# How do phones charge their battery at seemingly any rate with no issues? 500mA or 1A or 1.2A depending on the charger?

If most use some sort of li-ion battery, and those batteries like to be charged in CCCV format how can my phone charge just fine at 5V, 500mA or 5V, 1A or 5V, 1.2A just fine (actual current delivered, not power supply capabilities)? It seems the current is not constant. If I made a 5V, 1A battery charger it is recommended to not use a 5V, 0.5A supply since the charger was designed for 1A.

Do phones just have really complicated charging circuitry that differs from the standard li-ion charging ICs? Everyone can either charge their phone with USB at 500mA or use a wall adaptor for a faster rate.

• 1. How do you know what the ACTUAl current is? | 2. LiIon usually charges at a max of C (eg 1000 mA for a 1000mAh battery) BUT some manufacturers allow more - maybe 1.5C or 2C - and a few specify 0.5C. In CC mode it is usually OK to charge at less than or equal to the battery's maximum. In CV mode the battery sets the current. Jul 4 '14 at 14:57
• I have an ammeter in series with the charging cable output. I was more curious on how they did it, and I think one user said below that the charging load can be variable impedance to get the most juice allowed, up to a limit of course.
– ACD
Jul 7 '14 at 13:51

'constant current' just means the charger will act like a current source and provide the cell with a specific amount of current, at whatever voltage the cell wants to sit at. The current doesn't have to be 1A or 2A or whatever, the charger will use the largest current that it can get away with drawing from the source that the phone is connected to. I would imagine the charger has a configurable current limit, and the phone's CPU will adjust this depending on how much power it can request from the source.

• might be better to think of it as "current-limited mode" May 20 '16 at 14:21

Some of the charger chips can be programmed to produce 50mA to 1.2A. Some appear to have enumeration built-in. Some detect BC standard identification resistors. Others depend more on another MCU to do the smarts.

The charger (including the external MCU if that's involved) will determine the maximum current available by resistors attached to the USB data lines (BCv1.2 or whatever for dumb supplies), or by enumerating over the USB bus, asking for current and taking what is offered (or perhaps something different for non-compliant chargers).

It then uses the maximum available safe current that it can (it may not be capable of dealing with all possibilities optimally) for the battery, adjusting the parameters of the algorithm correspondingly. Of course it takes longer to charge at 500mA than at 1.2A.

Do phones just have really complicated charging circuitry that differs from the standard li-ion charging ICs?

Yes, phones do have a really complicated charging circuitry. The circuitry group is called "Power Management". It could be a single PMIC (power management IC), or can be a set of ICs under control of a microprocessor (usually embedded into main phone SoC).

The standard Li-Ion charger is only a part of it, which manages the output to battery cell.

The other part is a smart switch that balances battery charging with power delivery to phone's main CPU/SoC. When there is enough input power, the PMIC can do both functions - charge the battery, and feed the CPU. If not, phones might have different policies of how to handle weak battery.

Yet the other (and very important) part of phone's PMIC system is to identify capabilities of external power source, and manage its input consumption in accord with source limitations. A Phone usually identifies the source capability by reading its "charger signature" and/or engaging into some handshake protocol.

So yes, phones do have a really complicated charging circuitry.