Each parallel string of cells of a Li-ion pack needs independent voltage monitoring. The more cells that are connected in series, the more complex the protection circuit becomes. Four cells in series is the practical limit for commercial applications.
The internal protection circuit of a mobile phone while in the ON position has a resistance of 50 to 100 mW. The circuit normally consists of two switches connected in series. One is responsible for high cut-off, the other for low cut-off. The combined resistance of these two devices virtually doubles the internal resistance of a battery pack, especially if only one cell is used. Battery packs powering mobile phones, for example, must be capable of delivering high current bursts. The internal protection does, in a certain way, interfere with the current delivery.
Some small Li-ion packs with spinel chemistry containing one or two cells may not include an electronic protection circuit. Instead, they use a single component fuse device. These cells are deemed safe because of small size and low capacity. In addition, spinel is more tolerant than other systems if abused. The absence of a protection circuit saves money, but a new problem arises. Here is what can happen:
Mobile phone users have access to chargers that may not be approved by the battery manufacturer. Available at low cost for car and travel, these chargers may rely on the battery’s protection circuit to terminate at full charge. Without the protection circuit, the battery cell voltage rises too high and overcharges the battery. Apparently still safe, irreversible battery damage often occurs. Heat buildup and bulging is common under these circumstances. Such situations must be avoided at all times. The manufacturers are often at a loss when it comes to replacing these batteries under warranty.
Li-ion batteries with cobalt electrodes, for example, require full safety protection. A major concern arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. Such damage often causes the solid-state switches to fuse in a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but does not provide the required safety. If charged beyond safe voltage limits with a poorly designed accessory charger, the battery may heat up, then bulge and in some cases vent with flame. Shorting such a battery can also be hazardous.
Manufacturers of Li-ion batteries refrain from mentioning explosion. ‘Venting with flame’ is the accepted terminology. Although slower in reaction than an explosion, venting with flame can be very violent and inflicts injury to those in close proximity. It can also damage the equipment to which the battery is connected.
Most manufacturers do not sell the Li-ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits. Most battery assembling houses must certify the pack assembly and protection circuit intended to be used with the manufacturer before these items are approved for sale.