The folks who design and make phones, tablets, and the like know there are people who stay ‘plugged in’ wherever they can, so they design the charging in a way which doesn’t let that become a big problem. They’re going to avoid safety problems (big liability costs) and they’re going to avoid early battery failure (warranty costs, loss of customer to competitor). They can add another cell to compensate for a designed-in less than full charge if that’s what they need to do. Their customers are not expected to know anything about batteries and charging; that thinking has to be engineered in for them… Their battery charging parameters are far different that what we have with our discrete cell chargers .
We can’t add another cell and we want to access the full capacity, so fully charging to 100% capacity is what we get. Plus we’re going to be using many different cells for different capacity and resistance and terminating voltages. If we misuse our cells and have problems that is our fault, and there are no designed-in protections against that. We are expected to know what we’re doing and provide the thinking part since they can’t do that at a reasonable cost. An entirely different design parameter applies to us .
If we do the thinking well, we use the right charger and cells to get maximum performance without inducing unsafe conditions or unduly shortening cell life. Float charging to a low percentage of cell capacity probably doesn’t hurt much, Float charging to a high percentage of capacity does shorten cell life and possibly worse. If you really need to have full capacity accessible at all times such as what float charging is designed to accomplish then you need to use a different battery technology to achieve that goal . What the effect will be with LiIon is going to depend a lot on the cells being considered so no one answer can suffice. And since float charging LiIon isn’t done much in our usage there’s not going to be much info available for you. LiPo and NiMh would be more appropriate compact technologies to consider, while SLA and AGM technologies would be more optimum if there’s space available for their larger size.
If you’re “playing” design engineer the you have to do the thinking- all of it- for yourself. You are now the one responsible for any issues emanating from your choices. If your design harms someone now you’re personally liable for that, so whatever you choose to do keep a large safety margin in your design.
Phil