Saturday, November 9, 2024

Re: [electricboats] Cells (modules) are dying.

Nominal is a funny word. The relevant definition for batteries is, "Existing in name only; not real." For instance a "12 volt" lead acid battery is the name, but the voltage should be 12.5v or 13.7 or 14.4 when fully charged or when on life support or when charging (respectively). These voltage values change by manufacturer, by sealed or flooded, lithium or lead, by intended use case, by longevity requirements... the list really does go on and on. So point 1 is to start with the particular recommendations of the manufacturer for fully charged voltages and for storage voltages and for charging voltages.
Point 2: the idea of rust, deterioration. Humans deteriorate; metals deteriorate, some faster or slower than others. For every battery chemistry, there is a slowest-deterioration temperature and voltage. That voltage is chosen for how lithium chemistry batteries are shipped and stored. If the shipping voltage was any higher or lower, the batteries would degrade internally faster. So when you fully charge the batteries, use them quickly in hours or days, not slowly in weeks or months. The same use-it-or-lose-it idea also applies to batteries below their shipping voltage. In other words, for optimal longevity, minimize time spent above or below the shipping voltage. The bigger the deviation from the shipping voltage, higher or lower, the shorter the overall lifespan of the battery. That is why many people normally only charge to 70% or 80% of full, and then only right before they are going to use that particular electric vehicle or device. After use, leave at or very near shipping charge voltage. If you need a 100% charge every once in a while for maximum range, all the better (because it helps the BMS stay calibrated).
Question: while a lithium chemistry battery is being drained or charged, does it also degrade, or does it only degrade while sitting idle? I assume the active chemical reactions of charging or discharging take precedence over ambient degradation, so I'm guessing lithium batteries only degrade while sitting idle. Any chemists?
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