[AT&T Samsung Galaxy S II SGH-I777] gr8hairy1:
A number of people have had their phones get very hot and wonder, how hot is too hot?

The purpose of this thread is to answer that, as well as be a reference that can be linked to for providing a quick answer.

Before anything, download a temperature app so you can see exactly how hot your phone is getting.

Once you have that, continue reading.

People have reported phone temperatures being a hot as 100° f to 110° f (~38°c). That is hot but not that bad. Upper 40's celcius (47°-49° ) is not good. And 50°c is critical And can cause damage, even though 70°c is considered the 'kill' temperature for lithium ion batteries.

There are many reasons this could happen. The majority of people notice it after flashing a custom rom to their phone. This has been reported happening in many forums, not just this one. On the whole, people find that when this does happen after a flash, it usually occurs once or twice, and then does not happen again. There is no clear explanation that I know of that can explain it. It could be the phone 'relearning' or adjusting to the battery. It could be an app that is not working well with the new rom. That is why rom developers will sometimes say to not do a titanium restore on all but your most important apps, and to download the rest from the market. It makes for less chance of an app not working well with the rom.

'Rogue' apps are many times also to blame. A rogue app can be described as a malfunctioning app that is improperly working and as a result causing high CPU activity, which in turn causes the phone to heat up. It also can be a bad app that should be avoided, at least until the issues it has are worked out.

How to fix that is usually on a per phone basis, meaning most cases are unique, and what works for one may not work for another.

There are apps that can help track down the problem app. One of the better known apps is better battery stats.

https://market.android.com/details?id=com.asksven.betterbatterystats

Check here for a better explanation of it and how to understand it:

http://xdaforums.com/showthread.php?t=1179809

It has also been noted that using internet browsers cause the temperature to rise roughly 10°f (5°c). So if your phone is running hot, stay off the browser, it'll just make it worse.

However, while you're trying to figure out what the problem is, your phone may be overheating, or at risk of overheating. And a quick way to shorten the lifespan of electronic is to be too hot.

The first thing I recommend doing is downloading SetCpu.

https://market.android.com/details?id=com.mhuang.overclocking&hl=en

Once you have it, or a similar app, downclock the CPU to 500mhz, at least while charging it, to help keep the temperature low in case an app spikes CPU usage and you're not there to stop it.

Sometimes even flipping the phone so the back is exposed is enough to cool it down.

Keep in mind most electronics have solder that will melt between 120° to 180°c (248 - 356 farenheit). While your phone should never ever get close to that, 50°c pales in comparison. But it can still damage the battery.

I also recommend SetCpu as you can setup a profile for it to run your phone slower when it hits a certain temperature to help keep it cool. As well as sound a notification that it has past that temperature.

Charging your phone will raise the temperature of your battery, it just does. At times it may feel very hot. But again, use an app to see exactly what the temperature is. It's not uncommon for a phone to heat up to over 40°c while charging. Though 39°-42°c is towards the high end of what is acceptable, it's relatively normal for charging temperatures.

For my phone I have a SetCpu temperature profile set at 44°c. I'd rather catch it before it gets even close to bad.

Feel free to post about apps that have helped you to narrow down a problem that is causing your phone to get too hot. As well as any experiences you've had with overheating and the steps you've taken to stop it.