Why do chargers heat up?
Nothing is 100% efficient. And I think your answer is "both". The processes of converting voltages and controlling currents make the charger drop its efficiency and heat up. The heating is the product of power losses, which a re I2R losses. That means the losses due to the resistance of the elements. Even a short wire has a resistance.
However, most of the drop is in the power converter. Check out power supply efficiency using Google.
In an example, let's calculate the power loss in a MOSFET. Let's pick a ran dom MOSFET; IRF540PBF-ND. The properties of this MOSFET are: Rds On (Max) @ Id, Vgs : 77 mOhm @ 17A, 10V That means if you put 17A through this MOSFET, it will act as a resistor (w ell, at least to simplify) of a value of 77mOhm. That is 0.077 ohms! It may seem low, but let's calculate. Power loss in this MOSFET is approximately: I2R= 17.17.0.077 = 22.253 W atts! That means a lot of heat! Here is a 25W aquarium heater!
Abdullah has a good point about the efficiency of the power supply. The bat teries are also not 100% efficient in converting the charging current to ch emical energy. If when you say "charger", if you are referring to a single unit that also holds the batteries being charged, then some of the heat is coming from the batteries themselves. Once a battery gets full, it's charging efficiency drops to 0 by definition . In other words, no matter how much more current is pumped thru the batter y, no additional energy will be stored. That current times the battery's vo ltage at that current still represents power. That power has to go somewher e, which is to heat the battery when the battery isn't storing it chemicall y. Some batteries are not too tolerant of excessive charging or high heat, but that doesn't stop bad chargers from being sold. I saw such a bad charger o nce heat up some NiMH batteries so much that their outer plastic actually s tarted to blister. The person who bought it said they got that model becaus e it claimed to do the same thing as the others but faster. Lithium batteri es are less tolerant of that kind of abuse and chargers for them generally adhere closer to the specs. But there are still bad chargers out there that make great claims so you buy them but only find out too late that they are destroying your batteries.
I found this interesting. It made me decide to only buy ONLY protected batt eries.
Any comments ?
Andy