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Battery charger

A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery by forcing an electric current through it.

The charge current depends upon the technology and capacity of the battery being charged. For example, the current that should be applied to recharge a 12 V car battery will be very different from the current for a mobile phone battery.

Types of battery chargers

Simple

A simple charger works by connecting a constant DC power source to the battery being charged. The simple charger does not alter its output based on time or the charge on the battery. This simplicity means that a simple charger is inexpensive, but there is a tradeoff in quality. Typically, a simple charger takes longer to charge a battery to prevent severe over-charging. Even so, a battery left in a simple charger for too long will be weakened or destroyed due to over-charging. These chargers can supply either a constant voltage or a constant current to the battery.

Trickle

A trickle charger is a kind of simple charger that charges the battery slowly, at the self-discharge rate. A trickle charger is the slowest kind of battery charger. A battery can be left in a trickle charger indefinitely. Leaving a battery in a trickle charger keeps the battery "topped up" but never over-charges.

Timer-based

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.

Intelligent

Output current depends upon the battery's state. An intelligent charger may monitor the battery's voltage, temperature and/or time under charge to determine the optimum charge current at that instant. Charging is terminated when a combination of the voltage, temperature and/or time indicates that the battery is fully charged.

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, or "delta-V," charger, indicating that they monitor the voltage change.

However, the magnitude of "delta-V" can become small or even nonexistant if (very) high capacity rechargable batteries are recharged. This can cause even an intelligent battey charger to not sense that the batteries are actually already fully charged, and continue charging. Overcharging of the batteries result.

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.

Fast

Fast chargers make use of control circuitry in the batteries being charged to rapidly charge the batteries without damaging the cells' elements. Most such chargers have a cooling fan to help keep the temperature of the cells under control. Most are also capable of acting as a standard overnight charger if used with standard NiMH cells that do not have the special control circuitry. Some fast chargers, such as those made by Energizer, can fast-charge any NiMH battery even if it does not have the control circuit.

Pulse

Some chargers use pulse technology in which a pulse is fed to the battery. This DC pulse has 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. With pulse charging, high instantaneous voltages can be applied without overheating the battery. In a Lead-acid battery, this breaks-down stubborn lead-sulfate crystals, thus greatly extending the battery service life.

Several kinds of pulse charging are patented. 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.

Some chargers use "negative pulse charging", also called "reflex charging" or "burp charging". Such chargers use both positive and brief negative current pulses. Such chargers don't work any better than pulse chargers that only use positive pulses.

Inductive

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. It is commonly used in electric toothbrushes and other devices used in bathrooms. Because there are no open electrical contacts, there is no risk of electrocution.

USB-based

Since the Universal Serial Bus specification provides for a five-volt power supply, it's possible to use a USB cable as a power source for recharging batteries. Products based on this approach include chargers designed to charge standard NiMH cells, and custom NiMH batteries with built-in USB plugs and circuitry which eliminate the need for a separate charger. Moixa Energy patented a design of batteries, branded USBCELL, that incorporate their own USB chargers internally, complete with their own plugs. In the currently available AA battery design, the positive end of the battery doubles as a flip-cap for the built-in USB plug.

Charge rate

This is often denoted as C and signifies a charge or discharge rate equal to the capacity of a battery divided by 1 hour. For example C for a 1600 mAh battery would be 1600 mA (or 1.6 amps). 2C is twice this rate and 1/2C is half the rate.

Applications

Since a battery charger is intended to be connected to a battery, it may not have voltage regulation or filtering of the DC voltage output. Battery chargers equipped with both voltage regulation and filtering may be identified as battery eliminators.

Mobile phone charger

Most mobile phone chargers are not really chargers, only adapters that provide a power source for the charging circuitry which is almost always contained within the mobile phone. Mobile phones can usually accept relatively wide range of voltages, as long as it is sufficiently above the phone battery's voltage. However, if the voltage is too high, it can damage the phone. Mostly, the voltage is 5 volts or slightly higher, but it can sometimes vary up to 12 volts when the power source is not loaded.

Battery chargers for mobile phones and other devices are notable in that they come in a wide variety of DC connector-styles and voltages, most of which are not compatible with other manufactuers' phones or even different models of phones from a single manufacturer.

Users of publicly accessible charging kiosks must be able to cross-reference connectors with device brands/models and individual charge parameters and thus ensure delivery of the correct charge for their mobile device. A database-driven system is one solution, and is being incorporated into some of the latest designs of charging kiosks.

There are also human-powered chargers sold on the market, which typically consists of a dynamo powered by a hand crank and extension cords. There are also solar chargers.

China and other countries are making a national standard on mobile phone chargers using the USB standard.

Battery charger for vehicles

There are two main types of charges for vehicles:

To recharge a fuel vehicle's starter battery, where a modular charger is used.
To recharge an electric vehicle (EV) battery pack.

Battery electric vehicle

These vehicles include a battery pack, so generally use series charger.

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

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

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

On board 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 charging. Some isolated chargers may be used in parallel. This allows for an increased charge current and reduced charging times.

Non-isolated: the battery charger has a direct electrical connection the A/C outlet's wiring. Non-isolated chargers cannot be using in parallel.

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

Some battery electric vehicle charging devices includes:

Manzanita Micro Power factor correction (PFC) PFC series chargers (mid price range) (non-isolated) (90 to 240 VAC input, 12 to 366 VDC output).
Russco SC and DSO series chargers (low price range) (non-isolated) (120 VAC input, 72 to 120 VDC battery packs, 120 to 156 VDC pack require their AC input boost transformer).
Zivan NG series chargers (mid price range) (isolated).
BRUSA Elektronik AG (high price range) (isolated).

Charge stations

There is a list of public EV charging stations in the U.S.A.

Project Better Place is deploying a network of charging stations. It also subsidize vehicle battery costs through leases and credits.

Prolonging battery life

A short circuit (connecting the output terminals together) does not usually damage a simple battery charger. For that reason it is a suitable source of DC voltage for experimentation. It may, however, require an external capacitor to be connected across its output terminals in order to "smooth" the voltage sufficiently, which may be thought of as a DC voltage plus a "ripple" voltage added to it. To see the difference between connecting and not connecting a capacitor, connect also an oscilloscope across the output terminals. Note that there may be an internal resistance connected to limit the short circuit current, and the value of that internal resistance may have to be taken into consideration in experiments.

On the other hand, many rumors circulate about the best practices to prolong battery life. What practices are best depend on the type of battery. It is "rumored" that Nickel-based cells, such as NiMH and NiCd, need to be fully discharged before each charge, or else the battery loses capacity over time in a phenomenon known as "memory effect". However, this is only partially accurate: nickel alloy cells can be charged at any point throughout their discharge cycle ‏– they do not have to be fully discharged. Memory effect should instead be prevented by fully discharging the battery once a month (once every 30 charges). This extends the life of the battery since memory effect is prevented while avoiding full charge cycles which are known to be hard on all types of dry-cell batteries, eventually resulting in a permanent decrease in battery capacity.

Most modern cell phones, laptops, and most electric vehicles use Lithium-ion batteries. Contrary to some recommendations, these batteries actually last longest if the battery is not fully charged; fully charging and discharging them will degrade their capacity relatively quickly. Degradation occurs faster at higher temperatures. Lithium batteries degrade more while fully charged than if it is only 40% charged. The conditions of high temperature combined with full charge are exactly the scenario occurring when a laptop computer is run on AC power. Degradation in lithium-ion batteries is caused by an increased internal battery resistance due to cell oxidation. This decreases the efficiency of the battery, resulting in less net current available to be drawn from the battery.

Internal combustion engine vehicles, such as boats, RVs, ATVs, motorcycles, cars, trucks, and more use lead acid batteries. These batteries employ a sulfuric acid electrolyte and can generally be charged and discharged without exhibiting memory effect, though sulfation (a chemical reaction in the battery which deposits a layer of sulfates on the lead) will occur over time. Keeping the electrolyte level in the recommended range is necessary. When discharged, these batteries should be recharged immediately in order to prevent sulfation. These sulfates are electrically insulating and therefore interfere with the transfer of charge from the sulfuric acid to the lead, resulting in a lower maximum current than can be drawn from the battery. Sulfated lead acid batteries typically need replacing. Good ventilation and avoidance of ignition sources (e.g., sparks) is wise when recharging, since charging a lead acid battery generates highly explosive hydrogen gas.