![]() ![]() The other 3 series elements will still be around 3.6 or 3.7V.īut, the BMU tells the pack to stop charging, because one series element has hit the voltage limit of 4.2V. Now when you go to charge your battery pack, the weaker series element will hit 4.2V a lot sooner than the other 3 series elements because it has lost capacity, so it takes less energy to get to a certain voltage. Now, if you have one weak series element that has lower capacity than normal, let's say you have 3 of your 4 series elements at normal capacity, and 1 series element has degraded somewhat, and only has 80% of initial capacity. If they are all at 4.2V then the battery pack is at 16.8V ("max voltage"). When you charge up your battery pack normally, the cells all go up to 4.2V typically (max charge voltage is greater than nominal voltage). If certain cells age faster than others, the weakest series element in the pack is usually responsible for the most extreme capacity loss.įor instance, let's say you have 4 series elements, nominal voltage is about 3.6V so this gives your pack 14.4V nominal, or "faceplate" voltage. When the pack assembler puts them together, they try to match capacity and impedance of all the cells that go into the battery pack, so that all the cells will age uniformly. However, each battery pack and each cell is a little different and this is what complicates things. Then you can try (as cherjrw says) to estimate capacity based on "expectations" of how much capacity the cells start with. After brief periods of high current draw, you get a temporarily low voltage that can bounce back up, which also complicates things. ![]() While this sounds good on paper, most of the time you are using your battery, the voltage is fairly constant (it's a very flat 3.6-3.7V most of the time), so that doesn't always work so well. fully charged cells are typically at 4.2V, and fully discharged cells are typically at 3.0V. You can try to estimate fuel gauge by reading cell voltage. ![]() Accurate fuel gauging is actually fairly difficult for lithium ion batteries, particularly when the current draw is not a constant, but rather fluctuates depending on what you are doing with the computer (or Tesla Roadster, Nissan Leaf, whatever). The protection electronics make sure: the li-ion cells don't charge at high temperature, they don't get overcharged, or overdischarged, and so on.Īnother feature of a TI BMU is it has a fuel gauging chip. The BMU has many functions - First there are protection elements (every lithium-ion battery in an electronic device has a circuit board on it that protects the cell and regulates charging/discharging, the only exceptions are enthusiasts who use bare cells and have some kind of custom charger). Most laptop battery packs have a TI (Texas Instruments) BMU, or battery management unit on board. I work in this field - not super experienced yet, but I know my way around. ![]()
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