Battery Charging

The circuitry to recharge the batteries in a portable product such a mobile phone plays an important part in determining the battery longevity and the practicalities of using the product on a daily basis.

The charging protocol (how much voltage or current for how long, and what to do when charging is complete) depends on the size and type of the battery being charged.

The modern battery chargers adapt the charging parameters dynamically based on the level of charging the battery has reached. An empty battery can be charged faster without any safety risk. This is why most benchmarks for charging speed (ours included) quote the battery charging level reached after a 30-minute charging session on an empty battery.

With basic chargers outputting 5V/1A equalling to 5W of power, anything faster than that is considered quick or fast charging.

Quick charging

Quick charging field is still very much fragmented and almost every manufacturer has its own solution - most of the time, including proprietary tech.

The most common solution is the 5V/2A charging which delivers 10W of power and pretty much every phone other there supports this charging rate. The real quick charging starts from there and up.

Smartphones utilizing Qualcomm chipsets can make use of Qualcomm's Quick-Charge protocol. It's been through a few generations already with the latest one being Quick-Charge 4+. It is backward compatible with the previous generations and the most common implementations peak at 18W of power output. Motorola is using this standard for its phones even though they are marketing it as Turbo-Power and Quick-Charge is not mentioned anywhere.

Similarly to Qualcomm, MediaTek has also introduced its own charging standard called Pump Express, which is supported by phones using the company's chipsets and it requires its own set of proprietary chargers. The latest generation of the standard is Pump-Express 3.0 and it utilizes a USB-C connector for the charging cable. Pump Express+ 2.0 is available as well as a more budget solution and it allows the use of a micro-USB connector.

USB Power Delivery is another quick charging standard and this one is not limited to a particular hardware manufacturer. It doesn't require proprietary hardware though it does require the use of USB-C to USB-C cable. The maximum power output is 100W because there are even laptops that rely on this standard for charging. The current implementations in smartphones however only go as high as 18W of power output.

Oppo, Vivo and OnePlus share some of their intellectual property and R&D and as a result, their phones use similar quick charging solutions. Oppo calls it VOOC Flash charge, Vivo calls it just Fast battery charging, while OnePlus used to call theirs Dash charge (now renamed to just OnePlus Fast Charge for legal reasons). All three versions output 18-20W of power.

But since the three manufacturers are spearheading the quick charge revolution, in 2018 they came up with even faster implementations so they introduced new names as well making things a bit more confusing for the inexperienced users. Oppo's Super VOOC Flash charge can output 50W of power. Vivo's Dual-Engine Fast Charge can output 22.5W of power. And finally, OnePlus's Warp charge can deliver up to 30W.

Huawei also has a proprietary fast battery charging solution in their top-tier smartphones called Super-Charge, which is capable of outputting 40W of power but their more common implementations provide up to 22.5W of power.

Meizu's mCharge solution is proprietary as well and is already available in a few of their higher models. It can deliver up to 24W of power. Meizu has also demoed their future Super mCharge solution which can deliver up to 55W of power but it's yet to release a smartphone integrating it as of the time of writing this.

Wireless charging

Wireless (or inductive) charging uses an electromagnetic field to transfer energy between two objects through electromagnetic induction. Induction is achieved by placing a device which is equipped with the induction coil directly onto a dedicated charging station (or charging pad).

While there used to be at least a few competing wireless charging standards in the past, nowadays the entire mobile industry has moved to using Qi (pronounced "chee").

Much like with the regular wired charging - wireless charging can be performed at different rates. The nominal power output of a Qi charging pad is 5W but faster chargers can already pump out up to 15W of power to phones which support it.

Regardless of the maximum power output supported by the charging pad and the smartphone, the Qi standard demands that all the hardware is backward compatible so regardless the supported revision - any Qi pad is compatible with all Qi-enabled devices.

Basic Battery Charging Methods

Constant Voltage

A constant voltage charger is basically a DC power supply which in its simplest form may consist of a step down transformer from the mains with a rectifier to provide the DC voltage to charge the battery. Such simple designs are often found in cheap car battery chargers. The lead-acid cells used for cars and backup power systems typically use constant voltage chargers. In addition, lithium-ion cells often use constant voltage systems, although these usually are more complex with added circuitry to protect both the batteries and the user safety.

Constant Current

Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow, switching off when the voltage reaches the level of a full charge. This design is usually used for nickel-cadmium and nickel-metal hydride cells or batteries.

Taper Current

This is charging from a crude unregulated constant voltage source. It is not a controlled charge as in V Taper above. The current diminishes as the cell voltage (back emf) builds up. There is a serious danger of damaging the cells through overcharging. To avoid this the charging rate and duration should be limited. Suitable for SLA batteries only.

Pulsed Charge

Pulsed chargers feed the charge current to the battery in pulses. The charging rate (based on the average current) can be precisely controlled by varying the width of the pulses, typically about one second. During the charging process, short rest periods of 20 to 30 milliseconds, between pulses allow the chemical actions in the battery to stabilize by equalizing the reaction throughout the bulk of the electrode before recommencing the charge. This enables the chemical reaction to keep pace with the rate of inputting the electrical energy. It is also claimed that this method can reduce unwanted chemical reactions at the electrode surface such as gas formation, crystal growth and passivation. If required, it is also possible to sample the open circuit voltage of the battery during the rest period.

Burp Charging

Also called Reflex or Negative Pulse Charging Used in conjunction with pulse charging, it applies a very short discharge pulse, typically 2 to 3 times the charging current for 5 milliseconds, during the charging rest period to depolarize the cell. These pulses dislodge any gas bubbles which have built up on the electrodes during fast charging, speeding up the stabilization process and hence the overall charging process. The release and diffusion of the gas bubbles is known as "burping". Controversial claims have been made for the improvements in both the charge rate and the battery lifetime as well as for the removal of dendrites made possible by this technique. The least that can be said is that "it does not damage the battery".

IUI Charging

This is a recently developed charging profile used for fast charging standard flooded lead acid batteries from particular manufacturers. It is not suitable for all lead acid batteries. Initially the battery is charged at a constant (I) rate until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. This first part of the charging cycle is known as the bulk charge phase. When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. This second part of the cycle completes the normal charging of the battery at a slowly diminishing rate. Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalize the charge on the individual cells in the battery to maximize battery life.

Trickle Charge

Trickle charging is designed to compensate for the self discharge of the battery. Continuous charge. Long term constant current charging for standby use. The charge rate varies according to the frequency of discharge. Not suitable for some battery chemistries, e.g. NiMH and Lithium, which are susceptible to damage from overcharging. In some applications the charger is designed to switch to trickle charging when the battery is fully charged.

Float Charge

The battery and the load are permanently connected in parallel across the DC charging source and held at a constant voltage below the battery's upper voltage limit. Used for emergency power back up systems. Mainly used with lead acid batteries.

Random Charging

All of the above applications involve controlled charge of the battery, however there are many applications where the energy to charge the battery is only available, or is delivered, in some random, uncontrolled way. This applies to automotive applications where the energy depends on the engine speed which is continuously changing. The problem is more acute in EV and HEV applications which use regenerative braking since this generates large power spikes during braking which the battery must absorb. More benign applications are in solar panel installations which can only be charged when the sun is shining. These all require special techniques to limit the charging current or voltage to levels which the battery can tolerate.

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