How much exactly will using my laptop on AC destroy my battery?

November 25, 2006 8:20 PM

How much exactly will using my laptop on AC destroy my battery?

From what I've read, when the laptop is in AC with the battery inside, every now and then the battery gets below a certain threshold, and the computer tops off the battery (trickle charging). This "burns" a charge cycle, reducing battery life.

My question is: what's that threshold? and exactly how much time is this now and then? is it like once every five minutes, once every hour or once every day? And how is the effect in battery life compared to full cycles?

Justification: I use my laptop mostly at home, when I have AC always available. Right now I use it some 2 or 3 times a day, 2-3 hours each time. I really don't want to take the battery off, so, my decision is between plugging it on the wall whenever I'll use it, or alternating using it discharging the battery (until 10%) then charging again on AC (until 100%). I assume leaving it always plugged is not an option (I don't need it to be on when I'm not using it).

By the way, is there a problem if I leave the charger always plugged on the wall, even when it's not plugged on the computer?

If it matters, it's a HP Pavilion dv2000t. It was bought last month, so I think it should have all mainstream battery preservation technologies.

posted by qvantamon to computers & internet (15 comments total)

I have a three year old PowerBook G4 with a two year old battery (thanks to recall) and 90% of the time I run it off AC.

When I do run it off the battery, I get ~2hrs out of it. This is with the screen as bright as it will go, bluetooth on, and wireless enabled. I think that's pretty good.

The Lithium Ion/Polymer batteries don't have a memory effect. That is, if you discharge them half way then recharge that does not count as a "full charge". And when you're connected to AC the battery should hardly discharge at all (only due to internal resistance). The biggest draw will be from refreshing the RAM during sleep, and even that is pretty small.

I wouldn't worry so much about your battery. If you use AC most of the time then you'll get a decent life out of it (2-3 years).
posted by sbutler at 8:44 PM on November 25, 2006

I've had laptops at work for over 15 years - most of them Compaqs. I nearly always use them plugged into AC power. The first couple had battery problems, but none after that (we got new ones every 2 or 3 years). I don't think that you'll hurt the battery - modern chargers are smarter than that.
posted by rfs at 8:49 PM on November 25, 2006

I have a three year old PowerBook G4 with a two year old battery (thanks to recall) and 90% of the time I run it off AC.

When I do run it off the battery, I get ~2hrs out of it. This is with the screen as bright as it will go, bluetooth on, and wireless enabled. I think that's pretty good.

This is, to the word, exactly my experience.
posted by danb at 9:11 PM on November 25, 2006

Most people use laptops the same way you do, so it'll definitely be optimised not to do stupid things that will damage the battery.
posted by cillit bang at 9:24 PM on November 25, 2006

From what I understand, this problem is generally associated with older laptops. Laptops made in the past 5 or so years have a more sophisticated power distribution/charging system, and do not suffer from many of the problems older ones had.
posted by fvox13 at 9:25 PM on November 25, 2006

Dont worry about it. I had the same concern a while back, but as far as I can tell, it was groundless - I've kept an eye on when the battery is being topped up, and if yours is anything like mine (which it probably is) and you run it off AC most of the time (as I do), the top-up's are infrequent enough that the oxidation shelf-life of the battery will kill it long before you run out of charge cycles, so to speak. (Mine seems to let the battery get below 95%, probably lower, before topping off, and since the only major drain is when I unplug the running laptop to move to an AC outlet in a different room, it won't top off more than once a day, usually a few days between top-offs.

If you DON'T run it off AC, and thus deplete it and recharge one or more times a day, every day, you'll probably hit the charge cycle limit before the shelf-life limit.

The shelf-life limit can be pushed back a little bit by keeping the battery cool. If you wanted to be anal about it, you could store the battery in the fridge whenever you are using the laptop on AC or when it's not in use, but to my mind, at this level of effort, you're losing the point of having a laptop, which is a computer that better conforms to you, instead of the other way around.

By the way, is there a problem if I leave the charger always plugged on the wall, even when it's not plugged on the computer?"

No. It will drain a little bit of power, adding a tiny amount to your power bill, but it's not a problem.
posted by -harlequin- at 9:31 PM on November 25, 2006

I have no idea about the technicality of it all, this is just my own personal experience.

At the beginning of March 2005, I purchased myself a HP Pavillion zd8001ap. It's has a 17inch monitor, full keyboard/number pad and is very bloody heavy. It's name is goshzilla.

I run it 95% of the time off AC. When I did run it off the battery, I got about an hours use out of it. I have the brightness set towards the lower end of the scale, but I do have the wireless on all the time. I very rarely used the DVD/CD drive while on battery power, mostly because I couldn't do much before the battery ran down.
A few weeks ago, I discovered the battery life was down to about 5 minutes and a week later, the battery was completely and utterly DEAD.

I'm extremely annoyed with it, because on top of the dead battery, the connection of the power cord is quite tenuous and the slightest movement can cause the connection to be lost and power cuts out.
posted by goshling at 10:23 PM on November 25, 2006

I use my Thinkpad T40 about 99% of the time on AC, and leave it on and running 99% of the time. My main 9-cell extended battery's capacity has dropped to 50% of its design capacity over the course of a year. I also have a secondary Ultrabay battery, It has dropped to about 60% of its design capacity over about 9 months. Thinkwiki's page on battery treatment has some advice that seems difficult to follow:

• remove the battery when running on AC
• avoid charging the battery if nearly full
• never store the battery fully charged (40% is ideal)

This makes it awfully difficult to take off with a fully charged battery at a moment's notice. What I'd have to do is unplug, let it drop to about 40%, plug-in, remove the batteries, and set them aside (in the fridge). Then, if I need to take the laptop some place to run on battery, pull them out of the fridge, put them in, and let them charge up before I take off. How annoying, and definitely doesn't seem to be particularly geared toward the on-AC-most-of-the-time usage scenario.
posted by xiojason at 12:43 AM on November 26, 2006

I think the charging is infrequent and small enough that it won't make much difference compared to the temperature. Storing it at a higher temperature will reduce battery life. Depending on how hot the machine gets in operation, this can make a big difference.

I have two batteries for my laptop, one of which I leave in the machine when it's on AC (most of the time). That one is down to 50% of its original capacity after 18 months, the other one, mostly discharged once or twice a week and otherwise unused, is more like 85% or so.
posted by sfenders at 4:48 AM on November 26, 2006

My nickle's worth...

I have spent the last few weeks reviewing battery chargers for several battery species (sealed lead acid, NiCad, NiMH, LiPo, etc) Also, I've designed a number of chargers over the years. IAAEE, btw.

Your laptop, almost without doubt, has a sophisticated charger that maintains it fairly well...

The question you seemingly asked is does leaving it plugged into AC make a difference in long term life? How should you change your use to optimize battery life?

That's kind of a hard question, but remember, the designers had something in mind when they chose what they chose. Probably, it was portable operation off grid, which is not what you are doing. Your use pattern sounds like desktop, not laptop. (It prompts the question of why you care, almost. Mostly, you've got an integral UPS!)

Anecdotally, I use my 4 year old Thinkpad A31p 99% of the time plugged into AC, and that's 8 hours/day, 5 days a week on and plugged in the rest of the time. I have dual batteries. THey have always lasted an hour or two, but when I am portable, I am outside and I use the display on bright.

I'd run the battery down every month or two to see how it's doing, but I'd resist making any changes to personal use patterns. You've probably got a Lithium Ion battery pack, and the charger is going to take care of it just fine.

The normal curve applies to your individual battery (and computer for that matter) and you may have an outlier... a very good one or very bad one. (goshling's battery sounds like one, though his other problems sound suspcious for wiring/connector issues. Some of these batteries have integral temperature sensors and these are critical for maintaining proper charge profiles for some battery species. His unit could be actively killing his battery. )

Batteries are going to have a certain number of charge/discharge cycles, a certain shelf life charged/uncharged, and they are not going to last forever. Neither is the laptop. Budget for a replacement if you think you are going to keep it for a decade. You've got a brand name box, and a lot of engineering went into that part of the laptop, so worry is a little wasted, methinks.
posted by FauxScot at 5:29 AM on November 26, 2006 [2 favorites]

It really all comes down to which laptop you have. IBM/Lenovo, Gateway, and Dell tend to use -- in most models -- fairly shitty charge circuitry (and/or badly-chosen charging parameters) that eat away at the battery somewhat more aggressively when left to cook on AC. This is somewhat less of a problem for other brands.
posted by majick at 9:20 AM on November 26, 2006

For reference, I've done some tests (unplug the cable for a minute, plug back), and I've found out that:

1) If the cable's plugged and the battery is above the threshold, the charge reported in the windows power meter is 100%.

2) The threshold is 93-94%. After that, the laptop starts charging the battery, and the windows power meter shows the true charge level.

What I'm yet to test is the loss rate for the battery while the laptop is on AC. What I can say is that it is certainly less than 2% per day for my usage profile.

So, in the worst case, the battery will be topped off once every 3 days, which seems good enough for me. I wish this threshold were user-configurable, though. I gather that having it at 90% (or even 85) would fit my needs better, with no relevant impact on off-grid use.
posted by qvantamon at 10:43 PM on November 26, 2006

TomsHardware (err... MobilityGuru) just did a huge report on this here. Given that their "Lessons Learned" seem rather opposite to the replies here, you'll have to make a decision on which you trust more.

Personally, I have had great luck with my laptop batteries, but I don't leave my batteries in my laptop after they are charged. I recently retired my 4-year-old Dell Inspiron with dual batteries... I was getting about 50% battery life out of them at the time, which I feel is pretty awesome. When my brother's school forced him to buy a laptop (part of the nursing program), he asked me what he should do, I suggested removing the batteries if they are charged. At the end of his first year, he was the only one in class that could make it through the class without plugging into the wall... The school A) tells all students to remove batteries as soon as they are charged, B) no longer deals with Gateway computers.

I don't know why you wouldn't want to take the battery out... M.B. guesses that just keeping the battery at a high temp for a year will kill it, without charging/recharging it once. If you want battery life, take your battery out... it takes 5 seconds to pop it back in when you actually go somewhere, it's worth the extra time...
posted by hatsix at 5:44 PM on November 28, 2006 [1 favorite]

I've used Notebook Hardware Control on my laptop, which gives detailed information about the battery status (including charging rate, voltage, etc) and never observed this behavior before. Once charged, the battery stayed charged.
posted by slush at 9:13 PM on November 28, 2006

"It really all comes down to which laptop you have. IBM/Lenovo, Gateway, and Dell tend to use -- in most models -- fairly shitty charge circuitry (and/or badly-chosen charging parameters) that eat away at the battery somewhat more aggressively when left to cook on AC."

I was under the impression (from examining the charging circuits on a sample of laptop motherboards) that the designer pretty much slaps on one of the all-in-one charging ICs, like the DS2715 and they're done with it -- not much engineering goes on on the part of the laptop manufacturer.
posted by slush at 9:24 PM on November 28, 2006