Lithium Batteries: When Protection Circuits Fall Short
The landscape of lithium batteries is a complex one. Protection circuits, integral to the safety and longevity of these batteries, sometimes fall prey to subpar designs. For instance, a grey area emerges when you have a protection circuit (sometimes called a PCM) where the maximum continuous discharge currents are noticeably below the over-current protection values. What happens when the load or charge current exceeds the maximum continuous current but doesn't reach the over-current protection value? This grey area of operation is a cause of concern.
In fact, when I posed this question to one manufacturer, their unsettling answer was simply, "the board will char." Such an answer indicates a deeper issue: many battery designs opt for economical protection circuits, that is a poor choice for your product and your customer's safety.
A potential solution? Introducing a dual redundancy circuit. While adding a secondary layer of protection, say, a PTC, can potentially alleviate the problem, it's essential to ponder: What if one of the protection mechanisms malfunctions?
The IEC62368.1:2023, Annex M provides some clarity. It requires testing the battery system with one protection device intentionally deactivated, either through a short or open circuit. Subsequently, the battery is subjected to a controlled failure scenario, examining the effectiveness of the remaining protection circuit.
When you apply an overcurrent to the system after simulating a single point failure, you're testing how the rest of the system responds to such a condition without the protection. Ideally, for a system with well-implemented safety measures, there should be a secondary mechanism or redundant protection in place to handle or mitigate the overcurrent event in the absence of the primary protection.
In the case of the low cost protection circuit mentioned earlier, disabling the secondary protection device, say the PTC, still does not cover protection in the grey area that can char the board, which may lead to a hot battery and potential thermal runaway.
Indeed, when I tested a battery with the low cost protection circuit under a single fault condition, the FETs measured 137 °C within 55 seconds of introducing an overcurrent fault.
In the realm of battery protection and similar circuits, redundancy is highly valued. Common redundancies for overcurrent situations might include:
1. Thermal protection: If overcurrent causes excessive heat, a thermal cutoff (TCO) or a positive temperature coefficient (PTC) device might be in place to intervene.
2. Backup electronic protection: Another electronic circuit or controller that detects overcurrent and either shuts down the system or restricts the current.
3. Mechanical fuses: Traditional fuses that would blow under excessive current, providing a hard stop.
4. Software-based monitoring: Systems with microcontrollers or ICs that can detect overcurrent and respond accordingly.
When designing a product or system, especially something as critical as a battery or power management system, ensuring multiple layers of protection against possible failure scenarios is crucial. A secondary mechanism isn't just a nice-to-have; it's often a necessary backup to maintain safety standards and prevent catastrophic failures.
The bottom line is that every battery should be engineered with safety at its core, adhering to standards. IEC 62368 is a beacon in this regard, building upon multiple foundational standards pertinent to battery safety systems in Audio/Video , information and communication technology equipment.
So, when in the market for batteries for your new product, insist on designs that meet the requisite standards. Any esteemed battery manufacturer should possess and have a deep understanding of the standards pivotal to your industry.
