Feature: Batteries
Figure 7: TI’s BQ76PL455A can be used with fewer than 16 cells; in such cases, the unused cell inputs must be the highest ones in the chain
• Overtemperature aff ects the cell electrolyte material, reducing the SOC. It can also increase solid-electrolyte interphase (SEI) formation, resulting in increased and non-uniform resistivity and power loss;
• Under temperature is also a problem, as it can cause deposition of lithium, which also results in capacity loss;
• Overcurrent, and resultant internal heating due to uneven internal impedance and eventual thermal runaway; this can increase the SEI layers in the battery and increase resistivity.
T ere’s a conundrum here, since, for
example, it is fairly straightforward to accurately measure the voltage of an individual cell at the test bench. A designer just needs to connect a fl oating (non-grounded) or battery-powered digital voltmeter (DVM) across the cell of interest; see Figure 1. However, it is far more diffi cult for many reasons to do so with confi dence and safety in an electrically- and environmentally-harsh situation such as, say, an EV or HEV. T is is made clear by a representative EV power pack example comprising 6720 Li+ cells, managed by eight control modules; see Figure 2. Each cell has a capacity of 3.54Ah, resulting in a total nominal energy
Figure 8: TI’s BQ79616PAPRQ1 supports a bidirectional ring topology for an additional link connectivity path in case of a wire break or node failure
storage of 100kWh; i.e., 3.54Ah x 4.2V x 6720 cells. Each of the 96 series- connected rows is made up of 70 cells in parallel, for a battery voltage of 403.2V (96 rows × 4.2V), with a capacity of 248Ah (100kWh/403.2V, or 3.54Ah × 70 columns). Problems include:
• Providing the needed resolution and accuracy when measuring a low, single- digit voltage to get meaningful precision at several mV, due to the presence of a high common-mode voltage (CMV), which can overload the measurement system and aff ect reading validity. T is CMV is the sum of the voltages of all the series-connected cells, up to the one being measured, with respect to system common (also referred to as “ground” – although that is a misnomer). Note that in an EV, there can be up to as many as 96 or even 128 battery cells in series, yielding a CMV in the hundreds of volts.
• Due to the high CMV, it is necessary to galvanically isolate the cells from the rest of the system for both electrical integrity and user/system safety, since neither should be exposed to the full CMV.
• Electrical noise and surges can easily corrupt millivolt-range reading.
• T e multiple cells must be measured nearly simultaneously (within a few milliseconds), to create an accurate overall picture of cell and battery
pack status. Otherwise, time skew between cell measurements can result in misleading conclusions and resultant actions.
• T e large number of cells means that some sort of multiplexing arrangement is needed between the cells and the rest of the data-acquisition subsystem, or else the size, weight and cost of the interconnecting wiring becomes prohibitive.
• Finally, there are signifi cant and mandatory considerations related to safety, redundancy and error reporting that must be satisfi ed. T e standards diff er from industry to industry – industrial and power tools are very diff erent from vehicles, where they are the most stringent. In mission- critical automotive systems such as those related to battery management, a loss of functionality must not lead to a hazardous situation. In the case of a malfunction within the system, the “safe” state requires the electronics be switched off , and the vehicle driver alerted via a dashboard light or other indicator. For some systems, however, a malfunction or the loss of functionality can potentially lead to a hazardous event that cannot simply be switched off , so safety goals may include a defi ned “safety-related availability” requirement. In such cases, tolerance for some types of faults in
www.electronicsworld.co.uk June 2022 25
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48
