Why do mobile fast chargers sometimes stop charging suddenly?
No, fast chargers doesn’t stop charging suddenly instead they stop charging fast.
There are mainly 2 reasons for this:
1. Charging — Battery charging mechanism itself effects the fast charging because the charging cycle consists of 2 stages.
- Constant current variable voltage — In the 1st stage, current flowing into the battery is kept constant and voltage keeps increasing from low to high. Till the voltage reaches a peak value, the current flowing is high resulting high power. In this stage fast charging takes place (till 70–80%).
- Constant voltage variable current — In the 2nd stage, the voltage is kept constant and the current varies from high to low. This current almost becomes zero when the battery is full. Here the total power output is low due to low current (from80–100%).
2. Battery — Modern day smartphone uses a lithium-ion battery. In short, lithium-ion battery doesn’t charge quickly when it goes above 80% because above 80% the movement for lithium-ions from the cathode to anode becomes difficult. It is similar to packing a suitcase in (less) remaining space.
That’s the reason why charging in the 2nd phase is slow and it should be kept that way else the battery life decreases.
Coming to fast chargers, they charge the battery at high voltage with high current flow in the 1st phase and eventually it slows down in the 2nd phase. Due to this fast charge, the 2nd phase it feels like it is charging very slowly.
That raises another question — Why OEM’s didn’t use fast chargers right from the start?
It is because, most people these aren’t using their smartphone for more than 2 years so smartphone OEM’s were willing to make a compromise here, that is battery life. Fast charging doesn’t come for free without baking the battery life. That means, in long-term the battery degrades faster using a fast charger.
Bonus:
Another parameter that affects the fast charging is the temperature. When the smartphone or the charger gets hot the power rating is reduced automatically, credits to USB-PD (USB Power Delivery).
