I have a battery recharger that shows no indication when the batteries are fully charged. I'm supposed to assume they are fully charged after 8 hours.

If I leave the batteries to charge while I'm out during the day and there is a power cut while I'm out, they will not be fully charged after 8 hours, but I will not know this if the power is back on before I return.

Therefore it would be useful if I had a way to know whether there was a power cut while I was out.

The only method I can think of is to have a mains-powered alarm clock permanently on, so that a power cut will cause it to reset. But is there a better method?

  • 24
    Battery chargers that work like that are usually pretty low quality. They don't adapt their charge cycle to the battery type you have, so your batteries may not be fully charged after 8 hours. An intelligent charger measures your batteries and adapts its charge cycle. They also usually take less time to charge your batteries, and they indicate when charging has finished. So you could replace the charger instead of setting up a hack to monitor power cuts.
    – Hobbes
    Commented Jul 24, 2019 at 14:25
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    @EmmaV: With no problem at all. They don't need to remember their progress charging because they monitor the charge characteristics of the battery and deliver the right charging current for that, so they're largely stateless. Commented Jul 25, 2019 at 4:05
  • 4
    Surely you would just look at your UPS logs to see how long the power was off for. *8')
    – Mark Booth
    Commented Jul 25, 2019 at 17:07
  • 2
    Are you somewhere where power cuts are likely? I realise they are likely in some locations but, in most of the developed world, the answer to your question is "just assume there was no power cut". Commented Jul 26, 2019 at 14:04
  • 2
    @David Richerby, I was wondering that too. According to the Energy Information Administration, The most power outages that average US citizen can expect in a year is 2. eia.gov/todayinenergy/detail.php?id=27892#
    – Michael J.
    Commented Jul 26, 2019 at 15:44

13 Answers 13


Set the time on an electric clock that uses household current

Most households today have a microwave. This has a clock you can set. On nearly all of these the clock rolls back to 12:00 and flashes when the power goes out. Chances are you have one doing this right now. Set it to the correct time, if the time is still correct next time you look at it, there has not been a power outage.

VCR players used to have the same thing, but not many people have them any more.

Lastly just buy a small electric clock, get one with a radio to use in the kitchen.

  • 2
    A stove is also a common digital clock Commented Jul 24, 2019 at 13:49
  • 20
    Some clocks will flash 12:00 when the power returns and then keep time normally, but still flash the time. From this, you get an added benefit of knowing how long ago the power came back on, although you can't tell how long it was out for. Commented Jul 24, 2019 at 14:28
  • 9
    A mains-powered analog clock is even better: not only will it tell you that the power was out, it'll tell you how long it was out for.
    – Mark
    Commented Jul 25, 2019 at 2:27
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    Unless the outage happens at exactly 12:00
    – alex.k
    Commented Jul 25, 2019 at 7:25
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    @user1776237 Even if the power comes back on exactly at 12:00 after an outage, the devices which resets to this time generally flash - indicating that there was a power outage.
    – Alan Ball
    Commented Jul 25, 2019 at 11:22

Get a mechanical mains timer - they are very cheap and set it for your 8 hr cycle. It only progresses while there is power, so set it to the beginning of the cycle, plug it in and forget about it.

Something like this:


Image from https://cpc.farnell.com/pro-elec/pel00412/timer-mechanical-7-day/dp/PL15117

Even if there is a power cut, it will just resume charging when the power comes back on, and stop charging after 8 hrs of charging automatically - you don't even need to remember to unplug it after the 8 hrs, just grab the battery when needed.

  • 2
    Unless it's a countdown timer, you will still want to power it off once past 8 hours, otherwise it will automatically charge for another 8 hours every day until you do.
    – Mark Booth
    Commented Jul 25, 2019 at 17:03
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    @MarkBooth I've seen timers (not the one in the link) that use a design where you put separate "on" and "off" tabs on the clock, you could turn on the switch manually and place an off tab on the 8 hour mark with no on tabs.
    – Random832
    Commented Jul 25, 2019 at 20:54
  • @MarkBooth to add to what Random832 said, you're presumably charging it for some use - and you would have another 16 hrs to check on it and unplug it - still far easier than doing it manually. The ones We have at home are as Random describes, with pins to designate on and off times.
    – Baldrickk
    Commented Jul 26, 2019 at 9:05
  • Yup, most of my timers are of the type pictured, with on/off toggles, I have a few with both on and off pins, so you can use them as a countdown timer by just setting an off pin. The pin type is a lot more difficult to find though (in the UK). I've also used a timer to give a bunch of batteries a once a day 1/4 hour charge, rather than leaving a bunch of trickle chargers running all the time. I calculated at the time that over all the energy was lower.
    – Mark Booth
    Commented Jul 26, 2019 at 10:12
  • Helpfully, because the time also only advances when there is power, assuming the user sets the timer plug to the correct time of day, if there is a power cut then the timer will lag behind the real time. The time difference between the plug and the world indicates the duration of the power cut
    – Caius Jard
    Commented Jul 29, 2019 at 17:11

Visit antique shops and second hand stores to find an old style electromechanical clock -- the kind with a motor and gears. Put this near the battery charger, plugged into the same outlet. When you start the battery charging, you have two choices.

The simple way: set the clock to 12:00 (there'll be a knob to set with, or in some cases you can just push the minute hand around as needed). When the clock reads 8:00, your battery has gotten enough charge, regardless whether the power was out for a while.

More complicated way: set the clock to the correct time. When you come back, if it's incorrect, the amount of time it has lost is the duration of the power failure(s) for the day.

  • 1
    The simple way makes the assumption that the battery is capable of continuing its charge after a power cut. This is not how all batteries work and given the description of the charger and charge time I would say that this is not a battery type where the charge can be continued.
    – kenjara
    Commented Jul 25, 2019 at 11:06
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    @kenjara Where have you seen a battery that can't continue an interrupted charge? I'm not aware of such, and the description suggests older tech than lithium cells, all of which are happy enough with interrupted charge as long as the charger doesn't let the cell/battery discharge during a power drop (very simple to do, electronically).
    – Zeiss Ikon
    Commented Jul 25, 2019 at 11:28
  • Sorry what I was referring to was old non smart chargers. Not the battery itself but the charger.
    – kenjara
    Commented Jul 25, 2019 at 12:17
  • 4
    @kenjara Even still (back to NiCd and lead gel cell days) every one I've owned would charge any time it had power.
    – Zeiss Ikon
    Commented Jul 25, 2019 at 12:36

While the other answers here give some good suggestions for answering your literal question, I caution you that they likely won't solve your underlying problem. Knowing the power went out isn't really sufficient. The other critical piece of information is how long the power was out. A 3-second blip and a 3-hour outage mean very different things for your charging battery. A device that resets its state on power-up will (at best) only tell you how long it's been since the power was last restored.

If you need to charge for a certain length of time, I suggest connecting the charger to a mechanical timer similar to this one. Set the timer to turn "on" immediately and then turn "off" after 8 hours. Any interruptions in power will also stop the timer, so you'll get a full 8 hours no matter what happens.

For the best results, I recommend investing in a good quality battery charger. Along with handling situations like this intelligently, they can also help your batteries last longer. You can pick up fairly inexpensive but still quality chargers for AA/AAA batteries or "universal" chargers for non-standard battery shapes. For batteries that you can't remove for charging, I recommend getting a good battery tester or learning how to check battery levels using a multimeter.

  • That's an interesting outlet timing device, ultimately however I upvoted this because of the multimeter. Nothing beats actually knowing what you have. Basically if it isn't reading 5% over the nominal voltage rating then it isn't charged.
    – lucasgcb
    Commented Jul 26, 2019 at 9:06

Lifehack: Avoid the power cut all together. Connect your battery charger (and other critical devices) to a UPS (Uninterruptible Power Supply). It has a built in battery so it will continue to run your devices for the brief time while the power is down.


enter image description here

  • 2
    Or better yet, have a UPS connected to your computer and simply have it log every time it makes a switch to battery power. I have a NAS that even sends me an email when this happens, so even at work I know right away when a power interruption has happened.
    – J...
    Commented Jul 25, 2019 at 18:43
  • That's quite an expensive lifehack for the event you find your Xbox controller to be less charged than you'd hope for. On the upside, you can connect more critical devices to this if power cuts are that common.
    – lucasgcb
    Commented Jul 26, 2019 at 9:10
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    @J... just found it humorous because it really isn't a "hack" per se but effectively buying a battery for the battery.
    – lucasgcb
    Commented Jul 26, 2019 at 11:38
  • 1
    @lucasgcb You won't get an argument that 'lifehack' is a stupid term. OP's question is how to tell if they've had a power outage, though, and a UPS is the most common item in a residential household that has the capability to provide a reasonably detailed log of power quality in the home.
    – J...
    Commented Jul 26, 2019 at 12:01
  • 1
    @lucasgcb Yo, dawg, I heard you like batteries. Commented Jul 26, 2019 at 14:05

Get any multimeter and measure the voltage on the battery. Depending on the battery type, the open-circuit voltage can be a great indication of the charge of the battery. Example for 12 V lead-acid battery:

enter image description here

  • These charts are usable for lead acid batteries, where the voltage is closely correlated to the capacity left in the batteries, but it won't work for a lot of types of new rechargeable batteries (li-ion, ni-mh, etc ...) as some of them have a very flat discharge profile (i.e. the voltage hardly change for most of the time, and only collapse when the battery is empty). Besides, voltage variations due to temperature are sometimes of the same magnitude than the variations over a full discharge ... making it very difficult to assess if you don't know exactly all the specifications of your battery.
    – Hoki
    Commented Jul 26, 2019 at 11:40

At the risk of answering a question you haven't actually asked, it sounds like what you want is a good battery tester. It won't tell you whether there was a power cut while you were out, but it will tell you whether your batteries are charged or not, which is, I think, what you actually want to know.

And it's always possible that your batteries didn't charge for some other reason (a faulty charger, for example).

  • 1
    Cheap battery testers have a hard time distinguishing between partially charged and fully charged batteries.
    – Hobbes
    Commented Jul 25, 2019 at 13:22
  • Yes indeed. You get what you pay for. Commented Jul 25, 2019 at 13:43

If you only want to know if you had a power cut, an electric clock is your easiest choice.
If you want to know if you had a long power cut, here's another way:

  1. Get a small, transparent, capped bottle.
  2. Fill it with just an inch or two of water.
  3. Put it in the freezer and let it freeze.
  4. Before you leave the house, turn the bottle upside down inside the freezer. Now the frozen water should be on top. You can leave now (please don't forget to close the freezer :D).

When you come back, check the bottle in the freezer.

  • If the frozen water is still on the upper part of the upside-down bottle, you didn't have a power cut long enough for the water to start melting, and so your batteries are probably almost fully charged.
  • In any other case, i.e. there's some liquid water, or the water is now frozen at the bottom, you should check your batteries.

I use this when I go on vacation or a long trip, so I know if the food in the fridge/freezer is still safe to eat when I come back home.

  • 5
    Modern freezers are pretty well insulated, and a four-hour power cut wouldn’t melt the water, but it would significantly reduce the battery charging time.
    – Mike Scott
    Commented Jul 25, 2019 at 11:39
  • 2
    @MikeScott It also might be pretty sensitive to how much cold stuff you have in your freezer. If it's packed full, it would take a lot of heat seeping in to actually reach the melting point, compared to just the bottle in an empty freezer.
    – JMac
    Commented Jul 25, 2019 at 12:17

Ask your neighbors.

If you want a technological solution, you can more directly track your battery charging, and many other things, using a device such as the Kill-A-Watt. They run about $20 in the US. Plug it into a wall socket and plug your battery charger into it. It will record total energy used (Kilowatt-Hours) and total time plugged in. When the power goes out, both will reset.

There's also a version with a battery backup, then you can directly see how much energy has been used by the battery charger and get a better idea when they're charged.

enter image description here

Or you can spend the $20 on a better battery charger.


A slightly more concrete way (however, it will require more setup and is probably overkill for your use case) would be to use a Raspberry Pi, connected to the mains to ping a web server you had setup (on a seperate power supply). When the Pi stops receiving pings, it logs it. Then when it receives them again it logs that too having a concrete time difference. You could then email that to yourself from the web server. Again, overkill but just wanted to give a suggestion as to how I would do it.


Another version with the freezer. Put a plastic glass full of water in the freezer, when it's all frozen put a coin on top of the ice. If the coin sunk in the ice there was a power cut. With some testing you can even tell how long the power cut lasted, based on how much the coin sunk.

EDIT: Now I tested this and it's not going to work for the case in the question. It can work only for power cuts that last long enough to melt most of the water. The ice raft effect mentioned by @gburton definitely occurs. The water starts melting not only from the top but all around, so all the water goes to the bottom and makes the remaining ice float. This also means that it's never possible to make any consideration based on how much the coin sunk: it will sink only when almost all the ice melted.

  • 3
    Its clever. I'm curious if you've tried it and how long a power outage is necessary before you'd notice a change. Ice sublimates in a freezer which might distort things.
    – Schwern
    Commented Jul 25, 2019 at 23:22
  • @Schwern No, I didn't try, I have the glass in the freezer but I haven't had power cuts recently. I had not thought of sublimation, but if I understand correctly what that is (evaporation of ice) I should be able to notice that, I'll keep an eye on it, thanks for mentioning. Anyway I cannot claim any practical experience with this system. Commented Jul 26, 2019 at 7:04
  • I'd go with this before trying the answer with the UPS for sure
    – lucasgcb
    Commented Jul 26, 2019 at 9:12
  • 3
    Doubt this would work in practice unless the power cut was for quite a length of time. The freezer is full of air at a very low temperature, so the water isn't going to liquidate right away; also, ice floats, so the penny will just sit on top of an ever reducing iceberg and never actually sink below the surface. Commented Jul 26, 2019 at 10:39
  • 1
    @SantiBailors I am always in favor of practical science, I'd like to know if my theory is correct. I predict that the coin will stay on top (and get damp but not sink) until the ice "raft" is small enough that it capsizes. Commented Jul 26, 2019 at 10:58

There is another option, if you have a vendetta against alarm clocks, cookers or microwaves. Or you just happen to own none of those things and want a free way using something you already have.

If you have an internet router (and I presume you must, unless you are entirely reliant on mobile data, dial-up or someone else's connection), there is almost always a timer built into it indicating how long it has been powered on for, or how long the line has remained connected; if not both.

How you connect to it can vary a lot depending on how it has been set up. If you're using the one provided by your internet service provider, you can always look up the specific instructions online by searching for your ISP and router login. In general the procedure will be something along the lines of:

  1. Open your browser and connect to or similar IP address.
  2. Log in with details such as "Admin" and "Password", unless you or someone else has changed them from the default. It may also be the name of your ISP.
  3. Go to a section called router logs or maintaince, and look for the uptime of the router. If it's more than 8 hours, then you are good.

You can usually find the address you need to connect to using the command line, however this won't tell you the username and password, so it isn't much use on its own.

  • Most Unix-like systems have a last command which shows every shutdown and reboot, which includes every power outage, unless on a UPS. (Assuming it automatically reboots on power restore.)
    – WGroleau
    Commented Jul 27, 2019 at 14:03

If you have a desktop computer, just leave it working when getting out of the house.

Then when you get back home, several options:

  1. Computer still on, no reset was done: no power cut.
  2. Computer is turned off: there was a power cut.
  3. Computer still on, but you notice it was reset:
    • If planned reset, e.g. due to Windows Update, you can't know if there was power cut.
    • If unplanned reset, most likely due to a very short power cut.
  • 2
    That wastes a lot of energy.
    – Hobbes
    Commented Jul 24, 2019 at 14:26
  • 2
    @Hobbes lots of people (myself included) leave the computer working anyway. And when in sleep mode it doesn't consume much energy. Commented Jul 24, 2019 at 14:35
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    This is potentially very damaging to your computer; if you're not using an SSD, you can risk physical hard drive damage, as well as risking system damage due to interrupted write processes.
    – Allison C
    Commented Jul 24, 2019 at 15:52

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