Discover which charger is best for your application
A good charger provides the base for well-performing and durable batteries. In a price-competitive market, chargers often receive low priority. The battery and charger must go together like a horse and carriage (Figure 1), and this is not always the case. Engineers are often not aware of the complex power requirement of a portable device and the need to charge under adverse conditions. Battery and charger are often tacked at the end of a development project.
Figure 1: Battery and charger must go together like a horse and carriage
One does not deliver without the other.
Chargers are divided into personal and industrial, “smart” and “dumb,” slow, fast and ultra-fast types. Consumer products come with a low-cost personal charger that performs well when used as directed. The industrial charger is often made by a third party and includes special features, such as charging at adverse temperatures. Although batteries operate below freezing, not all chemistries can be charged when cold and most Li-ion falls into this category. Lead and nickel-based batteries accept charge but at a lower rate. (See BU-410: Charging at High and Low Temperature)
Some Li-ion chargers (Cadex) include a wake-up feature, or “boost,” to allow recharging if a Li-ion battery has fallen asleep due to over-discharge. A sleep condition can occur when storing the battery in a discharged state and the self-discharge brings the voltage to the cut-off point. A regular charger treats such a battery as unserviceable and the packs are discarded. Boost applies a small charge current to raise the voltage to between 2.20 and 2.90V/cell and activate the protection circuit, at which point a normal charge commences. Caution applies if Li-ion has dwelled below 1.5V/cell for a week or longer. Dendrites may have developed that could compromise safety. (See BU-803: “Can Batteries be Restored?)
Lead- and lithium-based chargers operate on Constant Current Constant Voltage (CC/CV) by which the voltage is capped when reaching a set limit. At this point of the charge cycle, the battery begins to saturate and the current drops. Full-charge occurs when the current drops to a set level. Lead acid requires a periodic full saturation charge to prevent sulfation.
Nickel-based batteries charge with constant current and the voltage is allowed to fluctuate freely. This can be compared to lifting a weight with an elastic band where the hand moves ahead of the load. Full charge detection occurs when observing a slight voltage drop after a steady rise. This method is known as delta Temperature over delta time, or dT/dt, and works well with rapid and fast charge. To safeguard against anomalies, such as shorted or mismatched cells, the charger should include a plateau timer to assure a safe charge termination if no voltage delta is measured. Temperature sensing should also be added.
A temperature rise is normal with nickel-based batteries, especially when reaching the 70 percent charge level. A decrease in charge efficiency causes this and the charge current should be lowered to limit stress. When “ready,” the battery must cool down. If the temperature stays above ambient, then the charger is not performing right and the battery should be removed. Extended trickle charge on nickel-based batteries inflicts damage. NiCd and NiMH should not be left in the charger unattended for weeks and months. If not required, store them in a cool place and apply a charge before engagement.
Lithium-based batteries should always stay cool on charge. Discontinue using the battery or charger if the temperature rises more than 10ºC (18ºF) above ambient when reaching full charge. Li ion cannot absorb over-charge and therefore does not receive trickle charge when full. It is not necessary to remove Li-ion from the charger, however, if not used for a week or more, it is always best to place the pack in a cool place and recharge before use.
The most basic charger is the overnight charger, also known as a slow charger. This goes back to the old nickel-cadmium days where a simple charger applied a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery was connected. Slow chargers have no full-charge detection; the charge stays engaged and a full charge of an empty battery takes 14–16 hours. When fully charged, the slow charger keeps NiCd lukewarm to the touch. Because of its reduced ability to absorb over-charge, NiMH should not be charged on a slow charger. Low-cost consumer chargers to charge AAA, AA and C cells often use this charge method, so do some children’s toys.
The rapid charger falls between the slow and fast charger and is used in consumer products. The charge time of an empty pack is 3–6 hours. When full, the charger switches to “ready.” Most rapid chargers include temperature sensing to safely charge a faulty battery.
The fast charger offers several advantages and the obvious one is shorter charge times. Short charge times demand tighter communication between the charger and battery. At a charge rate of 1C, (see BU-402:What is C-rate?) which the fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. As the battery approaches full charge, some nickel-based chargers reduce the current to adjust to the lower charge acceptance. The fully charged battery switches the charger to trickle charge, also known as maintenance charge. Most of today’s nickel-based chargers have a reduced trickle charge to also accommodate NiMH.
Charging Li-ion is most efficient and the coulombic efficiency is better than 99 percent. The charge time to 70 percent is typically less than an hour. The extra time is devoted for the long saturation charge that is not mandatory as it is for lead acid. In fact, it is better not to fully charge Li-ion as it will last longer. Of all chargers, the Li-ion charger is the most simplistic. No trickery applies to improve battery performance and longevity as is often claimed with chargers for lead- and nickel-based batteries. Only the basic CCCV method works.
Lead acid cannot be fast-charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14–16 hours; anything slower is a compromise. Lead acid can be charged to 70 percent in about eight hours; the all-important saturation charge takes up the remaining time. A partial charge is fine provided the lead acid occasionally receives a fully saturated charge to prevent sulfation.
The standby current on a charger should be low to save energy. Energy Star assigns five stars to mobile phone and similar small charger drawing 30mW or less on standby. Four stars go to 30–150mW, three stars to 150–250mW and two stars to 250–350mW units. The average is 300mW and this gets one star. Energy Star aims to reduce current consumption of personal chargers that are mostly left plugged in when not in use.
Last Updated 2015-09-29
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