BU-401: How do Battery Chargers Work?

A good battery charger provides the base for batteries that are durable and perform well. In a price-sensitive market, chargers often receive low priority and get the “after-thought” status. Battery and charger must go together like a horse and carriage. Prudent planning gives the power source top priority by placing it at the beginning of the project rather than after the hardware is completed, as is a common practice. Engineers are often unaware of the complexity involving the power source, especially when charging under adverse conditions.

Chargers are commonly identified by their charging speed. 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-ions fall into this category. Lead- and nickel-based batteries accept charge when cold 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 in which self-discharge brings the voltage to the cut-off point. A regular charger treats such a battery as unserviceable and the pack is often discarded. Boost applies a small charge current to raise the voltage to between 2.2V/cell and 2.9V/cell to activate the protection circuit, at which point a normal charge commences. Caution is required if a Li-ion has dwelled below 1.5V/cell for a week or longer. Dendrites may have developed that could compromise safety. (See BU-802b: What does Elevated Self-discharge Do? in which Figures 5 examines the elevated self-discharge after a Li-ion cell had been exposed to deep discharge. See also BU-808a: How to Awaken Sleeping Li-ion)

Lead- and lithium-based chargers operate on constant current constant voltage (CCCV). The charge current is constant and the voltage is capped when it reaches a set limit. Reaching the voltage limit, the battery saturates; the current drops until the battery can no longer accept further charge and the fast charge terminates. Each battery has its own low-current threshold.

Nickel-based batteries charge with constant current and the voltage is allowed to rise freely. This can be compared to lifting a weight with a rubber band where the hand advances higher than the load. Full charge detection occurs when observing a slight voltage drop after a steady rise. 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 detected. Temperature sensing should also be added that measures temperature rise over time. Such a method is known as delta temperature over delta time, or dT/dt, and works well with rapid and fast charge.

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 charger switches to trickle charge and the battery must cool down. If the temperature stays above ambient, then the charger is not performing correctly and the battery should be removed because the trickle charge could be too high.

NiCd and NiMH should not be left in the charger unattended for weeks and months. Until required, store the batteries in a cool place and apply a charge before use.

Lithium-based batteries should always stay cool on charge. Discontinue the use of a battery or charger if the temperature rises more than 10ºC (18ºF) above ambient under a normal charge. Li ion cannot absorb over-charge and 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 best to place the pack in a cool place and recharge before use.

Types of Chargers

The most basic charger was 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 charging AAA, AA and C cells often deploy this charge method, so do some children’s toys. Remove the batteries when warm.

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. This demands tighter communication between the charger and battery. At a charge rate of 1C, (see BU-402:What is C-rate?) which a 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.

Li-ion has minimal losses during charge and the coulombic efficiency is better than 99 percent. At 1C, the battery charges to 70 percent state-of-charge (SoC) in less than an hour; the extra time is devoted to the saturation charge. Li-ion does not require the saturation charge as lead acid does; in fact it is better not to fully charge Li-ion — the batteries will last longer but the runtime will be a little less. Of all chargers, Li-ion is the simplest. No trickery applies that promises to improve battery performance as is often claimed by makers of chargers for lead- and nickel-based batteries. Only the rudimentary 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 8 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 chargers and other small chargers drawing 30mW or less on standby. Four stars go to chargers with 30–150mW, three stars to 150–250mW and two stars to 250–350mW. The average consumption is 300mW and these units get one star. Energy Star aims to reduce current consumption of personal chargers that are mostly left plugged in when not in use. There are over one billion such chargers connected to the gird globally at any given time.

Simple Guidelines when Buying a Charger

• Charging a battery is most effective when its state-of-charge (SoC) is low. Charge acceptance decreases when the battery reaches a SoC of 70% and higher. A fully charged battery can no longer convert electric energy into chemical energy and charge must be lowered to trickle or terminated.

• Filling a battery beyond full state-of-charge turns excess energy into heat and gas. With Li-ion, this can result in a deposit of unwanted materials. Prolonged over-charge causes permanent damage.

• Use the correct charger for the intended battery chemistry. Most chargers serve one chemistry only. Make sure that the battery voltage agrees with the charger. Do not charge if different.

• The Ah rating of a battery can be marginally different than specified. Charging a larger battery will take a bit longer than a smaller pack and vice versa. Do not charge if the Ah rating deviates too much (more than 25 percent).

• A high-wattage charger shortens the charge time but there are limitations as to how fast a battery can be charged. Ultra-fast charging causes stress.

• A lead acid charger should switch to float charge when fully saturated; a nickel-based charger must switch to trickle charge when full. Li-ion cannot absorb overcharge and receives no trickle charge. Trickle charge and float charges compensate for the losses incurred by self-discharge.

• Chargers should have a temperature override to end charge on a faulty battery.

• Observe charge temperature. Lead acid batteries should stay lukewarm to the touch; nickel-based batteries will get warm towards the end of charge but must cool down on “ready.” Li-ion should not rise more than 10ºC (18ºF) above ambient when reaching full charge.

• Check battery temperature when using a low-cost charger. Remove battery when warm.

• Charge at room temperature. Charge acceptance drops when cold. Li-ion cannot be charged below freezing.