Battery chargers evolve with more intelligence

The rapidly expanding market for portable equipment is leading to continuing demands for batteries to deliver improved performance and longer operating life.

The rapidly expanding market for portable equipment is leading to continuing demands for batteries to deliver improved performance and longer operating life.

In line with these criteria, the capacity and performance of rechargeable cells have steadily increased, from the introduction of nickel cadmium and sealed lead acid batteries in the 1960s to the more recent arrival of nickel metal hydride and lithium ion technologies.

Each cell type has its own advantages and drawbacks, but the decision which to use in a particular application may well depend on considerations of cost as much as technical desirability.

Sealed lead acid batteries (SLA) require current limited constant voltage charging.

For SLA batteries in standby use, continuous charging at 2.25V/cell is adequate.

At this potential, the battery will not overcharge and can remain connected indefinitely without incurring damage.

The equivalent for nickel cadmium (NiCd) and nickel metal hydride (NiMH) is an overnight (14h) constant current charge at 0.1C.

Although most SLA and NiCd cells are suited to long term trickle charging, charging of NiMH cells must be terminated to avoid damage.

For fast charging, at rates of up to 2C, intelligent monitoring of battery parameters at all stages in the charging cycle is essential.

The latest generation of intelligent charger technology offered by Lawtronics can address the effects of protection circuitry and variations in operating temperature and contact resistance.

An essential requirement of fast chargers is an automatic reduction in charge rate before the onset of overcharging, with its consequent rise in temperature and pressure.

The charging characteristics of NiCd and NiMH cells are similar, although NiMH generates more heat during charge and peak voltage is less apparent.

Lawtronics can offer various constant current fast charge termination techniques in both cell types.

These include: dV/dt, the rate of change of voltage close to peak voltage; -dV, the fall in cell voltage beyond peak; and dT/dt, the rate of change of battery temperature as it approaches a fully charged state.

Circuitry for reforming and conditioning abused, unformed or old cells prior to bulk charging is also highly desirable for nickel-based cells.

All these features come with a price, but without this sophistication, serious damage to the cells can occur, affecting both capacity and lifetime, as well as creating safety issues.

The charging of lithium ion (Li-ion) cells is similar to that for SLAs, both requiring current limited constant voltage.

However, for Li-ion, it is vital that the charger incorporates end point current cutoff and a fail-safe timer for overall protection.

Due to the specific requirements for individual applications, Lawtronics recommends that Li-ion charging systems should be integrated with the battery pack design at the earliest opportunity in the design process.

Battery technology is continuing to evolve, and intelligent charger design is essential for ensuring that the maximum benefits are available to the user.

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