Why do cell phones get hot while charging? (Efficiency of charging)

[mike5757]

On my old iPhone 4, I noticed the back left side got hot while the right side remained cool. The right side is where the battery is located, and I read that LiPo batteries are very efficient at accepting a charge (>97%), so this makes sense. So the heat must be coming from the logic board.

Is this heat purely the energy loss of converting the 5V input to whatever voltage the battery needs to be charged or something else? I remember reading that stepping down DC power is very efficient, so this makes no sense, unless I made that up.

I found this blog entry where the author calculated the overall efficiency of charging the battery of the iPhone 3GS to be 61%. I have no idea how much of that is loss from the wall charger, though. http://kjordahl.net/blog/?p=82

 

[oynaz]

You already answered your own question. The efficiency is not 100%, which means that part of the energy ends up as heat. Personally, I think it would be much more fun if wasted energy ended up as kinetic energy instead (zoooooom!), but alas, the universe do not work that way.

 

[SecurityTheatre]

You already answered your own question. The efficiency is not 100%, which means that part of the energy ends up as heat. Personally, I think it would be much more fun if wasted energy ended up as kinetic energy instead (zoooooom!), but alas, the universe do not work that way.

A LiPo charging system also has some current limiters. This kind of circuit will put out some heat. I suspect a bulk of the heat is the charging circuitry. I have charged independent-pack LiPo batteries before and the packs don't get hot, but the chargers do.

A LiPo battery that's getting hot is a SERIOUS fire hazard.

 

[videogames101]

It's not the battery, as mentioned if the battery were getting warm you'd be in trouble.

Here's an example of an IC charging solution:

http://datasheets.maximintegrated.com/en/ds/MAX1551-MAX1555.pdf

The two power transistors (one for USB, one for A/C wall blocks), and the logic circuitry in this IC are likely the source of the resistance generating the heat you feel. The faster you charge the battery, the more current is passing through those transistors, the more heat will be generated. Keep in mind that this die is also likely to be very small, so temperature could get very high even if you aren't dissipating much power.

Maximum power dissipation for this IC is listed at 727mW. Not huge, not exactly tiny either.

Page 6 also mentions how the circuit reduces charge current when the IC reaches 110°C. You'd certainly feel that

I'm sure the iPhone 4 uses something similar to charge its battery.

Although 727mW doesn't seem like enough to me to make the phone feel warm, maybe there is another culprit.

 

[bryanl]

The charger can be 80-90% efficient, while the charging of the cells can be 60-70% efficient.

 

[Matt1970]

Every battery I have ever felt while charging gets warm.

 

[Sheep221]

The current that flows in, is quite high, most phones charge at 0.5 - 1A which will create enough heat for device to appear warm, it's normal, whether battery or circuit board is hotter, it is the current that is passing through both of them and generating the heat along the way. Unless the phone is so hot you can't basically touch it or hold in the hand, I wouldn't be worried. In case it does, I would definitely get rid of it or get the battery replaced or phone board fixed as you don't want it to strike the fire in your house.