I would like to know about the workings of mobile phone battery chargers. I know the basic principle, you take a transformer to bring it down to 9/6/3.7 volts then a bridge rectifier circuit and a capacitor input filter to rectify it into DC. Then you apply it to the battery. But I know this should not be enough, I need at least two things,

  1. Protection against back current flow
  2. Switch off power when the battery is fully charged.

Can someone point me to something simple (apart from this http://www.extremecircuits.net/2009/07/mobile-phone-battery-charger-circuit.html)

  • \$\begingroup\$ Are you talking about the wall adapter only, or also the battery charging circuitry that's built into the phone? \$\endgroup\$
    – endolith
    Oct 16, 2010 at 17:27
  • \$\begingroup\$ The battery charger circuit in the mobile will be extra credit yes. \$\endgroup\$
    – Rick_2047
    Oct 17, 2010 at 5:45

3 Answers 3


You are taking an AC signal from the wall and then as you say, the transformer, recitifier and capacitors are bringing it down to a lower DC voltage. From there, most charging circuits are either linear regulator type chargers or switch mode power supply type regulators (both of those are advanced type links, but just there to see they both exist). They are basically normal DC/DC power converters with some kind of monitoring circuit inside. These two components fulfill the needs you list in number 1 and 2. To learn more about how they work, I'd check out Dave's explanation of linear and switch mode power supplies.

If you're looking to build your own eventually, you'll need to figure out how to do the monitoring with a micro or otherwise and then control some kind of DC/DC power converter. Good luck!

  • \$\begingroup\$ These are very advanced type of things, What I need is a very simple circuit like that used in cheap chargers. \$\endgroup\$
    – Rick_2047
    Oct 17, 2010 at 5:48

The battery charging for Li-ion is a little more involved. IIRC The Li-ion chargers typically apply a constant voltage until the battery is charged to a certain level and then apply constant current until the battery is charged.

There are a number of Li-ion charger ICs that handle the various charging cycles and also have the safeties built in. One of the popular ones is the Maxim MAX1555. The application notes on the Maxim site will provide the details about Li-ion charging.

If you decide to build your own charger I would suggest purchasing a UL approved wall transformer that provides you an isolated DC output. Add your charging circuit to the output of the safe DC voltage.

  • 2
    \$\begingroup\$ The battery charging is usually built into the phone though. The adapter just supplies a fixed voltage for it to use. \$\endgroup\$
    – endolith
    Oct 16, 2010 at 17:26
  • \$\begingroup\$ Umm, BIG warning here, please look at jluciani 's post, you cannot charge a mobile battery directly, not only is there a little bit of math, but you HAVE to have a temperature sensor, every phone has one AFAIK, it's probably a legal requirement... anyway, there a chips that will do the whole trick for you, or you can use a microcontroller ,or 2 timers and a comparator or three ... + a temp sensor. DO NOT credit my answer click on jluciani! \$\endgroup\$
    – Conrad B
    Oct 16, 2010 at 19:10
  • \$\begingroup\$ I may have misunderstood the question. I thought the question was about how the battery charging works not how to supply the battery charger circuitry in the phone with DC voltage. \$\endgroup\$
    – jluciani
    Oct 16, 2010 at 21:42
  • \$\begingroup\$ SparkFun sells a Polymer Lithium Ion battery charger based on the MAX1555 IC ( sparkfun.com/commerce/product_info.php?products_id=726 ). The schematic is here: sparkfun.com/datasheets/Batteries/LiPo-USB-Charger-v13.pdf \$\endgroup\$
    – tcrosley
    Oct 19, 2010 at 3:31

For a cell phone, the wall adapter just supplies a constant voltage, like any other wall wart, and the phone has all the current regulation and charge monitoring circuitry internally.

If the charger is small, light-weight, and cool to the touch during operation, it probably has a switching regulator inside. If it's heavy, bulky, cheap, and warm to the touch during operation, it's probably a linear regulator.

5 V micro USB chargers will be the standard in the future. With any charger, the voltage applied to the phone is fixed, but the phone can adjust the amount of current it draws. Some supplies can provide a lot, but some (like plugging into a laptop's USB) can only supply a little. There are a few different ways to tell the phone how much current it's allowed to draw. The USB-IF battery charging standard has the charger short the D+ and D- pins together. iPhones, on the other hand, require certain voltages applied to the D+ and D- pins. Others use the 5th ID pin to indicate a dedicated charger. Otherwise, without some kind of identification of the charger, a USB-charged phone should only draw 100 mA, in case it's connected to a low-power computer port.

As for "Protection against back current flow", a simple Schottky diode will do. The charger circuitry drives current into the battery by applying a larger voltage than the battery's voltage.

And the phone's internal circuitry takes care of this, as well as switching off power (or applying a float charge?) when the battery is fully charged.


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