Battery Management Systems: Uses, Design and Working
A battery is essentially a box full of highly corrosive liquid called an electrolyte with metals or graphite(carbon) called electrodes dipped in it. The liquid reacts with the metals and this drives electrons from one metal to another creating an electrical current. Thanks to the fact that mother earth gave birth to men like Alessandro Volta and Giovanni Aldini, we learnt how to harness the power of an electrochemical reaction and developed batteries. These batteries weren’t powerful enough to power anything beyond a small flashlight bulb and we would have to wait for at least 170 years before Lithium-Ion batteries became a reality and a further 30 years before large scale Lithium-Ion batteries started coming around in the mid-2000s.
A typical Lithium-Ion battery pack consists of several tiny little batteries also commonly called as cells and they are connected together in series and parallel.
Connecting multiple cells in series increases the overall voltage of the battery pack and connecting them in parallel increases their capacity. To know the number of cells in series and in parallel for a given battery pack look for the “S-P” number for example a “2S-3P” connection means 2 cells in series and 3 cells in parallel. In case either of the alphabets is missing say only “2S” or “2P” is written on the pack, it means that there is no connection in the other combination i.e., 2S means 2 cells in series and 0 in parallel and correspondingly, 2P means 2 in parallel and none in series. There are a few terms associated with battery packs in general.
· State of Charge (SoC): This the amount of energy/electrical charge that is left in a given battery. Take a battery that is rated to have 4Wh of capacity an SoC of 35% would mean that out of the 4Wh of energy, only 1.4Wh remain. In EVs, the SoC is generally calculated as a function of energy whereas in case of smartphones and laptops SoC is calculated as a function of electrical charge. Take the 4Wh battery for example, let us suppose it is rated at 3.6V, this would give the battery a nominal capacity of 1.111Ah or 1111mAh and an SoC of say 30% would mean that out of the total charge that the battery can store, 0.3333Ah remain. The SoC of a battery depends upon several factors such as the voltage across its terminals, the instantaneous current, the temperature of the cells, pressure on the cells etc.
· State of Health (SoH): SoH is typically how much the battery has worn out after a given number of charge-discharge cycles. A battery rated for 4Wh might not be able to retain its capacity throughout its life. This happens because of several factors such as corrosion of the electrodes, development of unwanted precipitates inside the cell, thermal damage, dust, water etc. An SoH of 75% in a 4Wh battery means that the battery which could hold 4Wh of electrical energy at 100% SoC can hold only 3Wh now. Typically, a Lithium-Ion battery is considered useful for EVs until it reaches 80% SoH after which its capacity to hold electrical charge degrades too quickly to be used safely in an EV, such batteries are given a second life as inverter batteries in most cases or are recycled for precious metals such as nickel and cobalt.
Now that you’ve developed a rough idea of how Lithium-Ion batteries behave, let’s take a look at how they have to be managed.
A battery management system is essentially what is says it is, a system designed to effectively manage batteries. But how and more importantly why should batteries be managed. (A BMS is sometimes also referred to as an energy management system or an intelligent energy management system by several auto manufacturers)
Some of the functions of a BMS are,
· SoC, SoH measurement
· Offer protection from electrical faults
· Facilitate charging
· Cell balancing
The first 3 functions are pretty straight forward but what is cell balancing and how is it done?
Cell balancing is essential to ensure optimum life of a lithium-ion battery pack and this is simply because not all cells are created equal despite the best quality control processes set up and this may cause some cells to charge/discharge more than others, this results in more wear and tear on some cells than the rest and it reduces performance and can also cause protection circuits to trip.
Cell balancing is achieved in the following ways,
· Active cell balancing: Here the cell with the highest SoC is disconnected from the supply and the rest are allowed to get charged until they reach the same capacity. The key advantage of such a balancing system is that no energy is lost in the balancing process. The disadvantage is that this can only be implemented while charging.
· Passive cell balancing: Here the cell that has the least SoC is disconnected and other cells are discharged to the SoC of this cell. Alternatively, cells can be brought to an SoC which is in between the highest and lowest SoC depending upon the circuit used. In case of the former, the rest of the cells are discharged across resistors until their SoC reaches the minimum value. In the latter, the cells dump their extra charge into capacitor(s) which in turn charge cells that are at a deficit. This method can be used in both charging and discharging conditions but then a minimum of 50% of energy will be lost even in ideal conditions, so this is also generally preferred during charging only. Passive cell balancing is cheaper to implement than active cell balancing that is why low cost EVs generally use this method.
Now, it must be much easier to understand how Lithium-Ion batteries work and why they require fancy electronics to function properly. Do check out the following sources of information.
My published work on Lithium-Ion batteries.
1. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3808494
2. https://ieeexplore.ieee.org/abstract/document/9386861/
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