A good charger provides the base for well-performing and durable batteries. In a price-competitive market chargers often receive low priority. Engineers are often not fully aware of the complex power requirement of a portable device and the need to charge under adverse conditions.
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.
Some Li-ion chargers (Cadex) include a wake-up feature, or “boost,” to allow recharging if a Li-ion battery have 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.
Lead- and lithium-based chargers operate on Constant Current Constant Voltage (CCCV) 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.
Lithium-based should always stay cool on charge. Discontinue using the battery and/or charger if the battery heats up on 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 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 C AA and AAA cells often use this charger 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. 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, 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 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 charges are most efficient and charge the battery to 70 percent in 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. Only the CCCV method works.