battery going down while plugged in?

I have a new thinkpad t420. I keep it plugged in at all times for the most part right now because I'm only using it a home. However, I did unplug it for about 10 percent and it went down to 99%. I have had it plugged in for probably a week since then and the battery has actually gone down to 98%. I use it on very high performance but I'm thinking since it's plugged in, shouldn't the battery go up and not down? Or is it not drawing in enough power from the plug in?

 

The battery discharges over time due to the protection circuit. Lenovo batteries are smart enough to set charging thresholds so the battery doesn't try to stay at 100%, unlike most batteries. It won't recharge until it hits 96%.

 

That is correct. Unplug it and run it down and then replug it in and it should run back to 100 percent.

Recognize though, that your power settings may be set so that it does not start charging until you reach a certain level of discharge. I cannot recall the defaults, but I have mine set to recharge when I drop below 50 percent and stop at 90 percent. Repeated charge cycles are what cause a battery to deteriorate

 

not as much the cycles, but keeping the battery at full charge. My last one died with about 65 cycles or so, but I kept it at 100% non-stop. Well I got the lesson.

I also do 50 to 90% charge cycles.