80% of battery explosions and fires happen during the charging period, according to FEMA. The main cause of these failures in the battery was associated with the use of alternate chargers that were not originally sold with the unit. To control the charging current, some simple devices rely on the wall charger, whereas more complex devices contain their own charging IC. How using the wrong wall charger could incorrectly charge a portable medical device and lead to a battery is easy to understand.
In a compact bundle with a decent charge capacity and energy density, most portable devices need a light-weight battery. For portable devices, lithium-ion and lithium-polymer batteries are chosen because they have a high density of energy, are light weight and can be packed or formed in several ways. Lithium-polymer batteries are lower in energy density than lithium-ion batteries, but they are safer, lighter and extremely thin to make. The remainder of this article will concentrate on the Lithium-ion and Lithium-polymer battery technology.
Environmental conditions can lead to battery failures as well. Take into account the temperature limits that your portable medical system would be exposed to over its lifetime. For instance, because of the extreme temperature ranges inside a vehicle, a Bluetooth hands-free adaptor installed in a car containing a lithium-polymer battery may experience battery failure.The battery, with its small solar panel, was charged. When the sun was brightest, the rate of charge would rise to its limit. The temperature inside the car will, coincidentally, rise during the day. The battery began to swell and finally broke open the enclosure as a result of continuously charging the battery at peak temperatures.
Today, there are many chemicals available for batteries. For the correct application, it is necessary to select the correct battery. In cars, Lead Acid batteries are used because they are effective at producing large quantities of current. As they have high energy density (small) and are lightweight, lithium batteries are widely used in portable medical devices.
The battery chemistry will determine the following parameters of your battery.
- Charge capacity
- Charge density
- Discharge and charge currents
- Nominal voltage
- Charge voltage
- Self-discharge rate
- Operating temperature
Battery protection circuit module (PCM)
With a Safety Circuit Module, the key means of securing a battery is (PCM). This module limits the battery’s maximum voltages and currents for charge and discharge. If any of these conditions are met, the PCM will go into open circuit mode. Overcharging or even discharging a battery can lead to thermal instability, resulting in an abrupt release of energy (jet of fire or explosion).
A PCM should protect every cell in a battery pack. Usually, the PCM is placed directly on the cell, but can be combined with the circuit board for charging. To support a multi-cell battery pack, multiple PCM’s can be ganged together on a single PCB.
Lithium battery manufacturers must minimise the presence of impurities in the batteries during the manufacturing process, such as microscopic metal particles. Failure to do so may lead to a heat buildup resulting from an internal short circuit. A chain reaction also occurs where the internal heat causes the battery to become thermal unstable and causes the battery from the inside to self-destruct.This self-destruction may be as simple as gases that swell and hiss, or as dramatic as flames and explosions that shoot. The consumer may not be shielded from defective or impure battery chemistry by Safety Circuit Modules (PCMs).
Storage and Shelf Life
Lithium-ion and lithium-polymer batteries with around 40 percent charge should be kept at 15 ° C. By reducing the self-discharge volume, this increases the shelf life. After sitting in storage for 1 year a standard Lithium-Ion Polymer can hold more than 85 percent of its original charge. The retained charge would be decreased when a battery is held at a higher temperature.
Swelling during Charging
When a battery is charged, up to 7 percent of its original size can naturally swell. This must be integrated into its architecture by your enclosure. Ignoring this would put excessive mechanical stress on the battery and the enclosure, causing the battery and/or enclosure to suffer damage.
Charging a battery with the correct voltage and current in the correct sequence is critical. To safely charge them, lithium-ion and lithium-polymer batteries need a multistate charging profile. This profile is designed to maintain the battery’s durability while maintaining its protection without decreasing its efficiency.
In the market place, there are a large number of battery charger ICs that work well are safe and have been extensively checked. To prevent charging or discharging the battery when the temperature is greater than its maximum permissible charge/discharge temperature, many charger ICs often control the battery temperature.
No amount of testing and security will completely eradicate battery failures that result in damage or injury to property. Both precautions and safety standards should be respected by product designers and engineers. When designing portable medical equipment, take precautions and never underestimate the vast quantities of energy contained in the batteries.