Charging a lithium battery to 80% only?

Tesla suggests charging their new LFP type battery to 100% regularly. Why isn't this new type of battery more widely used?

1. All batteries have an aging characteristic whether they are charged or not (around 6% loss per year). For example, a home solar array or EV should prioritize the largest practical capacity budgets allow. Thus, the performance remains more consistent throughout the entire service life. This also reduces the discharge load stress on the cells reducing wear, and exposing greater energy capacity through efficiency gains.

2. There are several types of Lithium batteries with unique charge cycle limits. A common 4.2v cell is ideally rated for 8000 cycles if shallow cycled, and slow charged. Accordingly, deep cycling a LiPol or LiIon below 60% into the dead pre-conditioning zone or in the Constant-Current mode will reduce the life to well below 4000 cycles.

3. 80% or 100% full is meaningless because accessible energy depends on temperature (outside 4'C to 50'C is bad), chemistry, and loading. Typically, even the cheapest Lithium charge management ICs have a hysteresis built into the cycle, and will let the cells stand-by after charging until the cell internal resistance self-drains a bit of power in time before charging again.

4. Li batteries do not like trickle-charging or rapid-changing, and in a way the "100%" indication icon often means just that. Over-boosted the cell into the 112% capacity range to provide a short-term capacity boost. This wears out the cell 15% faster, but many tends to do this... so who would know the difference. I'd wager the Apple team added the 80% mode to turn off this boost trick, at the cost of "boosted" capacity.

5. There are consumer chargers for Lithium cells with temperature and impedance monitoring while charging. These are highly recommended if you have 21700 or 18650 cell based equipment. Unlike other options these chargers will show you a rapid impedance change when the cell has an internal problem (dendrite fusing in polymers etc.)

Read the manufacturers app notes for your cells. They will often disclose the conditions their chemistry is stable (Samsung and Sony are usually the best). There are so many various types of cells out there... most of them are crap.

This text is just a starting point, and should be assumed as another ill-informed internet opinion until validated under your own efforts. YMMV

Best regards =)

This is an excellent answer and explanation. It’s helpful for people that want a deeper understanding of what’s going on with lithium batteries and where the 80% recommendations come from.

But in terms of general advice that does the most good for the average person, I strongly recommend just doing whatever the manufacturer says. Just use the device and charge it when you want. Live. Some team at Apple, Tesla, Google, whatever, spent 9 figures plus figuring out how to charge the battery so that it meets lifetime guarantees under normal use. There’s too much bad advice out there. Don’t discharge fully every cycle/occasionally (that was NiCd specific, and it was bad advice then). Don’t obsess about turning things off, force quitting apps, putting your car in neutral, or whatever dumb thing your neighbor said.

That being said, the answer here is correct, but keep in mind how that works with the battery management your device already has. Tesla explicitly recommends a lower limit. Do it. Apple phones limit charge cap to 80% until before you wake up. That’s great.

No. It's better but still stresses the battery when loading to 100%. Best is to load slowly (or so that the battery doesn't get too warm) to 80% or 90%.