Chargers are often overlooked and labelled as "afterthoughts" in a price-sensitive market. A good battery charger serves as the foundation for long-lasting, high-performance batteries. The industrial charger is frequently manufactured by a third party and offers unique characteristics such as charging under extreme temperatures.
Lead and Nickel-based batteries receive charge at a slower pace in cold temperatures than lead- or nickel-metal hydride batteries, for example. Although batteries may be charged when cold, not all chemistries can, and most Li-ions fall into this group.
Lead or Lithium chargers use constant current and constant voltage (CCCV). The charge current is continuous, and when it reaches a particular level, the voltage is restricted. When a battery hits its voltage limit, it saturates; the current decreases until the battery can no longer absorb more charge, at which point the fast charge is terminated. Each battery has a unique low-current threshold.
Some Li-ion chargers (Cadex) offer a wake-up mechanism, often known as "boost," that allows recharging. A sleep situation might arise when storing a discharged battery. A standard charger considers such a battery to be unserviceable, and the pack is frequently discarded. Boost uses a small charge current to raise the voltage and activate the protection circuit to a point after which regular charging begins.
Lithium ion cannot absorb overcharge and does not get trickle charge when completely charged. If the temperature rises beyond 10 ° C (18ºF) during a usual charge, turn off the battery or charger. During charging, lithium-ion batteries should always be kept cold.
Nickel-based batteries charge at a steady current and enable the voltage to grow freely. When observing a little voltage decrease following a steady ascent, full charge detection happens. The charger should have a plateau timer that ensures a safe charge termination. Temperature sensing, which detects the rise in temperature over time, should also be included. This is known as delta temperature over delta time, or dT/dt, and it works well with quick and fast charging.
A temperature rise is usual with nickel-based batteries, especially when they reach 70% charge. This is caused by a loss in charge efficiency, and the charge current should be reduced to reduce stress. NiCd and NiMH batteries should not be left alone in chargers for weeks or months. Store the batteries in a cool area until needed, and charge them before using. If the charger detects "ready," it switches to trickle charge, and the battery must cool.
TYPES OF CHARGERS
- The most basic sort of charger is the overnight charger, often known as a slow charger. As long as the battery was connected, a basic charger provided a constant charge of around 0.1C (one-tenth of the rated capacity). Slow chargers lack full-charge detection; the charge stays active, and a full charge of an empty battery takes 14-16 hours. The slow charger keeps the NiCd lukewarm to the touch when fully charged. This charging method is used in low-cost consumer chargers that charge AAA, AA, and C batteries
- The rapid charger is utilised in consumer items and falls between the slow and fast chargers. An empty pack takes 3–6 hours to charge. When the charger is full, it flips to "ready." To securely charge a malfunctioning battery, most quick chargers contain temperature sensors.
- The obvious advantage of using a fast charger is that it charges faster. Fast chargers that draw 30mW or less on standby receive five stars from the Energy Star programme. Some nickel-based chargers limit the current as the battery near full charge to compensate for the reduced charge acceptance. When the battery is fully charged, the charger enters trickle charge mode, commonly known as maintenance charge.
- There is no greater need for ultra-fast charging than in the electric vehicle business (EV). Only at moderate temperatures is ultra-fast charging feasible. Li-ion batteries must be designed to charge in 10 minutes or less. As cells age, their capacity and resistance diverge, resulting in a mismatch and unnecessary load on weaker cells. The charging speed of an ultra-fast charger is governed on the battery's state.
The charge characteristics of lead, nickel, and lithium-ion batteries are summarised in the table below.
|Slow charger||NiCd Lead acid||0.1C||14h||0ºC to 45ºC
(32ºF to 113ºF)
|Continuous low charge or fixed timer. Subject to overcharge. Remove battery when charged.|
|Rapid charge||NiCd, NiMH, Li-ion||0.3-0.5C||3-6h||10ºC to 45ºC
(50ºF to 113ºF)
|Senses battery by voltage, current, temperature and time-out timer.|
|Fast charger||NiCd, NiMH, Li-ion||1C||1h+||10ºC to 45ºC
(50ºF to 113ºF)
|Same as a rapid charger with faster service.|
|Ultra-fast charger||Li-ion, NiCd, NiMH||1-10C||10-60 minutes||10ºC to 45ºC
(50ºF to 113ºF)
|Applies ultra-fast charge to 70% SoC; limited to specialty batteries.|
Simple Charger Guidelines
- Charge at a reasonable rate if feasible. When time permits, an ultra-fast charger should allow you to charge at a regular rate, minimizing stress.
- A fast or ultra-fast charge only partially fills the battery; a slower saturation charge completes the charge. Unlike lead-acid batteries, Li-ion batteries do not require a saturation charge, although their capacity is slightly reduced.
- When the battery is cold or hot, do not apply a quick charge. Only charge at room temperature. Avoid charging an old or underperforming battery quickly.
Simple Guidelines for Purchasing a Charger
- When a battery's state-of-charge (SoC) is low, charging it is most efficient. When the battery reaches a SoC of 70% or above, charge acceptance diminishes. A fully charged battery can no longer transfer electric energy into chemical energy and must be discharged or reduced to trickle charge.
- Excess energy is converted into heat and gas when a battery is charged past its full capacity. This might result in the accumulation of undesirable materials with Li-ion batteries. Prolonged overcharging results in lasting harm.
- Use the appropriate charger for the battery chemistry. Most chargers only support one chemical. Check that the battery voltage matches that of the charger. If it is different, do not charge.
- A battery's Ah rating may differ somewhat from what is specified. Charging a bigger battery pack will take longer than charging a smaller pack, and vice versa. If the Ah rating deviates too much, do not charge (more than 25 percent).
- A high-wattage charger reduces charging time, but there are limits to how quickly a battery can be charged. Stress is caused by ultra-fast charging.
- At fully saturated, a lead acid charger should transition to float charge; when full, a nickel-based charger should switch to trickle charge. Li-ion batteries cannot absorb overcharge and do not get trickle charge. Self-discharge losses are compensated for by trickle and float charges.
- To stop charging a bad battery, chargers should include a temperature override.
- Examine the charge temperature. Lead acid batteries should be lukewarm to the touch; nickel-based batteries will grow warm near the end of charge but must cool down before they are ready. When fully charged, lithium-ion batteries should not exceed 10oC (18oF) above ambient temperature.
- When using a low-cost charger, check the battery temperature. When the battery is heated, remove it.
- Allow to charge at room temperature. When it is chilly, charge acceptance decreases. Li-ion batteries cannot be charged below freezing temperatures.