Quote of Battery University article on Li-ion battery longevity vs. storage temperature: https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage . Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.
Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperatures. Elevated temperature hastens permanent capacity loss. Not all Li-ion systems behave the same.
||98% (after 1 year)
||94% (after 1 year)
||96% (after 1 year)
||80% (after 1 year)
||85% (after 1 year)
||65% (after 1 year)
||75% (after 1 year)
||60% (after 3 months)
Apple Care instructions on their Li-ion batteries: Batteries - Maximizing Performance - Apple (see section down page on Avoiding Extreme Ambient Temperatures)
2015 scientific battery examining the effect of temperature on Sony li-ion batteries (see Figure 5.): https://www.nature.com/articles/srep12967 (the paper was published in NATURE, one of the world’s premiere scientific journals).
As I mentioned in my post, I arbitrarily set 86 deg F as my cutoff point for wearing my HA’s outside for prolonged exposure to high temperature (hours at a time) as I didn’t want to go 95 deg F (35 deg C) or 104 deg F (40 deg C) to explore the effects of REALLY elevated temperatures on my expensive Quattro’s.
The Quattro manual says: “The operating temperature range for the charger and the hearing aids is 0 °C - 40 °C/ 32 °F - 104 °F.” but I don’t think ReSound is being as upfront as Apple in telling you that from all the experimental evidence, you’re going to get less li-ion battery lifespan from exposing your device to elevated temperature. Apple even recommends removing occlusive cases from one’s iPhone when charging to avoid heat build up.
Most people just treat their battery-powered phones as expendable items to be trashed anyway every few years for the NEXT BIG THING so for most people having enough charge to get through the day over that time span is all they care about and they don’t want to worry about how hot their phone is getting from charging, use, or the environment. The What-Me-Worry society…