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From my experience all chargers for Li-Ion batteries notably heat up when charging the battery.

Why does that happen? Is the charger acting as a not that efficient power supply or is there any process inside it that is specific to charging that leads to such heating up?

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    \$\begingroup\$ Unless heat generation is the actual purpose of a device it is always a sign of "not that efficient". \$\endgroup\$ – Curd Dec 20 '11 at 10:04
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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 are 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 random 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 (well, 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 Watts! That means a lot of heat!

Here is a 25W aquarium heater!

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Abdullah has a good point about the efficiency of the power supply. The batteries are also not 100% efficient in converting the charging current to chemical 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 battery, no additional energy will be stored. That current times the battery's voltage at that current still represents power. That power has to go somewhere, which is to heat the battery when the battery isn't storing it chemically.

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 once heat up some NiMH batteries so much that their outer plastic actually started to blister. The person who bought it said they got that model because it claimed to do the same thing as the others but faster. Lithium batteries 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.

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