In the previous part of this blog mini-series I have discussed the battery pack housed in the energy box. In this part safety will be the main topic.
Lithium-based batteries are quite nice. They can contain (fairly) large amounts of energy at a low weight will also being able to deliver lots of power. They do have their downsides however, for example they can’t be turned off. This is true for all chemical batteries but since they are so energetic it is a larger problem. Below you can see the energy that is delivered by one of our battery modules as a function of its voltage. Normally we would say that it is ’empty’ at 3.0 volts. However, this is not entirely true or rather it is entirely not true. As can be seen, the battery can be discharged even further (down at 0V when it is truly electrically empty). However, once a Lithium-ion battery is discharged below 2.4 volts it is irreversibly damaged and going down to 0 volts it is deemed dead. In most cases this very unwanted, but not very dangerous. However, if someone were to try and recharge the battery it could potentially catch fire or explode due to the chemical processes in the battery cell.
So there are two problems here. The first is that we can’t power off the battery and so the boat will continue to consume energy slowly discharging the battery. The second problem is that we need to shut down the battery in case of an emergency or when the voltage drops too low. We evade these problems by not actually shutting down the battery, but disconnecting it from the rest of the systems by means of two relays. One big Gigavac relay which can handle hundreds of currents for discharging and one smaller relay for charging. The relays are controlled by our energy management system and are wired through the emergency stop and deadman switches, interrupting the current whenever the pilot or the system detects possible danger.
Even though the relays can handle hundreds of amps and can interrupt even more, they are not meant for emergency in-operation switching. In the case something does awfully go wrong we also employ a Carling Technologies circuit breaker. This specific circuit breaker is designed to interrupt over a thousand amps. A circuit breaker acts like a fuse in the sense that it will stop the current flow when it senses the current is too high, but it has the nice ability that it is resettable and therefore does not need replacement. Unfortunately, if the pilot is too enthusiastic the circuit breaker might trip, setting the boat to a standstill. Normally it would be necessary for the pilot to open the hatch to the energy box and manually reset the breaker. This would cost precious racing minutes, so we fitted the circuit breaker with a remotely operated motor so it can be reset with just a simple press of a button.
So now we have a system that can interrupt the current flow out of the battery in any given situation, sounds safe right? Well technically yes, but very rarely there may still be a problem. Battery technology has rapidly advanced since the development of the first lithium based battery back in the 90s allowing us to build the energy box in a very compact manner. However there is still a very slim chance that a battery cell might be faulty coming fresh out of the factory, think of the recent problems with “hoverboards”. If this is the case, our relays and circuit breakers are useless because a battery cell might catch fire just out of nothing. It would be a waste of the energy box not to speak of the danger it might be to the boat and the pilot.
A battery based fire is not easy to control. Some people may remember the “triangle of fire”, the three necessities for a fire to break out. These are heat, fuel and oxygen. Unfortunately, all of these three factors are fed to the fire by the battery cell itself and therefore a fire cannot be controlled in the traditional manner. Fortunately, the “triangle of fire” is actually the “tetrahedron of fire”, which sounds way cooler but also shows us another factor which must be present for a fire to exist: A chain reaction. A fire is nothing more than the combining of oxygen to other molecules in a chemical reaction. If you could in some way block the reaction from happening, the fire will die out. This is exactly what our firepro fire extinguisher does. It uses an aerosol mixture that interrupts the chemical reaction and thus it is able to stop a battery fire. It even leaves all of the electronics intact, unlike powder extinguishers.
This is the end of part three of our mini-series on the energy box. Want to read more about our energy box and his friends energy box 2 and energy box 3? continue reading part four which is on the topic of fuel gauging.