My background so you know where I'm coming from. I'm an Electric Vehicle enthusiast with a degree in Physics. As you can imagine, I want to keep the $5000 battery pack in my car working as long as possible since, unlike a phone, it is quite expensive to replace. As such I have done significant study into the various battery technologies and definitely into the various Lithium based batteries.
While the batteries in my car are LiFePO4 (Lithium Iron Phosphate), the ones in cell phones are typically Lithium Polymer. They have a higher voltage and energy density which is what we want in our phones. In all Lithium type batteries the electrolyte starts breaking down the moment the battery is manufactured. The things that cause it to break down faster is high temperatures and high states of charge. There are other breakdown mechanisms in the batteries too. One of them is Lithium plating and another one is the breakdown of the crystal structure into which the Lithium ions intercalate (where they "park" on charge or discharge). As another poster mentioned, extreme states of charge decrease the life of the battery. Ideally the batteries should be kept between 80-90% and 20% SOC (State of Charge). That would make the battery last a significantly longer period of time. IIRC, Tesla Motors found that charging the laptop batteries used in their cars (Lithium Cobalt) to only 95%SOC extended the cycle and calendar life by a significant amount, 20-50% but I don't remember exactly. Various other studies, including a NASA study I read, showed that keeping a Lithium battery at 100% SOC, rather than charging it up just before it was needed, shortened the life of the battery.
In laptops the batteries rarely die of old age but instead of being held at 100%SOC and elevated temperatures for long periods of time. Time at 100%SOC, heat and full discharges are the killers of Lithium based batteries. Contrary to past practices with NiCd and NiMh batteries, fully cycling a Lithium battery does not make it last longer, it actually shortens its life. I don't know the mechanism used in cell phones to determine SOC so it is possible that a full cycle will calibrate the phone properly so it may give the illusion that the battery works better when in fact it is the electronics which are reporting properly. That being said, there is a slight increase in realized capacity during the first bit of use. You don not, however, have to do any special cycles to gain this increase. Normal use will automatically take care of it so why waste it with a discharging type app? The voltage drop vs. SOC of the batteries used in cell phones is quite steep compared to what I use in my car. This means it is possible to get a reasonable SOC value of a cell phone battery on voltage alone but I don't know if that is what is used. The voltage usually ranges from ~2.7V discharged to ~4.2V charged. (For comparison, the LiFePO4 cells in my EV are ~2.5V discharged and 3.38V charged but the useful range is between 3.0V and 3.38V so very little change. The 2.5-3V range has at most 5% of the total capacity if it is used
very slowly.)
What I have found with my Nokia Lumia 920 is that with the Nokia supplied wired charger (rated at 1.3A output) that my battery rarely feels warm, even when charged from a completely discharged phone. When using the wireless charging pad, however, the entire phone gets quite warm even though it is only putting out 0.75A (though I don't know if this is rated at 12V or 5V). Warm enough that I worry that the battery life is being noticeably shortened. I hope to take some surface temperature measurements to see if the temperature is getting into the unsafe range. Basically the battery will be safe below something like 60?C (140?F) but the electrolyte will be breaking down rather rapidly at that point. I don't even like to see my batteries get much above 30?C (86?F) but I think with a phone, especially if carried in a pocket or something that it is likely to be that hot quite often. They do best at temperatures that we are comfortable with so on the order of 20-25?C (68-77?F).
As for heat generation during charging, I have found that the most heat is generated at the end of charge when the battery is nearly full. Very little heat is generated in the vast majority of the SOC range until nearly full. On discharge it is the opposite. Most of the heat is generated near the empty end of the SOC range. If you study the physics and chemistry of how these particular batteries work it makes perfect sense that this should happen this way.
Here is what I have done with my previous 3 phones to make sure the battery easily lasts over 2 years, in the case of my iPhone 3GS it is still going strong well over 4 years later and is in the hands of a new owner. My 2.5 year old iPhone 4 is also still going strong and appears to out last my 2 week old Lumia 920.
- If I am going to use my phone extensively throughout the day where it is likely to be really low by the time I go to bed then I charge it sometime during the day. These batteries do better with many small charges and discharges because it keeps them away from the extreme ends of the SOC range.
- Unless the battery is really low I don't charge it until the morning when I get up. This keeps it from sitting at 100% SOC for very long.
- If my phone isn't fully charged by the time I'm ready to leave I don't worry about it. Stopping charging at 90-95% is actually good for the battery.
- Except in an emergency I do not let my phone get much below 20%SOC and usually try to stay above 40%SOC.
- I don't leave my phone in a hot car or in the sun.
- If my phone were ever to be cooled to below 0?C (32?F) I never charge it until it has warmed to above freezing. Charging at temperatures below this point significantly increases the chance that Lithium plating will be happening in the battery. Any Lithium ion that plates out as Lithium metal is no longer available to store charge. Discharging at very cold temperatures, however, does not cause any problem, just that the battery doesn't put out energy very fast. This is not likely to be a problem with the really slow rates our phones use the battery but it could be a problem in extremely cold situations.
- If I had the option, I use a lower rate charger which is easier on the battery. I usually use a 0.5A charger for my Bluetooth ear piece if I have the time and my phone doesn't complain.
Hopefully this wasn't too long for my first post but I thought people would like to hear from someone who has a fair bit of personal experience with Li batteries. Believe me, I want my $5k investment to last so I have spent countless hours on the battery care issue.
David D. Nelson
David Nelson's 2003 Gizmo
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