Does a device use battery or mains power when plugged in?

(Eolake Stobblehouse) #1

I have sometimes wondered: if a tablet or laptop is plugged in, does the power used come via the battery or directly from the plug?

I’m using a great keyboard now, which is not avaliable in a wireless version. So I have it plugged into my iPad Pro via a USB-Lightning adapter. I’ve had power plugged into the adapter, but last night I unplugged it to give the battery its monthly exercise. And almost all the iPad’s power drained overnight, though it was sleeping. Surely most of it went to the backlighting of the keyboard, but still.

Of course when I have it plugged in, the power use is not costly, but if the power drain goes via the iPad’s battery, then the battery’s lifespan may be shortened noticeably by all the extra draining and charging…

Can anybody enlighten me?

(Al Varnell) #2

Once the battery reaches full charge, then it likely is only using charger power, depending on how heavy the use is. I leave mine plugged in most of the time.

Although your battery does need to be exercised periodically, there it isn’t necessary to drain it.

My wife’s original iPad has been operated in this same manner and the battery is almost as good as new. Holds a full charge and can be used all day off the charger without being completely depleted.


(Eolake Stobblehouse) #3

Right, that’s good to hear, thanks Al.

But the draining keyboard is a specialised case, does it wear extra on the battery? If you’re right, if it takes the power from the charger when it’s plugged in, then no, I think. But I’m not yt convinced this is so. Some devices I can use the second they are plugged in, others can’t be used until the battery is starting to have some charge again, so…


(Tommy Weir) #4

I wouldn’t worry here about the keyboard. Have you checked the iPad’s battery health in Settings? Settings > Battery > Battery Health (Beta) should give you an overall percentage.

Keyboards, even with backlights use very little power.

(Eolake Stobblehouse) #5

Weird, I’ve seen the Battery Health thingie in my iPhone, but it’s not on my iPad.

I’m sure my battery is healthy though. It’s only like a year old and I’ve treated it right.

And I’ve never seen an overnight drop of charge like this with the keyboard plugged in with backlight. (And you’re right, it should not be that much. Only set at middling light too. On the back of the keyboard is says it is 4.75 volts.)

Yours, Eolake

(Eolake Stobblehouse) #6

Update: I tried again to unplug the power overnight, and again half the battery charge was drained overnight. And this time the keyboard backlight was turned off. I really don’t get why the durn thing uses so much power. Or even how.

(It does explain though why the iPad complains it uses too much power. I had to put in an unpowered USB hub between them to fool the iPad. Unlike most USB keyboards, which work great with a USB-Lighting adapter.)

What I wonder is if I should unplug it when not in use, to avoid unnecessary drain/recharge of the iPad’s battery, or if it takes the power directly from the wall when power is plugged into the USB/Lighting adapter.

(Tommy Weir) #7

Is it syncing a lot of material? It could be that you have a large Photos library that’s trying to catch up or a few huge files that it struggles to upload or download?

I know I had to actively keep my iPhone awake and connected to get a set of databases on DevonThinkPro Office synced fully.

(Eolake Stobblehouse) #8

Thanks, Tommy, but no, I don’t use it for anything either processor- or bandwidth-intensive.

Yours, Eolake

(frederico) #9

It sure sounds to me like you’ve got more background processes than you are aware of, or something is stuck. I presume you have tried shutting it down and restarting?

Your battery health also tries to tell you which apps used the most power in the previous 24 hours, but I’ve found that to be incomplete and sometimes misleading. I had a weather app go wonky in background and was ultimately the issue of a few days or battery murdering; it didn’t even show up in Battery Health/Use because I hadn’t even opened it.

Modern battery lore suggests that modern battery “exercising” isn’t necessary anymore; Apple says that your battery is good for up to 1000 complete cycles, and will start to lose full capacity after 500 cycles. They further state there is no damage or advantage to charging from 0% or 99%, but they are completely unclear is charge cycles from 99% count the same as from 0% in terms of total cycles of life.

Older MacBooks, as I’m sure you’re aware, would run the battery down to 97% while on the charger, and then top it off, then repeat; and with those older batteries, it was actually stated that the optimal mark to store/recharge your battery was 87%; but that wasn’t at all practical to leave your battery already below capacity.

I heard rumors that Apple adjusted the battery level indicator to read 100% when those batteries were actually at 87%, just to extend the life and let people think they were at top level.

I wouldn’t be at all surprised if all modern Apple devices practice similar reporting, if the battery is made healthier as a result.

Anyway, as far as I know, all iOS devices basically draw from battery, because if you’re on a charger that is under-amped for your device, you may, at best, not lose charge level, but likely lose charge under heavy use. The fact the battery area on an iPad gets hot when under load, even on a charger, where it doesn’t get hot just charging while asleep supports this.

I’m sure someone somewhere has published on this subject with better authority; but I cant find the article I remember from within the last year.

(Eolake Stobblehouse) #10

Thank you. Appreciated. Good points.

Steven K. Roberts ( told me that the iPad does draw directly and not over the battery. And it does not have a hotspot on the back. So I just don’t know.

(Eolake Stobblehouse) #11

It seems there’s a dramatic difference in drainage when the keyboard is plugged in.

(frederico) #12

4.75 volts isn’t necessarily a small amount; it’s important to know the amps/watts.

I have a beast of a keyboard (Kinesis Edge), and if it isn’t attached to a quality powered USB 2.0 hub, or to a USB 3.0 port, if I turn on the backlight, I gets very unreliable, especially if I also mount its internal USB storage to program it.

Can you disable backlight to test if that’s a factor?

(Eolake Stobblehouse) #13

Interesting, thanks.

To my surprise, turning off the backlight made little difference. I don’t know where all that power goes, it’s not like the keyboard gets hot.

(Adam Engst) split this topic #14

A post was split to a new topic: Any reason this problem isn’t a dying battery?