…the technology that goes into battery protection and management
A while ago, I began working on one of the most ambitious projects I’ve started to date. While I still can’t share the details, the project requires a large battery pack, with custom-designed electronics. We’ve all heard the horror stories of large battery packs inside EVs, electric bikes, and other electric vehicles exploding and causing lithium fires. In this blog post, I’m going to discuss some of the reasons for these failures, as well as how electronics like what I’m developing can be designed to mitigate these kinds of risks.
What is a lithium-ion battery?
A lithium-ion battery is a type of battery that stores electrical energy by moving lithium ions between the anode and cathode electrodes of the battery. Typically, these batteries are made with an electrolyte and separators for the anode and the cathode. The important thing to note about lithium-ion batteries is their ability to deliver incredibly high amounts of current over very small amounts of time. A standard AA battery, for example, can only supply about 2-3 amps at 1.5 volts. A single lithium-ion cell can supply hundreds of amps at much higher voltages, depending on how the lithium-ion batteries are connected.
I’m not going to go too deep into the science behind why these batteries work. I’m more focused on the practical uses of such batteries in high-current applications and how to protect them from catastrophic failure.
How do batteries fail?
First things first – why do we even need protection for these batteries? Why don’t we just hook them up to a load and leave it at that? Well, lithium-ion batteries aren’t immune to failure. The main reason lithium batteries fail is because of heat. Lithium batteries will explode at around 500 degrees Celsius, or 932 degrees Fahrenheit. While these temperatures seem very difficult to reach in practical applications, typically temperatures such as these are reached by failure of another part of the circuitry.
Short-circuits
A short circuit is simply when the positive and the negative poles of the battery are connected directly to each other, with no resistance. This leads to a monstrous amount of current to be pulled from the battery, which can very quickly heat the battery up to unsafe levels. The best way to prevent short-circuits is to simply protect any exposed wire ends in your circuitry with insulating material. Whether that’s electrical tape, insulating spray foam, or another type of insulator, it reduces the chance of your circuitry getting hit or knocked around and short circuiting.
Charging failure
A lithium-ion battery will eat whatever voltage you give it, at whatever current you give it. Therefore, the charger that is used for these batteries must have current-limiting capability. If these chargers fail, the batteries can be hit with a large surge of current, leading to the next, biggest reason why batteries fail…
Thermal runaway
While a lithium battery may explode at 500 degrees Celsius, this does NOT mean it is safe at any temperature below 500 degrees Celsius. There is a phenomenon known as thermal runaway, where if the battery exceeds a certain safe temperature it will enter a self-heating chain reaction that eventually leads to explosion. In reality, thermal runaway can occur if the battery heats up at a rate quicker than 20 degrees Celsius per minute, or if the battery reaches ~125 degrees Celsius. These numbers are much more likely to be reached in typical applications! Therefore, great care must be taken to protect batteries from this kind of failure.
Okay… how do you protect batteries from overheating?
The best way to protect a battery pack from overheating is to understand the maximum amount of current the batteries can provide safely. Lithium-ion batteries are not 100% efficient, and have a nonzero impedance. This causes them to heat up when being charged or discharged. Battery manufacturers will always provide a maximum current rating with their battery, and on most consumer batteries a sticker can be found with the maximum safe current draw.
Over-current protection
By using shunt resistors (very low value, high current resistors) and an op-amp, current can be accurately measured through measuring voltage drop. This allows us to use a microcontroller to detect when the current being drawn from a battery pack reaches a threshold, and cut the battery pack off with a power MOSFET.
Over-temperature protection
Temperature sensors can also be positioned in a battery pack to determine if the battery pack is at a safe temperature or not. If not, the microcontroller can, again, cut the battery pack off.
Charge monitoring & Over-voltage protection
A lithium-ion battery may continue to try and charge, even if the battery is at full charge. This can damage the battery and lead to possible thermal runaway.
Closing Thoughts
I’m still working on the electronics for this project of mine, which is why I didn’t go much into detail on them in this blog post. However, this research I’ve done into battery protection has helped me move forwards with electronics design and work to make my battery pack as safe as I possibly can.
Until next time.
