoff level. From end-user perspective, this can look like the device just seems to survive in any temperature without any issues… until the instant shutdown occurs.
If you are looking for good and reliable performance in cold, the solution is always the same – keep the battery warm to avoid the impedance rise. You can keep the battery warm by keeping the device in the inside pocket of your jacket. Or use a power bank to power the device, it will generate heat and warm up your battery.
Stay tuned for part two!