Home > Portable power principles

Portable power principles

Supplier News

Calling a battery an energy storage system adds it to a group that includes such things as flywheels and clock springs. In the context of modern technology, however, batteries are better described as portable, self-contained chemical systems that produce electrical energy.

One-way batteries (called non-rechargeable or primary cells) create electricity from a chemical reaction that permanently transforms the cell. Discharge of the primary cell leads to a permanent and irreversible change in the cell chemicals. Rechargeable batteries (or secondary cells), on the other hand, can be charged by a charger as well as discharged by the application. Thus, secondary cells “store” rather than “generate” energy.

Charge or discharge current is usually expressed (in Amperes) as a multiple of the rated capacity (called the C-rate). For example, a C/10 discharge current for a battery rated at one ampere-hour (1 Ahr) is 1 Ahr/10 = 100 mA. The rated capacity of a cell or battery (in Ahr or mAhr) is the amount of electricity that it can store (produce) when fully charged under specified conditions. Thus, the total energy of a battery is its capacity multiplied by its voltage, resulting in a measurement of watt-hours (Whr.

Measuring battery performance

The chemistry and the design of a battery cell together limit the current it can source. Barring the practical factors that limit performance, a battery could produce an infinite current, if only briefly. The main impediments to infinite current are the basic reaction rates of the chemicals, the cell design and the area over which the reaction takes place. Some cells are inherently able to produce high currents. Shorting a nickel-cadmium cell, for instance, produces currents high enough to melt metals and start fires. Other batteries can produce only weak currents.

The net effect of all chemical and mechanical factors in a battery can be expressed as a single mathematical factor called the equivalent internal resistance. Lowering the internal resistance enables higher currents.

No battery stores energy forever. Unavoidably, the cell chemicals react and slowly degrade, causing a degradation in charge stored by the battery. The ratio of battery capacity to weight (or size) is called the battery’s storage density. High storage density enables the storage of more energy in a cell of given size or weight.

Table 1 list nominal voltage and storage density (expressed in watt-hours per kilogram of weight, or Whr/kg) for the major chemistries used in storage batteries for personal computers and mobiles

You may ask “why not always choose secondary cells, if primary and secondary cells fulfil the same purpose?” The reason is because secondary cells have drawbacks: they lose their electrical charge relatively quickly, through self discharge; and they must be charged before use.

A question of chemistry

A new rechargeable battery or battery pack (several batteries in one package) is not guaranteed to be fully charged; in fact, it is likely to be nearly discharged. The first thing to do, therefore, is to charge the battery/pack in accordance with the manufacturer’s chemistry-dependent guidelines.

Every charging operation applies voltage and current in a sequence that depends on the battery chemistry. Thus, a look at battery-cell chemistries reveals different requirements to be met by the charger and the charging algorithm. The terms most commonly found in battery charging are constant current (CC), used for NiCd and NiMH cells, and constant current/constant voltage (CC/CV), applied to the lithium-ion and lithium-polymer cells now typically used for portable devices.

NiCd cells are charged by applying a constant current in the range 0.05C to more than 1C. Some low-cost chargers terminate the charge by means of absolute temperature. Though simple and inexpensive, that method of charge termination is not accurate. A better choice is to terminate charging when the condition of full charge is indicated by a drop in voltage. The -δV phenomenon is most useful for charging NiCd cells of 0.5C or greater. The -δV end-of-charge detection should be combined with battery-temperature measurement as well, because aging cells and mismatched cells can reduce the voltage delta.

You can achieve a more precise full-charge detection by sensing the rate of temperature increase (dT/dt), and that method of charge detection is kinder to the battery than a fixed-temperature cutoff. Charge termination based on a combination of δT/dt and -δV cut-off enables a longer life cycle by avoiding overcharge.

Fast charging improves charge efficiency. At 1C, the efficiency is close to 1.1 (91 percent), and the charge time for an empty pack will be slightly more than one hour. When applying a 0.1C charge, the efficiency drops to 1.4 (71 percent) with a charge time of about 14 hours.

Because the charge acceptance of a NiCd battery is close to 100 percent, almost all energy is absorbed during the initial 70 percent of charging, and the battery remains cool. Ultra-fast chargers use this phenomenon to charge a battery to the 70 percent level within minutes, applying currents equal to several times the C-rating without heat buildup. Above 70 percent the charging continues at a lower rate until the battery is fully charged. Eventually, you top off the battery by applying a trickle charge in the range 0.02C to 0.1C.

Charging nickel-metal-hydride cells

Though similar to NiCd chargers, an NiMH charger employs the δT/dt method, by far the best method for this type of cell. The end-of-charge voltage depression for NiMH batteries is smaller, and for small charge rates (below 0.5C, depending on temperature) there may be no voltage depression at all.

New NiMH batteries can show false peaks early in the charge cycle, causing the charger to terminate prematurely. Moreover, an end-of-charge termination by -δV detection alone almost certainly ensures an overcharge, which in turn limits the number of charge/discharge cycles possible before the battery fails.

It seems there is no available -dV/dt algorithm that works well for charging NiMH batteries under all conditions: new or old, hot or cold, and fully or partly discharged. For that reason, don’t charge an NiMH battery with a NiCad charger unless it uses the dT/dt method for end-of-charge termination. And because NiMH cells do not absorb overcharge well, the trickle charge must be lower (about 0.05C) than that recommended for NiCd cells.

Slow-charging a NiMH battery is difficult, if not impossible, because the voltage and temperature profiles associated with a C-rate of 0.1C to 0.3C do not provide a sufficiently accurate and unambiguous indication of the full-charge state. The slow charger must therefore rely on a timer to indicate when the charge cycle should be terminated. Thus, to fully charge a NiMH battery you should apply a rapid charge of approximately 1C (or a rate specified by the battery manufacturer), while monitoring both voltage (δV=0) and temperature (dT/dt) to determine when the charge should be terminated.

Efforts should be made to charge at room temperature. Nickel-based batteries should only be fast-charged between 10°C to 30°C. Below 5°C and above, the charge acceptance of nickel-based batteries is drastically reduced. Table 2 summarises the charging techniques for the various battery types.

Charging lithium-ion and -polymer cells

Whereas chargers for nickel-based batteries are current-limiting devices, chargers for lithium-ion batteries limit both voltage and current. The first lithium-ion (Li-ion) cells called for a charge-voltage limit of 4.1 V/cell. Higher voltage means greater capacity, and cell voltages as high as 4.2 V have been achieved by adding chemical additives. Modern Li-ion cells are typically charged to 4.2 V with a tolerance of ±0.05 V/cell.

Full charge is attained after the terminal voltage has reached the voltage threshold and the charging current has dropped below 0.03C, which is approximately 3 percent of Icharge (see Figure 1). The time for most chargers to achieve a full charge is about three hours, though some linear chargers claim to charge a Li-ion battery in about one hour. Such chargers usually terminate the charge when the battery’s terminal voltage reaches 4.2 V. That kind of charge determination, however, charges the battery only to 70 percent of its capacity.

A higher charging current does not shorten the charge time by much. Higher current lets you reach the voltage peak earlier, but then the topping charge takes longer. As a rule of thumb, the topping charge will take twice as long as the initial charge.

Li-ion batteries offer reasonably good charge performance throughout the temperature range, but below 5°C the charge rate should be less than 1C.

Because overcharging (or overdischarging) a Li-ion cell can cause it to explode and injure people, safety is a major concern in handling this type of storage cell. As a result, commercial Li-ion battery packs contain a protection circuit such as the Maxim (distributed by Arrow ) DS2720 (see Figure 2), which provides all electronic safety functions required for applications involving rechargeable Li-ion batteries: protecting the battery during charge, protecting the circuit against excess current flow, and maximising battery life by limiting the level of cell depletion.

DS2720 ICs control the conduction paths for charge and discharge currents with external switching devices such as low-cost n-channel power MOSFETs. The IC’s internal 9 V charge pump provides high-side drive to the external n-channel MOSFETs, yielding lower on-resistances than do the same FETs operating in a more common low-side protection circuit. FET on-resistance actually decreases as the battery discharges.

The DS2720 lets you control the external FETs from the data interface or from a dedicated input, thereby eliminating the redundant power-switch controls otherwise required in a rechargeable Li-ion battery system. Through its 1-wire interface, a DS2720 provides the host system with read/write access to the status and control registers, instrumentation registers and general-purpose data storage. A factory-programmed 64-bit net address allows each device to be individually addressed by the host system (see Figure 3).

The DS2720 provides two types of user memory for battery-information storage, EEPROM and lockable EEPROM. EEPROM is a true non-volatile (NV) memory whose contents (important battery data) remain unaffected by severe battery depletion, accidental shorts or ESD events. When locked, a lockable EEPROM becomes a read-only memory (ROM) that provides additional security for unchanging battery data.

Protection modes

Overvoltage: If the cell voltage sensed at VDD exceeds the overvoltage threshold VOV for a period longer than the overvoltage delay tOVD, the DS2720 shuts off the external charge FET and sets the OV flag in the protection register. The discharge path remains open during overvoltage. The charge FET is re-enabled (unless blocked by another protection condition) when the cell voltage falls below the charge-enable threshold VCE, or discharging causes VDD - VPLS > VOC.

Undervoltage: If the cell voltage sensed at VDD drops below the undervoltage threshold VUV for a period longer than the undervoltage delay tUVD, the DS2720 shuts off the charge and discharge FETs, sets the UV flag in the protection register and enters sleep mode. After the cell voltage rises above VUV and a charger is present, the IC turns on the charge and discharge FETs.

Short circuit: If the cell voltage sensed at VDD drops below the depletion threshold VSC for a period of tSCD, the DS2720 shuts off the charge and discharge FETs and sets the DOC flag in the protection register. The current path through the charge and discharge FETs is not re-established until the voltage on PLS rises above VDD-VOC. The DS2720 provides a test current through internal resistor RTST (from VDD to PLS) to pull up PLS when VDD rises above VSC. This test current allows the DS2720 to detect removal of the offending low-impedance load. In addition, it enables a recovery charge path through RTST from PLS to VDD.

Overcurrent: If voltage across the protection FETs (VDD-VPLS) is greater than VOC for a period longer than tOCD, the DS2720 shuts off the external charge and discharge FETs and sets the DOC flag in the protection register. The current path is not re-established until the voltage on PLS rises above VDD-VOC. The DS2720 provides a test current through internal resistor RTST (from VDD to PLS) to detect removal of the offending low-impedance load.

Overtemperature: If the DS2720 temperature exceeds TMAX, the device immediately shuts off the external charge and discharge FETs. The FETs are not turned back on until two conditions are met: cell temperature drops below TMAX, and the host resets the OT bit.

Newsletter sign-up

The latest products and news delivered to your inbox