Laptop Batteries/AC power question

I know that some years ago, I read that one should not run a laptop on AC power but should instead charge the battery, then use it til its almost discharged, then charge it again, and so on. The idea was that somehow, the AC power caused the battery to go through a discharge/charge cycle over and over again very rapidly (charging up to full immediately after discharging just a little bit) and that since the battery has a lifetime defined in terms of the number of charge cycles it goes throough, this is harmful to the battery.

My question is, does this still apply to laptop batteries? Or have things changed since I read that?


You’re partially correct in your recollection. It was better for older batteries to fully discharge them before recharging, but the reason was not because the alternating nature of AC current subjects them to many small charge/discharge cycles. The AC power is converted into DC before it ever gets to the battery - that’s what the power adapter does.

The reason for the recommendation was that older batteries (NiCd’s, specifically) exhibited a memory effect: if you ran the battery down halfway, and then charged it fully, the next time you discharged it would only last half as long as it should. The solution was to always fully discharge the battery.

However, this is not necessary now with Lithium-Ion and NiMH batteries.


To be clear: I wasn’t saying it had anything to do with the alternating nature of AC power.

The idea was that a battery has only a certain number of discharge/charge cycles it can go through before “wearing out,” whether or not it discharged fully or only just a tiny bit before being charged. It was said that a battery on AC power repeatedly discharges a tiny bit, then recharges–each such mini-cycle subtracting one cycle from your battery’s available lifetime.

Is this no longer true, assuming it ever was true?

I remember wondering why they don’t just design laptops to bypass the battery completely when it is fully charged and there is AC power available.


Oh, I understand what you’re asking. What you describe was never true: a laptop running on AC power is not constantly charging and discharging the battery. The battery is essentially bypassed. In fact, this does not require any effort on the part of the designer: designing a charging circuit that doesn’t bypass the battery, instead charging and discharging it in little steps like you describe, would be more complicated.