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

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%.