How to Make a Battery Protection Circuit (over-discharge protection)

How to Make a Battery Protection Circuit (over-discharge protection)

Powering homemade projects with rechargeable
batteries is actually more difficult than it sounds. This is because most battery types
cannot be over-discharged, as doing so may damage the battery and make it unstable.
Consumer electronics overcome this issue by using clever battery protection circuitry
that constantly monitors the battery’s voltage, automatically switching the device off if
it gets too low. Unfortunately these protection circuits are
not readily available separately, so in this video I’m going to show you how to construct
one yourself, allowing you to make your own protected battery pack to use in your own
projects. To make the circuit you’ll need the following
items. The finished circuit is compatible with most
lithium based batteries, which is a very common battery type used in devices from smartphones
to radio controlled models. I’ll be using a 3 cell 8000mah battery pack
for this project. It cost only $42 from Hobby King, so it’s a lot of power for the money.
It’s intended for use with RC models, which means it has an extremely high current capability,
making it ideal for use with high power projects like my 100w LED light panel.
One thing to keep in mind is that lithium batteries are usually wired in series to increase
their voltage. Each battery is referred to as a cell, usually having a nominal voltage
of 3.7v. As mine has got 3 cells, and each cell has a voltage of 3.7v, the total pack
voltage is 11.1v. Lithium batteries like this usually have balance
connectors. They’re basically little wires going to the positive end of each cell, and
are required for this build. If you want to learn more about battery packs
and how to calculate how long they’ll power your project for, Afrotechmods has an excellent
video on the subject, a link to which is in the description.
To keep the project simple we’re going to use a battery voltage alarm as the circuit’s
base. This alarm beeps loudly when the battery drops below a value you select, so we’re going
to repurpose this action to trigger a latching relay, which when triggered will disconnect
the load from the battery. Before we begin it’s important to note that
lithium batteries if abused can be dangerous and even catch fire in some circumstances.
You must always use a proper charger and set it up correctly for your battery pack, and
exercise extreme caution against short circuits, using a multimeter to check everything before
you hook it up to a battery. I’ve put a link in the description to an article all about
how to care for these batteries correctly, so I highly suggest giving it a read if you’re
new to them. So, with that out of the way, let’s begin!
The circuit has two options. One is the simple option, which just includes the cutoff circuitry,
while the other adds an on-off button, which is ideal for self contained packs but makes
things more complicated. Many of you will be fine working of this diagram, which you
can also find in the description, but for those of you who are less confident I’ll walk
you through it step by step. So the first thing to do is prepare the battery
alarm by removing the two buzzers. To do this without any desoldering tools, grab the buzzer
and touch your soldering iron to each of its contact points whilst pulling the buzzer away
from the circuit board. You’ll need to alternate between each contact point until it comes
free. Now it’s time to solder some wires to these
contact points. As the buzzers were wired up in series, we need to use these two contact
points. So solder on a black wire to the negative pad, and a red wire to the positive pad.
Next trim down your breadboard to measure 13 holes wide by 28 holes long, with the traces
running the longer length of the board. Now you can bend the pins on the alarm downwards
using a pair of pliers and insert it into the breadboard. Make sure that its far left
pin is in the third hole from the bottom and the fourth hole from the left, and solder
it in place. Now we need to break nine pins off our PCB
header strip and solder it inline with the alarm but on the bottom row of holes.
Because the alarm beeps when it’s first plugged in, we need to add a push button to act as
an on trigger. This pin on the left is the negative pin of
the alarm, so we’ll push the momentary push button in line with this pin above the alarm.
We can now flip the board over and use a knife to break the copper strip between the button’s
pins, and then solder it in place. Now it’s time to add the latching relay. This
particular relay has six pins. Two are the switched contacts, and the others are coils.
If either of these coils is given brief power, it will switch the relay either on, or off,
depending on the polarity. As we want the push button to turn the relay
on, we need place the relay on the board above the alarm, with the second set of coil contacts
in line with the push button. When the button is pressed, a circuit will
be made with the second cell, sending a current to the coil and switching the relay on.
So we’ll again scratch the copper off between these pins, extending the cut right to the
edge of the board, and solder it in place. To complete the circuit we need to bridge
to the third pin of the alarm, which is the positive contact of the battery’s second cell.
As the relay is rated at 5v, but the second cell is 7.4v, I used a diode to drop the voltage
down slightly. You can also use a 10 ohm resistor, or even just a wire bridge if you want, as
it should still be able to handle it for brief periods. We can also cut through the four copper strips
below the switched contacts, but we won’t solder them quite yet. Now we can wire up the alarm to the other
coil, so trim down the wires we added earlier and poke them through the board, with the
positive wire going next to the upper coil pin, and the negative wire going next to the
lower coil pin. We can now bridge the wire contacts to the adjacent coil contacts. To smooth out the alarm’s voltage pulse, we
need to add a 100uf capacitor. It just goes parallel to the coil, with the polarity matching
the wires we just added. So now we can give it a go. So if we plug
the battery’s balance connector into our circuit, we should hear a little click as the relay
switches ‘off’, assuming that it was on to begin with. This is because the alarm beeps
when it is first given power, and we’ve configured the beep pulse to trigger the relay off. So
to switch the relay on, we can press the push button. Again, we can hear a little click
as it switches on. So with the basic circuit complete, we can
now solder a power wire to each side of the relay’s switched contacts.
One of these wires connects directly to the battery’s negative terminal, and the other
can continue on to the device that needs the power. The battery’s positive wire can just
be connected directly to the device without anything in between.
So we’ll plug in the battery’s balance lead, and press the push button to trigger the relay
on. The alarm has been set to trigger at 3.7v,
and sure enough, as soon as the alarm detects that one of the cells has dropped below this
value, it trips the relay and the battery successfully disconnects from the load.
For general use you should set the voltage cut off to 3.5v, at which point you can simply
disconnect the battery and recharge it. So this is a really practical circuit that
will protect lithium based batteries from being over-discharged. But what if you want
to take it to the next step by adding a switch to turn it on and off, rather than having
to unplug the battery? Well, to do this we need to add a six pole
latching changeover switch between the balance connector and our circuit. The idea is that
when in the off position, it disconnects all of the balance leads from the circuit, and
also triggers the relay off so that the battery is also disconnected from its load.
So to add a switch for this we need these extra components, the most important of which
is the six pole latching changeover switch. A six pole latching changeover switch basically
has six completely separate on-on switches inside it. Each switch has three sets of pins.
When it’s turned on, the middle pin of each set is connected to the pin just behind it.
When it’s turned off, the middle pin gets connected to the pin just in front of it. The first thing we’ll do is break off five
pcb pins and solder some coloured wires to them, preferably matching those of your battery’s
balance lead. Why five pins? Well, as we’ve only got six poles, or switches, to work with,
it means we have to limit the switch to work with four cell batteries or less. You can
always use a nine-pole changeover switch if you need to use it with a higher cell count
battery. To keep it neat we can use some heatshrink
to cover the joints. Now we can solder the other ends of these
wires to the middle pin of each pole on the latching changeover switch.
Now we can get the pcb socket and solder five coloured wires to it, taking extra care to
use the same wire colours in the same order as the previously made connector, with the
black ground wire on the outer edge. Just like with the previous connector, we
can we can wrap some electrical tape or heatshrink around the joints to protect against shorts.
The leads of this new connector can then be soldered to the latching changeover switch.
They need to be wired up to the rear pin of each pole, so that when the switch is on they
are connected to the wires of the previous connector. We need to match the colours too. So the finished switch should look like this,
and when connected to the alarm and balance lead, it should turn the circuit on and off.
However, once the relay is on, turning the switch off doesn’t turn the relay off, meaning
that the battery is left connected. What we need to do is get the switch, when
turned off, to trigger the relay also off. We’ll do this by reversing the voltage applied
to the second coil. To do this, all we need to do is solder a
new wire to the circuit’s ground output. This is the one that goes to the device that needs
the power, rather than the one that goes to the battery.
This new wire can then be soldered to the middle pin of the last remaining pole on the
switch. Another wire of the same colour can then be soldered to the first pin of the same
pole, so that when the switch is off it is connected to the other wire. Here you can
see that I’ve actually soldered it to the top pin of the switch – this is because these
pins just go straight through, so it doesn’t actually matter which you solder them to. The other end of this wire can then be soldered
to the circuit, above the relay and in line with the second coil.
Now we have one last wire to add. This wire needs to be inserted between the push button
and the relay’s second coil. The other end of this wire needs to be connected
to the last cell of the battery to trigger the relay off. As I’ll be using a 3 cell battery,
that means I’ll have to connect it to the balance lead’s fourth wire, which in my case
is red. If you’re using a 4 cell battery, it would
have to be connected to the balance lead’s fifth wire.
Again, I’m going to connect it to the upper pin as it’s getting a bit cramped on the other
side. Remember, it needs to be connected to the pole’s front pin, so that it only carries
current when the switch is turned off. To avoid overloading the coil, we can use
an 82 ohm resistor to drop the voltage, which is around 12v for a 3 cell battery, to 5v
which the coil is rated for. So now the circuit is complete! Let’s give
it a test. We’ll turn on the circuit with the multi pole
switch, and then trigger the relay with the push button.
When the voltage drops below the set value, the battery disconnects just as before. But
now if you want to turn it off beforehand without unplugging the battery, it’s just
a case of turning the circuit off, which also now disconnects the battery.
So the last step is to build it into a custom hardcase to protect everything. I used 6mm
MDF to make mine, and as you can see, I’ve also added an additional balance lead and
power connector, so that it can be charged easily. These are basically extensions of
the original connectors on the battery, before any circuitry. Now you can safely power your projects without
worrying about damaging your batteries. If you’ve found this video particularly helpful,
I’ve actually set up a PayPal donation account if you feel inclined send a tip my way. All
donations go straight back in to buying the resources needed to make more videos like
this one, and are warmly appreciated. So, thanks for watching, and I hope I see
you in my next video where I’ll be showing you how to build a solder fume extractor to
help keep you safe when soldering. Bye for now!

100 Replies to “How to Make a Battery Protection Circuit (over-discharge protection)”

  1. Excellent idea and well explained instructable. I especially liked how annotations and switch animation remained aligned with objects during camera rotation. Can I ask what software was this done with?

  2. hey man would this work instead of the relay that you used.

    Look at this on eBay

  3. Hi, i made a same, but another relay, but when the beeper on, the relay is thicking, not stay on.
    I check the beeper voltage, and its going up and down.

  4. Amazing video. I couldn't find any other videos that covered assembling the circuit, so thanks for showing the assembly process.

    What relay could I use to control a 12v lead/acid battery?

  5. Hi, I'm looking to make a power swap automatic system, maybe that's not the correct name for it but is basically a system that works on two independent batteries and will use one at a time, and then I will disconnect one of them and the system will switch automatically to the other battery always keeping the system energized. I'm new about electronics so it will be helpful if you give me a link that explains how to make something like that. Thanks.

  6. Great DIY. One question though, instead of this main latching switch , why not just use normal one just on first (negative) wire ?
    Thx and cheers !

  7. great video!!! My only question is What is the voltage of the capacitor? i know is 100 uf , but the voltage? thanks

  8. Is it possible to do this with a two cell 7.4v battery? If so, how would I modify this to accomodate my needs?

  9. Even after breaking main circuit, battery is still drained up by lipo monitor. What about "over-discharge protection" which will cut off absolutely current? Battery will be leave for few days without supervision.

  10. This is great. Even a lunk like me could follow this. My real issue is that I want a protection circuit for overcharging as well so I can keep the battery connected to the item I'm powering.

  11. I'm getting on and my eyesight isn't great ,, is there anywhere in the uk that I can buy something like this ,,, I'm tinkering with 18650 cells ,, great video subbed and thumbs up will send you couple of quid in near future cheers Shane warks

  12. why 3.7v/3.5v cut off volatage? I thought that normaly it get's down to 2.5v till it's shutoffs

  13. if have tought of a different setup for the switch for people that might put the battery away for a long time. You dont want it to drop to low when you put it away for a few months. My idea was to put the sensor ground trough the relay also, so if the voltage drops to low and the relay cuts it off and sensor board will be of too. And then when you connect the charger there should be voltage and the sensor board gets power again because that is all connected to the charger side of the relay then. And because the board has power the relay will close the connection and turn the battery back on.

    And if you want to use this system in a power tool better or something make a push button switch on it that gets pushed in when the battery is in place instead of the on off switch DIY Perks suggests.

  14. use a bms it is far better and you only have to waire it and thats it will keep your battery healthy!! and it can even charge it!!

  15. I don't know what the situation was back in 2014 when this video was made, but these days (2017) you can get complete battery charger and protection circuits from China through sites like eBay for peanuts (or pretty near).

    I built a supplementary external battery pack for my laptop using a balance charger+protection board. The pack uses a dozen 3700mAH 18650 cells in a 4P3S arrangement (4 cells in parallel in each group, and each group in series for 11.1V at 44400mAH). The output goes through a double-pole switch to a buck-boost converter which provides a steady 19V output at a maximum of 8 amps, so it runs cool.

    The only real down-side is that using a standard laptop charger which provides only 4.5 amps, it takes around 12 hours to fully charge my battery pack. :p

    But it works, and I made it myself! 🙂

  16. What connections are what on those little buzz alarms? Been thinking along similar lines myself, but there is never any info on these splendid gizmo's!! So puts me off buying them, as just purchased one of those capacity controller meters and unable to use it, as' there is no plug or leads or any info of how it is used!! At least your video with those pin connectors have give me an idea of the type of plug the meter takes, just a matter of finding what connections are what on it now!! Any ideas on that too?

  17. As I understand, you want to protect the battery from dicharging by the circuit protection board.
    But for that purpose you are using the last cell of the battery and keep the relay coil drawing current from it until we are switching it back on.
    So you are creating a uneven discharge of your cells just to keep the circuit protection off.
    Would not have to be better to play with capacitors to do this job once and not in a permanent way ?

  18. You make the best videos! Brief but thorough description of the underlying basics, great links to resources and background info, very clear description of steps in the project with caveats. I watched many of your videos even though I dont even consider building the shown projects, but just for the knowledge gained and your way of explaining things.
    Thank you.

  19. hmm, I notice that the battery protection circuit requires power from the battery in order to function. Even when you have not used the battery. Would this be problematic if you were to leave the circuit connected to the battery for extended periods of time? IE: a year or two?

  20. Couldn't you add an inductor instead of the switch so that as the alarm is sounded when it's first turned on or plugged in it doesn't reach the relay? as there is no current allowed initially? and then once the inductor is charged it will just act as a wire?

    Just something that occurred to me, i think it would make the whole circuit a little bit easier to use, appreciate if anyone has any feedback as i'm looking to make something similar myself

  21. very inventive, but just a voltage meter ($3-5) across the battery output terminals will let you know where your battery level is at.
    high output current terminals should NEVER be that close, uninsulated. was there even 1mm separation?
    5-20 Amps where you'd probably be using this circuit. hmmm I had to -1 this bcs basically that invites disaster, esp when this vid is intended for those not familiar with electronics and battery circuits

  22. Very nicely done Sir! Also, thank you for taking the time to make this very complete and easy to follow. Such a necessary item!

  23. So what kind of current can this thing handle? I want to build something that can handle say, 6 or 7 amps of current draw for a discharger.

  24. Why bother using a strip board if you're just going to bridge adjacent strips? From the looks of it you could've directly used the strips to connect components in many places for a neater, easier job.

  25. Great video! Would this work as a prebuilt protection unit?

  26. AHH! I SEE!! These thick wires are "safety fuse wires". They will vaporize and cut open the circuit, whenever your device goes haywire. NICE SAFETY TOUCH!!! 😉 – NOT! – Include Fuses kids!!

  27. This is a great tutorial. Does anyone know of an equivalent relay that can be used? I'm a complainer- the only places i can find this part charge far too much for it, plus far too much for the s&h, and they always end up sending you a huge box that's several dozen times overkill (which you pay for) and i guess not enough people care since it's been that way for years. Sometimes they even put a little piece of candy in, some people seem to love it, i'd rather just get the part without all of the extras and frills.. I usually buy small parts on ebay just to avoid fluff, but I can only find it at one place there, and the seller actually wants $18.24US plus $23.57US for shipping. Well it's in australia and i'm in the US, so naturally. I guess.

  28. this video show how to build overdischarging protection. How about overcharging protection? please make the video about that

  29. You have a dead laptop battery, car battery or just about any other commonly used battery I can show you how to easily bring it back to life [Check Details Here>>>> ].

  30. I'm curious, can this be done to make an overcharge protector as well? arguably, overcharging is more dangerous than over discharging, but I'd love to be able to make one of each type of protectors, but preferably be able to calibrate the overcharge protection so it does not trigger to early or too late (My multimeter I use is calibrated professionally)

  31. Pretty much ALL batterys have protection on the actual battery now. Using common sence and using the proper charger to charge your lipo batterys. Trying to charge any battery incorrectly is asking for more trouble than your fire extinguisher can handle.

  32. hi everyone ,if anyone else wants to uncover how to recondition old batteries at home try Panlarko Recondition Planner ( Check Details Here>>> ) ? Ive heard some super things about it and my co-worker got excellent success with it.

  33. Nice movie. However you should take instructions from Avasva website if you want to learn how to make it easily.

  34. You can take each cell down to 3.2v easily and lowest at 3v per cell. I take mine down to 3.05 when I use mine

  35. if you want to know how to make it yourself , just go to Avasva webpage. There you'll find your answers 🙂

  36. Are these absolutely necessary?
    I’m new to drones and I’d just purchased a SKYRC IMAX B6AC V2 and two Hubsan batteries for my Hubsan 501S X4 drone. I’m trying to buy the NECESSARY items to keep my batteries and drone in optimal shape, but every time I buy something I see something else that SEEMS like it must be purchased…?
    So, I’m just trying to see if this is something I truly need? Or, is it an option? I’m all for making life easier, by having the right tools, etc. but I don’t even know how to use the darn charger yet (on order) and I still keep finding “goodies” to purchase for my and my daughters new hobby.
    Is it really NEEDED? And where could I find more info in regards to these?

    Thanks everyone!

  37. why not just add a bms to the battery with a 5-pole latching switch? it has built in low voltage protection and nowadays even a 4s 100A costs just a few Dollars from China..

  38. Hi super cool love it and like the fact it cuts off with individual low cells . What's your theory on charging large packs of 18650 13s 4p the diy paperwall YouTubers are building.. I've asked them the question about only charging from one end of bus bars, my opinion amps are going to go to first batteries in pack. They don't seem to see my theory of having imputs each end of bus bars maybe you could shine light on it and perhaps a video.
    18650's triple there lives running 3.9v max maybe a voltage cutoff 10s at 39v 5amp charge maybe. Thankyou in advance live your videos your a smart cookie keep it up.

  39. Well, definitely it will consume too much to finish the battery off after it disconnects the load. This relay consumes 12.5 to 100 mA when it's turned on.

  40. I don't understand the point of this. You can buy a proper BMS board for less than the cost of this which will have a lot more protection functions including balanced charging.

  41. Aren't there like a billion battery protection circuits on eBay? At least I found a ton of them for 3.7v li-ion batteries.

  42. Uhhhh maybe in 2014 they weren't but BMSs most certainly are readily available separately. Appreciate the content and free knowledge but in case making this is too intimidating or difficult for some viewers, they should know Amazon, eBay and a lot of other places sell all kinds of Battery Management Systems and PCBs

  43. Dear Mr. DIY Perks,
    I am writing to you my message hoping to find some answers to my strange problem. Based on your video i built some 4S battery packs using just the Li Po alarm modules. The circuit is simple. I connected the balance leads of the battery packs to the LIPO alarm module and put a switch on the negative wire, 1st pol. of the LIPO alarm module. By this i disconnect the LIPO alarm when the pack is not used…..i was thinking.
    This build i made 4 years ago and 1 year ago i noticed that all my battery packs, all 6 of them, died. Strange thing is that all the packs died in the same way. The cell number 3 got to 0V. Rest of the cells are all ok. When i noticed this issue i changed the LIPO cells and not long after, maybe around 11 months the new cell packs died again with the same simptom. cell nr. 3 close to 0V.
    I have no idea what can cause this……having in mind the following aspects:
    – the battery packs were all lipo, new ones and brand ones.
    – the alarm was connected one on one to the pack, no other electronics between – except the switch on the black lead.
    – usage of packs was 1-2 cases / 3 months and they were stored charged at around 4V / cell. I checked this every time when i put the packs away.
    – during usage i did not notice any unbalanced behavior. Battery packs performed well
    – the 3rd cell dying simptom occured on all packs with different brand and capacity of cells.
    – each dying happened while the switch from negative wire was off, so while the battery packs were not used.
    First i was thinking that i had the luck getting bad batch of battery packs on the first round but now after 3 other battery packs died just over 11 months in the same way, the situation is strange and i suspect that the LIPO alarm modules have something bad in their design.
    I have no idea how they behave after you cut the negative lead (so display and buzzer goes off)

    Thank you for your kind response and wishing you a nice day.


  44. Good Job giving the data in this simple format. >>> The kids are going to love it an be inspired by this.. I am going to set up a few things with this cheap solution and all these haters can go back to whatever school they think they graduated from.. They have never been to the school of Hard Knocks..Go SeaBees..

  45. No consideration for safety.
    Wires potentially almost touching each other.
    Bad Soldering
    Work Not professionally carried out .
    Quick fix video. Then ask for money.

  46. There is actually a low voltage cutoff available on eBay for LiPo batteries, and features a brownout delay (it doesn't switch off if the voltage dips below the set cutoff momentarily). Just search for LiPo low voltage cutoff. Be advised, the model I found is for 2S LiPos. There possibly are different models for more cell count.

  47. hey this is a really nice video, have been following your videos for a while and this is one of the best projects i have seen thus far. i have been looking for something like this for a while to use with my powerwall setup as off the shelf bms are too expensive or doesn't have a way to set the disconnect voltages. i would like to see you make an improvement on this using the isdt- bc-8s battery checker as this battery checker would provide more options as it can be programmed for over voltage, under voltage and voltage delta. i am going to follow your video to try and may a 200amp version with that battery meter. if you have the time and would like to make a 2.0 version i would really like to see what you come up with.

  48. Why all the cheap pcb for dangerous li-ion, but no cheap circuits for safe lead acid??? I don't want to buy a $20 charge controller for a 2 dollar solar light!

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