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In order to assemble our pack, we purchase individual 18650 cells. We use Panasonic NCR18650BsLG MJ1, which have a nominal voltage of 3.7V 635 V and a capacity of 3400mAh3500 mAh. Then, we assemble these cells in parallel to form modules or strings. Putting cells in parallel increases the capacity and maximum discharge rate of the assembled module, but each module still has a nominal 3.7V635 V. To raise the voltage to our operating voltage, we place modules in series to form the bare battery pack. These assembled packs are normally denoted by the number of cells in series and parallel. For example, our pack in MSXII is designed to contain 1296 cells with 36 cells per module and 36 modules in series. This is referred to as a 36s36p pack. Note that it is critical that cells of the same capacity and chemistry are used, and that all modules contain the same number of cells in parallel. We want all cells to experience the same load, and the overall performance of the pack is only as good as that of its weakest cell.
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- Over/Under-voltage: Each cell must stay within its safe operating voltage. Too low and we cause irreversible damage to the electrodes, permanently reducing its capacity. Too high and we risk thermal runaway in addition to electrode damage. We monitor each module individually and normally try to keep them between 32.0V 5V and 4.2V.
- Over current: Drawing too much current or overcharging can cause thermal runaway and reduces the capacity of the pack. A separate current limit is defined for charging and discharging.
- Over/Under-temperature: Lithium-ion cells operate optimally within a certain temperature range. Too cold and we lose capacity and risk electrode damage. Too hot, and we risk thermal runaway. We monitor the temperature of the cells to watch out for thermal trends.avoid charging or discharging the cells when any module is over-temperature. We currently do not monitor for under-temperature since we assume that as a solar car, we'll most likely always operate when it's relatively warm outside.
We cannot directly map a lithium-ion battery's voltage to remaining life, especially since they have non-linear charge and discharge curves. Instead, we monitor the battery's state of charge (SOC), which is defined as the available capacity as a percentage of either its rated or maximum effective capacity. This is a relatively arbitrary measure without units. There are a number of different methods of SOC estimation:
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