Battery charger

A simple charger works by supplying a constant DC or pulsed DC power source to a battery being charged. A simple charger typically does not alter its output based on charging time or the charge on the battery. This simplicity means that a simple charger is inexpensive, but there are tradeoffs. Typically, a carefully designed simple charger takes longer to charge a battery because it is set to use a lower (i.e., safer) charging rate. Even so, many batteries left on a simple charger for too long will be weakened or destroyed due to over-charging. These chargers also vary in that they can supply either a constant voltage or a constant current, to the battery.

Simple AC-powered battery chargers usually have much higher ripple current and ripple voltage than other kinds of battery chargers because they are inexpensively designed and built. Generally, when the ripple current is within a battery's manufacturer recommended level, the ripple voltage will also be well within the recommended level. The maximum ripple current for a typical 12 V 100 Ah VRLA battery is 5 amps. As long as the ripple current is not excessive (more than 3 to 4 times the battery manufacturer recommended level), the expected life of a ripple-charged VRLA battery will be within 3% of the life of a constant DC-charged battery.[6]

Fast chargers make use of control circuitry to rapidly charge the batteries without damaging any of the cells in the battery. The control circuitry can be built into the battery (generally for each cell) or in the external charging unit, or split between both. Most such chargers have a cooling fan to help keep the temperature of the cells at safe levels. Most fast chargers are also capable of acting as standard overnight chargers if used with standard NiMH cells that do not have the special control circuitry.

To accelerate the charging time and provide continuous charging, an intelligent charger attempts to detect the state of charge and condition of the battery and applies a 3-stage charging scheme. The following description assumes a sealed lead acid traction battery at 25 °C. The first stage is referred to as "bulk absorption"; the charging current will be held high and constant and is limited by the capacity of the charger. When the voltage on the battery reaches its outgassing voltage (2.22 volts per cell) the charger switches to the second stage and the voltage is held constant (2.40 volts per cell). The delivered current will decline at the maintained voltage, and when the current reaches less than 0.005C the charger enters its third stage and the charger output will be held constant at 2.25 volts per cell. In the third stage, the charging current is very small 0.005C and at this voltage the battery can be maintained at full charge and compensate for self-discharge.

Inductive battery chargers use electromagnetic induction to charge batteries. A charging station sends electromagnetic energy through inductive coupling to an electrical device, which stores the energy in the batteries. This is achieved without the need for metal contacts between the charger and the battery. Inductive battery chargers are commonly used in electric toothbrushes and other devices used in bathrooms. Because there are no open electrical contacts, there is no risk of electrocution. Nowadays it is being used to charge wireless phones.

Example of a smart charger for AA and AAA batteries

A "smart charger" should not be confused with a "smart battery". A smart battery is generally defined as one containing some sort of electronic device or "chip" that can communicate with a smart charger about battery characteristics and condition. A smart battery generally requires a smart charger it can communicate with (see Smart Battery Data). A smart charger is defined as a charger that can respond to the condition of a battery, and modify its charging actions accordingly.

Some smart chargers are designed to charge:

• "smart" batteries with internal protection or supervision or management circuitry.
• "dumb" batteries, which lack any internal electronic circuitry.

The output current of a smart charger depends upon the battery's state. An intelligent charger may monitor the battery's voltage, temperature or time under charge to determine the optimum charge current and to terminate charging.

For Ni-Cd and NiMH batteries, the voltage across the battery increases slowly during the charging process, until the battery is fully charged. After that, the voltage decreases, which indicates to an intelligent charger that the battery is fully charged. Such chargers are often labeled as a ΔV, "delta-V," or sometimes "delta peak", charger, indicating that they monitor the voltage change.

The problem is, the magnitude of "delta-V" can become very small or even non-existent if (very) high capacity rechargeable batteries are recharged. This can cause even an intelligent battery charger to not sense that the batteries are actually already fully charged, and to continue charging. Overcharging of the batteries will result in some cases. However, many so called intelligent chargers employ a combination of cut off systems, which are intended to prevent overcharging in the vast majority of cases.

A typical intelligent charger fast-charges a battery up to about 85% of its maximum capacity in less than an hour, then switches to trickle charging, which takes several hours to top off the battery to its full capacity.[7]

Linear induction flashlight, charged by shaking along its long axis, causing magnet (visible at right) to slide through a coil of wire (center) to generate electricity

Several companies have begun making devices that charge batteries based on human motions. One example, made by Tremont Electric, consists of a magnet held between two springs that can charge a battery as the device is moved up and down, such as when walking. Such products have not yet achieved significant commercial success.[8]

A pedal powered charger for mobile phones, fitted into desks has been created by a Belgian company WeWatt, for installation in public spaces, such as at airports, railway stations and universities have been installed in a number of countries on several continents.[9]

Some chargers use pulse technology in which a series of voltage or current pulses is fed to the battery. The DC pulses have a strictly controlled rise time, pulse width, pulse repetition rate (frequency) and amplitude. This technology is said to work with any size, voltage, capacity or chemistry of batteries, including automotive and valve-regulated batteries.[10]

With pulse charging, high instantaneous voltages can be applied without overheating the battery. In a Lead–acid battery, this breaks down lead-sulfate crystals, thus greatly extending the battery service life.[11]

Several kinds of pulse charging are patented.[12][13][14] Others are open source hardware.

Some chargers use pulses to check the current battery state when the charger is first connected, then use constant current charging during fast charging, then use pulse charging as a kind of trickle charging to maintain the charge.[15]

Some chargers use "negative pulse charging", also called "reflex charging" or "burp charging". Such chargers use both positive and brief negative current pulses. There is no significant evidence, however, that negative pulse charging is more effective than ordinary pulse charging.

Varta Solar Charger Model 57082 with two 2100 mAh Ni-MH rechargeable batteries

Solar chargers convert light energy into low voltage DC current. They are generally portable, but can also be fixed mount. Fixed mount solar chargers are also known as solar panels. Solar panels are often connected to the electrical grid via control and interface circuits, whereas portable solar chargers are used off-the-grid (i.e. cars, boats, or RVs).

Although portable solar chargers obtain energy from the sun only, they still can (depending on the technology) be used in low light (i.e. cloudy) applications. Portable solar chargers are often used for trickle charging, although some solar chargers (depending on the wattage), can completely recharge batteries. Other devices may exist, which combine this with other sources of energy for added recharging efficacy.

The output of a timer charger is terminated after a pre-determined time. Timer chargers were the most common type for high-capacity Ni-Cd cells in the late 1990s for example (low-capacity consumer Ni-Cd cells were typically charged with a simple charger).

Often a timer charger and set of batteries could be bought as a bundle and the charger time was set to suit those batteries. If batteries of lower capacity were charged then they would be overcharged, and if batteries of higher capacity were charged they would be only partly charged. With the trend for battery technology to increase capacity year on year, an old timer charger would only partly charge the newer batteries.

Timer based chargers also had the drawback that charging batteries that were not fully discharged, even if those batteries were of the correct capacity for the particular timed charger, would result in over-charging.

A trickle charger is typically a low-current (usually between 5–1,500 mA) battery charger or one which has a trickle charging operating mode. A trickle charger is generally used to charge small capacity batteries (2–30 Ah). These types of battery chargers are also used to maintain larger capacity batteries (> 30 Ah) that are typically found on cars, boats, RVs and other related vehicles. In larger applications, the current of the battery charger is sufficient only to provide a maintenance or trickle current (trickle is commonly the last charging stage of most battery chargers). Depending on the technology of the trickle charger, it can be left connected to the battery indefinitely. Some battery chargers that can be left connected to the battery without causing the battery damage are also referred to as smart or intelligent chargers. Some battery types are not suitable for trickle charging. For instance, most Li-ion batteries cannot be safely trickle charged, and the damage caused can be enough to cause a fire or even an explosion.

The most sophisticated types are used in critical applications (e.g. military or aviation batteries). These heavy-duty automatic “intelligent charging” systems can be programmed with complex charging cycles specified by the battery maker. The best are universal (i.e. can charge all battery types), and include automatic capacity testing and analyzing functions too.

Australian and New Zealand power socket with USB charger socket

Since the Universal Serial Bus specification provides for a five-volt power supply (with limited maximum power), it is possible to use a USB cable to connect a device to a power supply. Products based on this approach include chargers for cellular phones, portable digital audio players, and tablet computers. They may be fully compliant USB peripheral devices adhering to USB power discipline, or uncontrolled in the manner of USB decorations.

A power bank is a portable device that can supply power from its built-in battery through a USB port. Power banks are popular for charging USB charged devices and can be used as a power supply for various USB powered devices such as lights and small fans. They usually recharge with a USB power supply. The power bank includes a control circuit that both regulates charging of the battery and converts the battery voltage to 5.0 volts for the USB port.

USB power bank

There are two main types of chargers used for vehicles:

• To recharge a fuel vehicle's starter battery, where a modular charger is used; typically an 3-stage charger.
• To recharge an electric vehicle (EV) battery pack; see Charging station.

Chargers for car batteries come in varying ratings. Chargers that are rated up to two amperes may be used to maintain charge on parked vehicle batteries or for small batteries on garden tractors or similar equipment. A motorist may keep a charger rated a few amperes to ten or fifteen amperes for maintenance of automobile batteries or to recharge a vehicle battery that has accidentally discharged. Service stations and commercial garages will have a large charger to fully charge a battery in an hour or two; often these chargers can briefly source the hundreds of amperes required to crank an internal combustion engine starter.

Electric vehicle battery chargers (ECS) come in a variety of brands and characteristics. These chargers vary from 1 kW to 7.5 kW maximum charge rate. Some use algorithm charge curves, others use constant voltage, constant current. Some are programmable by the end user through a CAN port, some have dials for maximum voltage and amperage, some are preset to specified battery pack voltage, amp-hour and chemistry. Prices range from $400 to $4500.

A 10 amp-hour battery could take 15 hours to reach a fully charged state from a fully discharged condition with a 1 amp charger as it would require roughly 1.5 times the battery's capacity.

Public EV charging stations provide 6 kW (host power of 208 to 240 VAC off a 40 amp circuit). 6 kW will recharge an EV roughly 6 times faster than 1 kW overnight charging.

Rapid charging results in even faster recharge times and is limited only by available AC power, battery type, and the type of charging system.[16]

Onboard EV chargers (change AC power to DC power to recharge the EV's pack) can be:

• Isolated: they make no physical connection between the A/C electrical mains and the batteries being charged. These typically employ some form of inductive connection between the grid and a charging vehicle. Some isolated chargers may be used in parallel. This allows for an increased charge current and reduced charging times. The battery has a maximum current rating that cannot be exceeded
• Non-isolated: the battery charger has a direct electrical connection to the A/C outlet's wiring. Non-isolated chargers cannot be used in parallel.

Power Factor Correction (PFC) chargers can more closely approach the maximum current the plug can deliver, shortening charging time.

Project Better Place was deploying a network of charging stations and subsidizing vehicle battery costs through leases and credits until filing for bankruptcy in May 2013.

Auxiliary charger designed to fit a variety of proprietary devices

Researchers at the Korea Advanced Institute of Science and Technology (KAIST) have developed an electric transport system (called Online Electric Vehicle, OLEV) where the vehicles get their power needs from cables underneath the surface of the road via inductive charging, (where a power source is placed underneath the road surface and power is wirelessly picked up on the vehicle itself.[17]

Micro USB mobile phone charger

Mobile phone charging station

Most mobile phone chargers are not really chargers, only power adapters that provide a power source for the charging circuitry which is almost always contained within the mobile phone. Older ones are notoriously diverse, having a wide variety of DC connector-styles and voltages, most of which are not compatible with other manufacturers' phones or even different models of phones from a single manufacturer.

China, the European Commission and other countries are making a national standard on mobile phone chargers using the USB standard.[18] In June 2009, 10 of the world's largest mobile phone manufacturers signed a Memorandum of Understanding to develop specifications for and support a microUSB-equipped common External Power Supply (EPS) for all data-enabled mobile phones sold in the EU.[19] On October 22, 2009, the International Telecommunication Union announced a standard for a universal charger for mobile handsets (Micro-USB).[20]

Telecommunications, electric power, and computer uninterruptible power supply facilities may have very large standby battery banks (installed in battery rooms) to maintain critical loads for several hours during interruptions of primary grid power. Such chargers are permanently installed and equipped with temperature compensation, supervisory alarms for various system faults, and often redundant independent power supplies and redundant rectifier systems. Chargers for stationary battery plants may have adequate voltage regulation and filtration and sufficient current capacity to allow the battery to be disconnected for maintenance, while the charger supplies the DC system load. Capacity of the charger is specified to maintain the system load and recharge a completely discharged battery within, say, 8 hours or other interval.