Protection Circuits

-Battery University

Batteries can release high power, and most packs include protection to safeguard against malfunction. The most basic safety device in a battery is a fuse that opens on high current. Some devices open permanently and render the battery useless; others are more forgiving and reset. The Polyswitch™ is such a re-settable device. It creates a high resistance on excess current and reverts back to the low ON position when the condition normalizes. A third method is a solid-state switch that measures the current and disconnects on excessive load conditions. All switching devices have a residual resistance during normal operation, which causes a slight increase in overall battery resistance and a subsequent voltage drop.

Intrinsically Safe Batteries

Intrinsically safe (IS) batteries contain protection circuits that prevent the formation of high currents, which could lead to excess heat, sparks and explosion. Authorities mandate IS batteries for two-way radios, gas detectors and other electronic instruments operating in hazardous areas such as oil refineries, chemical plants and grain elevators. There are several levels of intrinsic safety, each serving a specific hazard level, and the requirements vary from country to country. The provisions are in addition to the protection circuit for lithium-ion, and the approval standards are rigorous. This results in a high price for the battery.

Making Lithium-ion Safe

Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes: [1] a built-in temperature switch called PTC that protects against high current surges, [2] a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and [3]a safety vent that releases gas in the event of a rapid increase in cell pressure.

In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.30V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from over-discharging, a control circuit cuts off the current path at about 2.20V/cell.

Each cell in a string needs independent voltage monitoring. The higher the cell count, the more complex the protection circuit becomes. Four cells in series had been the practical limit for consumer applications. Today, new chips accommodate 5–7, 7–10 and 13 cells in series. For specialty applications, such as the hybrid or electric vehicle delivering several hundred volts, specialty protection circuits are made, which sharply increases the overall cost of the battery. Monitoring two or more cells in parallel to get higher current is less critical than controlling voltages in a string configuration.

Protection circuits can only shield abuse from the outside, such as an electrical short or faulty charger. If, however, a defect occurs within the cell, such as contamination caused by microscopic metal particles, the external protection circuit has little effect and cannot arrest the reaction. Reinforced and self-healing separators are being developed for cells used in electric powertrains, but this makes the batteries large and expensive. While a Li-ion for a laptop provides a capacity of 170–200Wh/kg, the EV Li-ion has only 100–110Wh/kg.

The gas released by venting of a Li-ion cell as part of pressure buildup is mainly carbon dioxide (CO₂). Other gases that form through abusive heating are vaporized electrolyte consisting of ethylene and/or propylene. Burning gases include combustion products of the organic solvents.

Li-ion commonly discharges to 3.0V/cell. This is the threshold at which most portable equipment stops working. The lowest “low-voltage” power cut-off is 2.5V/cell, and during prolonged storage, the self-discharge causes the voltage to drop further. This causes the protection circuit to turn off and the battery goes to sleep as if dead. Most chargers ignore Li-ion packs that have gone to sleep and a charge is no longer possible.

While in the ON position, the internal protection circuit has a resistance of 50 to 100mOhm. The circuit typically consists of two switches connected in series; one is responsible for the high cut-off, and the other for the low cut-off. The protection circuit of some smaller cellular batteries can be relaxed, and some get away with only the cell’s intrinsic protection and/or an external fuse. The absence of a full protection circuit saves money, but a new problem arises. Here is what can happen.

Some low-cost chargers rely solely on the battery’s protection circuit to terminate charge current. Without a functioning voltage termination switch in the battery, the cell voltage can rise too high and overcharge the battery. Heat buildup and bulging are early indications of pending failures before potential disintegration occurs. Figure 1 shows a battery that has fragmented while charging in a car.

A concern also arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. This can fuse the solid-state switches into a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but fail to provide the required safety.

Low price makes generic replacement batteries from Asia popular with cell phone users. While the quality and performance of these batteries is improving, some do not provide the same high safety as the original branded version. A wise shopper spends a little more and replaces the battery with an approved model.

I receive many questions on www.BatteryUniversity.com from visitors wanting to know why the aftermarket does not provide low-cost laptop batteries as readily as cellular batteries. This is mainly due to safety. While a 1,400mAh cellular battery stores only 4Wh of energy, a laptop battery holds about 60Wh, 15 times more. Many manufacturers of consumer batteries protect the batteries with a security inscription that very few can break. Although aftermarket batteries are available, many do not offer all the functions of the branded version. Typical problems are fuel-gauge errors and not being able to charge correctly.

Manufacturers of lithium-ion batteries do not mention the word “explosion” and refer to “venting with flame” or “rapid disassembly.” Although seen as a slower and more controlled process than explosion, venting with flame, or rapid disassembly, can nevertheless be violent and inflict injury to those in close proximity. The court hears many legal cases involving laptops and other batteries that are said to have caused property damage, fire and personal injury. This is also a large concern in the aviation industry. Most of the batteries for consumer products are shipped by air just in time for improved inventory control.

Simple Guidelines for Using Lithium-ion Batteries

• Exercise caution when handling and testing lithium-ion batteries.
   
• Do not short-circuit, overcharge, crush, drop, mutilate, penetrate with foreign objects, apply reverse polarity, expose to high temperature or disassemble packs and cells.
   
• Use only lithium-ion cells with a designated protection circuit and approved charger.
   
• High temperature during charge or discharge may hint of pending failure. Discontinue using the battery and/or charger.
   
• The electrolyte is highly flammable and battery rupture can cause physical injury.
   
• Use a foam extinguisher, CO₂, dry chemical, powdered graphite, copper powder or soda(sodium carbonate) to extinguish a lithium-ion fire. Only pour water to prevent the fire from spreading.
   
• If the fire of a burning lithium-ion battery cannot be extinguished, allow the pack to burn out on its own in a controlled and safe way.

*   IATA (International Air Transport Association) works with airlines and air transport industry to promote safe, reliable, secure and economical air travel.