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Hitech-Battery-ChargeThe performance and longevity of rechargeable batteries are to a large extent governed by the quality of the charger. In a price-competitive world, battery chargers are often given low priority, especially as consumer products. Choosing a quality charger is important considering the cost of battery replacement and the frustration poorly performing batteries create. The charger should serve as a quintessential master and guardian angel to protect the environment and save money by extending battery life.


There are two varieties of chargers: the personal chargers and the fleet chargers. For cell phones, laptops or digital cameras, manufacturers include personal chargers. These are made for one battery type, are economically priced and perform well when used for the application intended.

The fleet charger serves employees in a team environment and often has multi bays. The original equipment manufacturer (OEM) sells the chargers and third parties also provide them. While the OEMs meet the basic requirements, third-party manufacturers often include special features, such as a discharge function for battery conditioning and calibration.

Some manufacturers of third-party chargers have become creative and offer advanced charge methods for lead- and nickel-based batteries. While pulse charging may be beneficial for nickel-based batteries, this method is not recommended for Li-ion. The voltage peaks are too high and cause havoc with the protection circuit. Battery manufacturers do not support alternative charging methods and say that pulse charging could shorten the life of Li-ion.

There are many valuable additional features for chargers, and hot- and cold-temperature protection is one. Below freezing, the charger lowers or prevents charge depending on the type of battery. When hot, the charger only engages when the battery temperature has normalized to a safe level. Advanced lead acid chargers offer temperature-controlled voltage thresholds, as well as adjustments to optimize charging for aging batteries.

Some chargers, including Cadex chargers, feature a wake-up feature or “boost” to allow charging Li-ion batteries that have fallen asleep. This can occur if a Li-ion battery is stored in a discharged condition and self-discharge has depressed the voltage to the cut-off point. Regular chargers read these batteries as unserviceable and the packs are discarded. The boost feature applies a small charge current to activate the protection circuit to 2.20–2.90V/ cell, at which point a normal charge commences. Caution should be applied not to boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer.

There are two common charge methods, which are voltage limiting (VL) and current limiting (CL). Lead- and lithium-based chargers cap the voltage at a fixed threshold. When reaching the cut-off voltage, the battery begins to saturate and the current drops while receiving the remaining charge on its own timetable. Full charge detection occurs when the current drops to a designated level. 

Nickel-based batteries, on the other hand, charge with a controlled current and the voltage is allowed to fluctuate freely. This can be compared to lifting a weight with an elastic band. The slight voltage drop after a steady rise indicates a fully charged battery. The voltage drop method works well in terminating the fast charge, however, the charger should include other safeguards to respond to anomalies such as shorted or mismatched cells. Most batteries and chargers also include temperature sensors to end the charge if the temperature exceeds a safe level. 

A temperature rise is normal, especially when nickel-based batteries move towards full-charge state. When in “ready” mode, the battery must cool down to room temperature. Heat causes stress and prolonged exposure to elevated temperature shortens battery life. If the temperature remains above ambient, the charger is not performing right and the battery should be removed when “ready” appears. Extended trickle charge also inflicts damage, and nickel-based batteries should not be left in the charger for more than a few days.

A lithium-based battery should not get warm in a charger and if this happens, the battery or charger might be faulty. Discontinue using the battery and/or charger. Li‑ion chargers do not apply a trickle charge and disconnect the battery electrically when fully charged. If these packs are left in the charger for a few weeks, a recharge may occur when the open circuit voltage drops below a set threshold. It is not necessary to remove Li-ion from the charger when full; however, if not used for a week or more, it is better to remove them and recharge before use.

A mobile phone charger draws about 2 watts on charge, while a laptop on charge takes close to 100 watts. The standby current must be low and Energy Star offers mobile phone chargers drawing 30mW or less five stars for high efficiency; 30–150mW earns four stars, 150–250mW three stars, and 250–350mW two stars. The industry average is 300mW on no-load consumption and this gets one star; higher than 500mW earns no star. Low standby wattage is only possible with small chargers, such as the four billion mobile phone chargers that are mostly plugged in.

Simple Guidelines When Buying a Charger

  • Use the correct charger for battery chemistry. Most chargers serve one chemistry only.
  • The battery voltage must agree with the charger. Do not charge if different.
  • Within reasons, the Ah rating of a battery can be higher or lower than specified. A larger battery will take longer to charge than a smaller one and vice versa.
  • The higher the amperage of the charger, the shorter the charge time will be. There are limitations as to how fast a battery can be charged.
  • Accurate charge termination and correct trickle charge prolong battery life.
  • When fully saturated, a lead acid charger should switch to a lower voltage; a nickel-based charger should have a trickle charge NiMH; a Li-ion charger provides no trickle charge.
  • Chargers should have a temperature override to end charge on a malfunctioning battery.
  • Observe the temperature of the charger and battery. Lead acid batteries stay cool during charge; nickel-based batteries elevate the temperature towards the end of charge and should cool down after charge; Li-ion batteries should stay cool throughout charge.

Slow Charger

Also known as an “overnight charger,” the slow charger goes back to the old nickel-cadmium days and applies a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery is connected. Slow chargers are very simple; they have no full-charge detection, the charge current is always engaged, and the charge time on an empty battery is 14 to 16 hours.

When fully charged, a slow charger keeps NiCd lukewarm to the touch. Some overcharge is acceptable and the battery does not need to be removed immediately when ready. However, the pack should not stay in the charger for more than a day or two because of “memory,” also known as crystalline formation.

A problem arises when charging a battery with a lower mAh rating than specified. Although the slow charger will charge the battery normally at first, higher than 0.1C current for this smaller battery will heat up the pack towards the full-charge state. Because there is no provision to lower the current or terminate the charge, excessive heat will shorten the life of this pack. Observe the battery temperature while charging and remove the battery when warm to the touch. Most slow chargers have no “ready” light.

The opposite can also occur when the slow charger charges a larger battery. In this case, the battery may never reach full charge and remains cold. Performance is poor because the battery does not receive a full charge. A nickel-based battery that is undercharged will eventually lose the ability to accept a full charge due to crystalline formation.

Slow chargers are found in cordless phones, electric toothbrushes and children’s toys. A slow charger works well for these products because battery and charger are harmonized. Chargers servicing a broader range of batteries need some intelligence to supervise the charge, control the current when full, and provide safety if an anomaly occurs.

Rapid Charger

The rapid charger falls between the slow and fast chargers and services nickel- and lithium-based batteries. Unless specially designed, the rapid charger cannot service both nickel- and lithium-based chemistries on the same platform; it needs a designated platform.

The rapid charger is most commonly used for consumer products. The charge time of an empty pack is 3 to 6 hours (less for a partially charged battery), and when the battery is full, the charger switches to “ready.” Most rapid chargers include temperature protection to safeguard against failures. This and other features offer improved service over the slow charger, and batteries tend to perform better. Although they are more expensive to build, high-volume production makes the rapid charger available at a moderate price.

Fast Charger

The fast charger offers several advantages over others. The obvious one is shorter charge times. Because of the need for a larger power supply and more complex control circuits, fast chargers are reserved for commercial application, such as medical, military, communications and power tools. Let’s not forget the hobbyist who must rely on a fast charger with minimal interruptions.

The faster the charge time gets, the better the communication must be managed between charger and battery. At a 1C charge rate, which the fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. As a battery approaches full charge, some nickel-based chargers reduce the charge current to adjust to the lower charge acceptance, and when the battery is full the charger switches to trickle charge.

Most nickel-based fast chargers accommodate NiCd and NiMH batteries on the same algorithm, but not Li-ion. To service a Li-ion pack, specialty dual-mode chargers can read a security code on the battery that allows switching to the right charger setting.

Lead acid batteries cannot be fast-charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14 hours; anything slower may be a compromise. As with all chemistries, lead acid can be charged relatively quickly to 70 percent; the all-important saturation charge consumes the remaining time. A partial charge at a high rate is fine provided the battery receives a fully saturated charge once every few weeks to prevent sulfation. Fast chargers serve the medical, military, power tool and communication markets. Let’s not forget the hobbyist who wants minimal disruptions caused by flat batteries.

Ultra-fast Chargers

Ultra-fast chargers have been around for many years. Large NiCds and specialty Li-ion batteries can be charged at a very high rate up to 70 percent charge. At a rate of 10C, or 10 times the rated current, a 1A battery could theoretically be charged in six minutes, but there are limits. To apply an ultra-fast charge, the following conditions must be observed:

  • The battery must be designed to accept an ultra-fast charge.
  • Ultra-fast charging only applies during the first charge phase, and the charge current must be lowered once the 70 percent state-of-charge (SoC) threshold is reached.
  • All cells in the pack must be balanced and in good condition. Older batteries with high internal resistance would heat up and are no longer suitable for ultra-fast charging.
  • Ultra-fast charging can only be done under moderate temperatures. Low temperature slows the chemical reaction, and energy not absorbed causes gassing and heat buildup.
  • The charger must include temperature compensations and other safety provisions to halt the charge if the battery gets unduly stressed.

An ultra-fast charger is similar to a high-speed train that is capable of traveling at 300km per hour (188 mph). Although made to go fast, the conditions of the tracks and not the machinery govern the maximum speed. Adding power to a charger is relatively simple; the intelligence to prevent overcharge on anomalies is the more complex task. A properly designed ultra-fast charger only provides the amount of charge the battery can absorb safely. Older batteries with high internal resistance and unmatched cells would not be suitable for ultra-fast charging.

If at all possible, do not charge batteries the ultra-fast way. This causes undue stress. If time permits, charge the battery at a more moderate current. An ultra-fast charger should offer user-selectable charge rates to optimize the charging requirements.

Figure 1 compares the cycle life of a lithium-ion battery when charged and discharged at 1C, 2C and 3C rates. Although the battery performs best at a gentle rate of 1C and less, we must keep in mind that many applications require high rate-charging and discharging, and hence the user must take the shorter life expectation into account.


Figure 1: Cycle performance of Li-ion with 1C, 2C and 3C charge and discharge
Charging and discharging Li-ion above 1C reduces service life. Use a slower charge and discharge if possible. This applies to most batteries.

Simple Guidelines on Chargers

Table 2 shows the benefits and limitations of different charge methods.






Charge termination

Slow charger

Lead acid



0ºC to 45ºC
(32ºF to 113ºF)

Continuous low charge or fixed timer. Subject to overcharge. Remove battery when charged.

Rapid charger

NiCd, NiMH,



10ºC to 45ºC
(50ºF to 113ºF)

Senses battery by voltage, current, temperature and time-out timer.

Fast charger

NiCd, NiMH,



10ºC to 45ºC
(50ºF to 113ºF)

Same as rapid charger with faster service.

Ultra-fast charger

Li-ion, NiCd, NiMH


10-60 minutes

10ºC to 45ºC
(50ºF to 113ºF)

Applies ultra-fast charge to 70% SoC; limited to specialty batteries.

Table 2: Charger characteristics. Each chemistry uses a unique charge termination.


Turn the portable device off while charging. A parasitic load confuses the charger. 
Charge battery at a moderate rate. Ultra-fast charging causes undue stress.
Ultra-fast charge only fills the battery partially. A slower charge does the rest.
Do not apply ultra-fast charge when the battery is cold or hot. Charge batteries at moderate temperatures only.
Do not apply ultra-fast charge to low-performing batteries. Very few chargers are able to assess battery condition and govern charge accordingly.

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