Does overcharging a MacBook degrade its life?

I have an early 2011 13" MacBook Pro and I use it almost like a desktop. I leave the power cord connected all the time. Is this OK? Am I ruining the computer or diminishing its lifespan?

All Macs cease charging and let you know this by changing the "orange" LED in the charging cable to "green" when the battery is topped off and you are merely running the Mac - not charging it.

The question you ask would be more precisely worded - what harm is it in keeping the battery constantly and always topped off to full?

Your early 2011 13" MacBook Pro has a lithium ion battery. Luckily, lithium ion batteries have an internal circuit to prevent the battery from being charged over 100%. There is a very slight chance that something can go wrong with the charging mechanism in the laptop, which would cause the battery to overcharge, but I wouldn't be concerned with that.

Years back, laptop batteries were made from nickel-cadmium or nickel metal hydride. It was recommended that these batteries were regularly fully discharged then fully charged so they'd continue to hold a full charge, though even following these practices they would eventually lose capacity.

Apple still recommends that you fully discharge your battery monthly, and never store your laptop with a charge below 50% for an extended period of time. In short, it's perfectly acceptable to leave your laptop plugged in while it's in use but let it fully discharge once a month for optimal battery health.

There are mixed opinions about this. Some say it'll shorten the battery's lifespan, others say the intelligent charging circuitry will prevent that from happening. I've always discharged the battery totally at least once a month as per Apple's guidelines. You can also see they do not recommend leaving it charged in all the time. Even their own guidelines aren't clear.

Here's my insights from eight years of laptop using experience.

The laptops that I've mainly used as a desktop (and left plugged in for the majority of the time) have all had their batteries lose a considerable amount of charge capacity within one year. A couple of batteries lost 80% of their charge just past a year of use.

When I've used them more on-the-go, I saw no giant reductions in charge capacity over a two year span.

I now try to fully discharge the battery at least once per month. Six months with a late 2011 MBP 15", and I see no difference in capacity.

So, my history indicates two points:

• fully discharging the battery will help to some degree
• batteries and charge controllers are better now than 5 years ago