Like others, I’ve been wondering about the “80%” ish rule of thumb for charging phone and car batteries. Specifically I’m wondering, is the electric potential stored evenly across the entire battery? Or do some cells/areas not get exercised when only charging to 80%?
I read the other posts about batteries, but couldn’t find this answer. Thanks in advance.
In: Technology
Yeah it’s evenly distributed and nothing needs to get “exercised” in modern batteries. There was some old tech for batteries 20 years ago that had a “memory” and wanted you to exercise them by fully discharging and charging but that stuff is all long gone
It depends.
A ton of lithium batteries are “single cell” in which there’s one physical battery that acts as a complete unit. A lot of devices that are just “rechargeable” have one battery like electric toothbrushes, rechargeable flashlights, Bluetooth speakers/headphones, etc. In these devices, the actual battery cell is one unit that just gets charged and discharged over and over again.
But a lot of bigger devices like laptops, power tools, electric cars, power banks, bigger Bluetooth speakers, etc. have multiple battery cells. Each battery cell functions independently from every other cell, so they’re all a little bit different from the next in terms of exactly how it was built, so we can’t always trust that cells are completely identical in how much they wear over time, how much energy they hold, or how fast they discharge. How well the cells align with one another is generally called “balance”, and it takes into account the voltage and discharge/charge characteristics of each individual battery cell.
In some cases where there’s only 1 or 2 individual cells like a power bank or Bluetooth speaker, it can be assumed that the few number of cells will remain somewhat similar over the device’s lifetime. Compared to other stuff, there’s not a big worry that the cells will get out of balance with one another. In cases like this, it’s basically considered free extra capacity by combining two battery cells together, and the balance is never even considered throughout the life of the product.
But in much bigger devices like power banks, power tools, and electric cars the “balance” between cells can be a huge deal. If two cells are too far out of balance it will at best reduce the usable life of the battery and at worst start a huge fire. So, these types of batteries employ what’s known as a “battery management system” or BMS.
A BMS takes in all kinds of data about a battery. It measures the voltage of each individual cell or small groups of cells, it determines how much charge the battery needs based on the current balance, it automatically discharges batteries that are above a set threshold so the battery stays better in balance, it can calculate the overall wear and tear on a battery, and much more. A lot of higher-end BMS controllers even have temperature sensors to detect if a battery or part of a battery is getting too hot and it will reduce the rate of charge/discharge to prevent the battery from catching fire!
The 80% rule generally applies to bigger batteries. Devices with a small single-cell battery don’t generally bother with the calculations to determine the best life of a battery. They just charge and discharge the battery between 0% and 100% without care for the battery. With bigger devices like electric cars, the 80% rule is more applicable and a BMS will generally optimize the battery based on that rule or similar specifications to prolong the life of the individual cells.
Overall, it really depends on the type of battery and exactly how it works. There’s a ton of different ways that manufacturers can configure a lithium battery, and it all comes down to specific requirements based on what is being powered!
Batteries are made up of several individual cells. An 18v pack can have five 3.7v cells in series.
Good multicell battery packs have equalizer circuitry in the pack to even out the “level” of charge across the pack so one cell isn’t at 90% when the others are at 60%. The circuit charges each cell individually so that a fully charged cell won’t overheat and catch the pack on fire.
There used to be a memory effect in older (nicd) batteries where the pack “remembered” its charge point so you periodically had the discharge it to zero to get the full capacity back.
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