Treat Li-Ion batteries like people to make them last

I put a link to Battery University at the end. Some of the numbers came from there. Others come from cell data sheets (best ones from Panasonic). A lot of this is synthesis over many data sheets and white papers, none of which would by themselves be illuminating to the average battery user, because so much of the data would be cell-specific.

You're right that device manufacturers choose what to name "100%". Generally, cellphone makers are most attuned to people running out of power, so their 100% is pretty close to the real top charge level (not overcharge). They are rated and tested and reviewed based on tests like Walt Mossberg's run standard processes and see how long it runs until empty. The incentive provided by such make-or-break tests is to allow users to fully charge and fully discharge.

Better control and information would be provided if you gave the actual voltages and chemistries, but both are beyond what a normal user would be comfortable using on a daily basis. Internally, we almost always refer to state of charge in cell voltage terms.

Tesla and BMW provide the ability for the user to know and adjust the top of the charge. Tesla's approach is fairly user-friendly, and BMW's is a little more powerful, albeit less user-friendly.

This post was not about boats; it's about li-ion batteries in general. We learned the lessons when developing our outboard motor battery pack, but the batteries we're using are the same cells used in laptops, and the same chemistry as used in cellphones, tablets, etc.

There is no specific charge level at which battery use becomes costly. It's just more costly to charge the higher the voltage to which you charge. Of course, overcharging (over the rated voltage) is extremely costly (and dangerous). Under that level, the device maker has to make an arbitrary choice about how to balance cycle life with calendar life, and that choice is usually near (but not at) top voltage.

Our tests have found that charging a cell to 80% instead of 90% can extend its life by more than 50% under the same conditions of use, sometimes much more (lots of factors: charge/discharge rates, temperature, time between cycles, etc.). I'm sure no cellphone/tablet maker is limiting charge to 80%.

All of these things are built into consumer devices already.

0% maps to about 5%, 100% maps to 90-100% depending on where in the charge cycle the device is (it will stop charging for a while to discharge some of the battery before starting charging again).