Keep your battery between 40-80% if possible, unless you plan to need extended battery use, occasionally.
Here's why:
(Note: Circuitry or software inside the battery, your PC, or the software added might already keep your battery below 80%, and report 100%, but I doubt it. As mentioned here by Daniel B, your software may stop charging after, or before it reaches 100%, until it falls to x%, which will extend the life of the battery.)
Both PC and phone manufacturers promote the best usable battery time, per charge, at the expense of your batteries' life expectancy, which could otherwise last almost indefinitely. For instance, the Chevy Volt never allows any of it's lithium cells to be charged over 80%. (There's a great 1 hour YouTube video of a lithium cell manufacturer's FAE, (Field Application Engineer), discussing what and why they advised Chevy during the Volt design.) Here's what I learned:
It isn't the circuitry that is at risk--it's the Lithium battery cell itself, which has different risks than NiMH or NiCad batteries. Lithium cells only work so well and risk-free today, because of layered intelligent circuitry that protects both you and the cell, to some extent. Some intelligence is inside the cell itself, the battery pack, your PC, and some in the reporting software. We can guess or imply how your Dell PC acts, but not definitively, based on design criteria. Here's what it takes to keep cells in such safe and perfect condition, they will safely live almost forever:
1-- Keep as cool as possible: Heat kills cells as the #1 cause, so let them cool before charging, if possible.
2-- Charge as slowly as possible: Charging slower is simply cooler, thus your batteries will live longer.
3-- Never completely drain to near 0% charge or store your battery below 40%: All batteries lose their charge over time, and once it dips below 10%, it will never charge again.
4-- Seldom charge to 100% and never store your battery above 80%. The longer and more often the cells are at 100%, the shorter your cell's life expectancy and available power.
Never allow your battery to reach 0% charge state, or store your battery below 40% Cells are designed to never be allowed to get to 0%, because they would take so much current to start charging. Then there's a high risk of explosion, or at least overheating. So, the cells' internal circuits themselves will generally prevent charging when below 10%.
If you've ever had a cellphone battery, or laptop that won't charge, it is very likely the battery itself, and not charger. Instead, you likely have one or more individual cells, or the battery pack that will prevent ever turning back on, once they are below 10%, without specially designed circuit to safely jump-start them. It's estimated that over 60% of the "dead" cellphone batteries could be revived to a normal life with a specially designed (and available) charging device.
Never store batteries below ~40%. Since all batteries will lose their charge with no use, over time they will all dip below that 10%, and sooner or later will become unchargeable. (Never discharge a power drill battery without recharging very soon after use, or it will never charge again).
Seldom charge your battery to 100% or store a battery at 100% As I learned, batteries at 100% charge will develop little shards of copper, which shorten the life of the battery, or eventually, kill it completely. Almost all devices we use will NOT have protection against this, (as does the Chevy Volt's cells).
There are often times when it is well worth charging to 100% when we know we will be away from a charger for a long time. We often choose which device to buy, based on how long we can use it between charges. So, while we need to occasionally charge to 100%, the cost is slightly reduced battery life. IMO, I'm speculating that how long it stays at 100% determines the extent of harm more than how often.