Introduction: Programmable Battery Protection [Open Source]

Every battery needs protection.

When I couldn't find a board that suited my needs, I set out to create one myself.

If you discharge a battery completely it can be permanently damaged. This PCB monitors the voltage (and thus the State Of Charge) and if it dips below the programmed voltage it will disconnect the load. For a lead-acid battery this is ~11.5V for example. Please note that this battery monitors total pack voltage, it doesn't offer individual cell protection. It can be used on any battery chemistry you want, as long as the pack voltage is between 5.5 and 29V.

This solution is based on an ATTiny microcontroller which remains in very low power shutdown unless the button is pressed. If the battery voltage is above the programmed treshold the load will be switched 'on' using a mosfet.

A couple of key features include: reverse battery protection, very low power consumption, able to switch high current loads, compact board, cheap to make.

In the video you will see the design process step by step. A more detailed explanation in text format can be found on the following pages. On the last page you can download all the files, including schematic, layout, GERBERs and code.

Step 1: R&D

Picture of R&D

The three main things I wanted to achieve:

-low cost

-small footprint

-high flexibility

This project doesn't require many pins, so I chose the popular ATTINY85 microcontroller (6 I/O's) in a SOIC package. An ATTiny gives a very flexible solution that is cheap and requires almost no external parts.

For the mosfet I needed something that was small, cheap and can switch 8A+. N-Channel would be the most practical, because then you can connect one side to the ground plane of the PCB. It can switch about 13A continuous and 110A (!) pulsed.

For the voltage regulator I needed something that has low quiescent (idle) current, because the power usage has to be almost zero when 'off'. The LM2936 satifies this requirement with only 15uA of idle current.

Step 2: Schematic

Picture of Schematic

I was able to keep the required part count very low.

Couple design notes;

-The voltage divider on the input has to have high resistance to keep power dissipation low

-Max input voltage times the divider ratio should be about 3.3V (Vcc)

-1%+ tolerance resistors should be used to guarantee precision

-Pull down resistances prevent parasitic toggling of the mosfet and switch

-Small decoupling capacitors keep voltage stable

-LED current limiting resistors can be increased to decrease power consumption at the cost of brightness

I was not aware that the ATTiny came in a different SOIC package than the mosfet and regulator. That's why they don't fit properly on my PCB's. It's fixed in the downloadable files at the end of this Instructable.


Step 3: Breadboard Testing (AutoCAD)

Picture of Breadboard Testing (AutoCAD)

I have made a 'breadboard schematic' of how I tested my plan. This setup allows me to see if my powerdown, treshold and mosfet output is working. It's best to use a labbench to power the circuit. This way you can vary the voltage and test the switching treshold.

Step 4: Layout

Picture of Layout

Layout is pretty simple. It's a 2 layer PCB with stitched groundplane on both sides.

In/Output terminal on the left, programming header on the right.

JST 2P for the switch at the top, JST 3P for the bicolor status LED on the bottom

Step 5: Code

Picture of Code

A microcontroller in powerdown uses almost no current (I measured ~300nA). That's a lot less than the self-discharge of the battery itself. You can wake a microcontroller from its sleep by using a pin-change interrupt. I commented my code so it's easier to understand.

Current consumption when active is 2,06mA. It's the same at all voltages because a linear voltage regulator is used.

There is a download link to the .INO arduino file on the last step. Attached code doesn't have the lines for the led control, I might change the file later. If you need help with that, message me.

The Arduino UNO can be used as ISP to program the ATTiny as seen here:


Thanks to user frarugi87 for noticing some flaws in the code I uploaded. The picture above and the file in the download now have been corrected to a fixed and optimized version of the code.

Step 6: BoM and Files Download

Picture of BoM and Files Download

Bill of Materials (ordercode at Farnell):

-ATTiny85 [1455164]

-Mosfet, N-Channel SOIC [2322618]

-Resistors, 1% tol 0805 [9332405; 9333606; 9333509]

-Capacitors, 35V+ 0805 [2496944]

-Terminal block, 6way 5mm [9633006]

-Regulator, 3.3V SOIC low IQ [2492178]

I got my JST 2P and JST 3P connectors plus cables from eBay.

As you can see, I listed the key specs so you can find replacement parts if needed/wanted.

Minimum PCB order quantity is 10 pieces. At that quantity BoM is around 5$ per board.

Files can be downloaded here

-Production ready GERBERs

-.INO Arduino C code

-Schematic and Layout [DipTrace]

If you need help with ordering pcb's, modifying code, programming the chip or anything else; send me a message.


MensjeN (author)2017-08-08

What is the code for a momentary button?

I made the circuit board and uploaded the code. with the current code the mosfet turns on when pushing the button. when i release the button the mosfet turns off.

i tried to change the digitalread(sw) to LOW. but then it works vice versa

redkurawa (author)2016-12-10

Hi, i cannot found link for the .ino file

lutkeveld (author)redkurawa2016-12-11

It's in the zip file on the last page of the instructable

djimenez9 (author)2016-11-16

Great project! I found an error in the schematic picture, in the voltage divider it says 820 and it must be 820K.

lutkeveld (author)djimenez92016-11-16

Thanks, fixed it! That's a big difference :)

WannaDuino (author)2016-11-13

toevallig nederlander?

wow great ible man and what a good video.

lutkeveld (author)WannaDuino2016-11-13

Nederlander inderdaad :) Thanks!

WannaDuino (author)lutkeveld2016-11-15

kun je me mailen zof dat we via PM ff praten?
ik zou graag zoon pcb willen kopen. of info
dan vertel ik daar wel waar ik weg kom enze.
ik wil nix op het net hebben, snappie

lutkeveld (author)WannaDuino2016-11-15

Begrijp ik, stuur maar een PM via de Instructables site

-AFS- (author)2016-11-15

A great project!

I was thinking to make something extremely similar to this just a a few days ago, and I was already designing it.

This will help me a lot. Thank you!

sockit (author)2016-11-14

Perhaps in your intro you could elaborate about what batteries you are protecting, and why. I hope I didn't just miss that part. Apologies if I did.

Apparently everyone else who has posted comments already knows about this - sorry for my ignorance.

None of my batteries are protected that I know of.


lutkeveld (author)sockit2016-11-15

Thanks for the feedback, I added some lines to the intro post

Wrrr 10-G (author)2016-11-14

Very cool of you to open source this hands-on design.

Thanks Lutkeveld!

JesN1 (author)2016-11-14

Wondeful ! What is your design software and how did you print your circuit ?

lutkeveld (author)JesN12016-11-14

I used DIPTrace for pcb schematic/layout. You can order them using Seeedstudio or Elecrow (about 15$ for 10pcs with standard shipping)

JesN1 (author)lutkeveld2016-11-14

thanks Iutkeveld

frarugi87 (author)2016-11-14

If I understood your program correctly, there are a couple of failure points (or at least I haven't understood everything).

1) if you press the button and the battery is discharged, your load will not turn on (ok), but the attiny will never enter the low power mode until the battery is charged again. This is a bad behavior (it should enter the sleep phase immediately)

2) as soon as you leave the button the load will be shut down (maybe this is the wanted behavior, though, but I think it is more useful if the button can act as a turn-on - turn-off button)

3 and most important) while the load is powered, if the battery gets discharged the attiny will stop monitoring the button, thus leaving the load powered on even if the button is released. This is a real problem...

I think that the first and third points can be solved modifying your code in

digitalWrite(mosfet, HIGH);

while (systemstate == 1) {

if (analogRead(voltdiv) < threshold)

systemstate = 0;

if (digitalRead(sw) == HIGH)

systemstate = 0;


digitalWrite(mosfet, LOW);

Moreover you can avoid using the systemstate variable, modifying the loop in

while (1) {

if (analogRead(voltdiv) < threshold)


if (digitalRead(sw) == HIGH)



Best regards

lutkeveld (author)frarugi872016-11-14

I think you misunderstood the code. The ATTiny is always in sleep mode, but when the state of pin 3 changes the interrupt will wake the system up. Then it'll go through the loop: 1) check if switch is in 'on'-state 2) check if input voltage is high enough 1) if yes: mosfet enable.

The code I posted isn't suited for momentary buttons, but it can be modified to work with those.

As long as the battery voltage is higher than 5.5V, the ATTiny will be able to turn on. Just as long as the voltdiv is below treshold, it won't.

I think I don't fully understand your third comment. The ATTiny is always in sleep mode (veerrry low power consumption). Once a pin-change interrupt happens, it'll go through the loop and do the checks. If the requirements to turn on aren't met, it'll go to sleep again.

frarugi87 (author)lutkeveld2016-11-14

Uhm.. Maybe there are two versions of your code, but in the image posted in step 5 there is a while loop (while (systemstate == 1)). You exit this loop (and consequently power off the MOS and enter the lop power state) only when the voltage is greater than the threshold AND the button is not pressed. If you enter this loop and the voltage decreases, you will never come out of it..

As for the button/switch, sorry, I misunderstood ;) yes, with a switch it will work

lutkeveld (author)frarugi872016-11-14

I see what you mean, I might have accidently uploaded an early version of the code. It should go through a while loop which loops while systemstate=1, in the loop it checks if both the button is pressed and the voltage is above the treshold. At the end of the routine it should check if the switch is low, if yes: systemstate=0 (breaks the while loop) after which it writes the mosfet low and enters low power untill an interrupt is fired. I'll look into this when I get home.

jtechian (author)2016-11-13

Have you ran 10 amp on this for a very long time as it looks like your traces need to be beefed up a bit more. I always try to err to the overkill direction.

Considering that amp draw may burn the trace if ran for more then a minute or two once it gets hot and releases the bond to the PCB. I do like your idea and also wonder: when the micro is in standby, does it still monitor and react to the battery voltage changes or only when you wake it with the pin change button ?

lutkeveld (author)jtechian2016-11-13

I think it can easily handle 10A, but I haven't tried yet. As you can see in the layout, there are four parallel 0.6mm traces to the mosfet of only 12mm, and the other end is tied to the ground plane on the bottom (whole bottom is copper). I'll put it on a CC psu sometime soon to test.

jtechian (author)lutkeveld2016-11-13

I would still for safety use a bigger trace rather then the parallel traces, and also the ground side tied to the ground plane - I would make the hole tied solid to ground rather then the cross hair solder tail. Best to over do it then under. I'm thinking of doing a similar protector for multi cell l-ion packs as IMO battery management boards really are not needed if your cells are all new or good shape. Also packs I want are for like solar power wall or EV.

I want just the monitor protector to open a relay if the cell condition is out of tolerance. Yes a simple 555 or comparator would do that but an attiny would be cooler :)

lutkeveld (author)jtechian2016-11-13

Yeah I know, that would effectively half the resistance of the positive trace but I think the mosfet is already the bottleneck here. What do you mean with ''cross hair solder tail"? The negative side of the mosfet is tied to the top ground plane, which is stitched with the bottom ground planes with a lot of via's. This gives the lowest resistance possible.

Especially for multi-cell systems a microcontroller is the way to go. My next board will be a battery protector for 2-6S lithium, so I can use those cheap RC packs in my projects.

jassing (author)2016-11-13

@ac1ac: SMD's are not that hard -- google soldering drag method. these size components should be easy to do with hand soldering. For thru hole of similar design: (I've built a number of these, but the BOM from the files are wrong)

This SMD version is much nicer/compact in that it is slimmer and I think can slip into existing project boxes a little easier.

Good job!


@jtechian: I agree, that high current needs thicker traces & a heat sink on the mosfet, but I think it wiser to just use it to drive a relay. I was going to order some boards off oshpark, but they complained about the gerbers, (oshpark lets you choose thicker weight traces.) I would modify the code the to use a watchdog timer to wake instead of disabling the watchdog, so it spends most of it's time asleep and wakes up ever few seconds to do it's tests.

lutkeveld (author)jassing2016-11-13

The top and bottom have stitched groundplanes which double as a heatsink. They only need to dissipate 0.07W (mosfet ESR times current).

You can use WDT (also interrupt based) but a Pin-Change Interrupt reacts instantly and then you can also use momentary push-buttons. With a WDT it's possible to be asleep during the button press. Also it'll use more power because it's constantly checking in intervals.

ac1ac (author)2016-11-13

I am not great with SMD's, but I am planning to do this project with through hole components. Especially since I already have all of them on hand except the pcb.

lutkeveld (author)ac1ac2016-11-13

That's also possible, but SMD is very easy. These are all pretty 'big' components. Easiest way is to apply paste on the pads, put down the components and use a hot air gun. But even with a soldering iron it's very do-able. If you need help with the layout for the through hole version; send me a message

andyk75 (author)2016-11-13

Very good documented! Thanks for sharing!!!

NizzyAmps (author)2016-11-11

Excellent work!!!!

Eric Brouwer (author)2016-11-10

Very nice PC Board.

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