Universal LiPo/Li-ion UVLO (undervoltage lockout)

Feature-packed over-discharge protection. Set the cell-count (2S to 7S), minimum cell voltage and dip-time below threshold. Fully hackable!

Similar projects worth following
This board will solve one problem in my (any maybe your!) workshop and projects:
Using whatever battery you want for your projects without it being deeply discharged and damaged! The system automatically disconnects your load when the battery drops below the set voltage per cell for the set period of time.

ONE board for all your needs from 8.4V to 29.4V (2 ... 7 cells). The real beauty is in the users freedom of choice. You:

- want to discharge your cells to 2.8V or better just to 3.2V to be safe? Go for it.
- expect heavy load-spikes near the cutoff-voltage which would set off any usual load-disconnect early? Set the dip-time to 5s, so your batteries can recover from the heavy load without the system turning off!

A reset from the trip-point can only be done manually by the user.

Short summary

Use this board for all your battery powered projects, especially those using lithium ion or lithium polymer cells. It cuts the load from the battery if the user-set parameters are met and prevents the cells from deep-discharge, which can and will destroy the battery in the long run.

Key features 

  • Works from 2 to 7 cells.
    • Battery-voltages vary depending on charge:
      • 5.4 … 18.9V @ 2.7V/cell (max. allowed repeated discharge)
      • 7.2 … 25.2V @ 3.6V/cell (nominal voltage)
      • 8.4 … 29.4V @ 4.2V/cell (fully charged)
  • cuts off load when the battery voltage
    • sags below a user set threshold
    • for a user-set duration
  • no automatic reset, user manually re-enables the device
  • super low Rds_on N-Channel MOSFET for output switching
  • supply current (ON, working): 1mA
  • quiescent current (OFF): < 1µA

Basic mode of operation

With three potentiometers you set the cell-count, the cut-off voltage per cell and the time the voltage needs to be below the threshold to cut the load from the battery.

The battery is connected with an standard XT60 connector, which is common for Li-Ion and Li-Po batteries. The load is connected on the opposite side of the board with screw terminals. It is initially off.

The board is activated by a short press of a button and the load is switched on if the battery voltage is high enough.

  • 02 – v0.2 boards fully functional!

    Jan6 days ago 1 comment

    In my last log I mentioned just a few quick things. Now it's time to tell you the following:

    My v0.2 boards are fully functional without any modification. Can't remember when this happened the last time, if ever :)

    A few notes:

    • current consumption when active in the single digit mA range
    • current consumption when OFF: 0.45µA
    • wake-up from OFF-mode only by pushing a button connected to the board
    • pushing reset == OFF

    I wrote a quick first-try program for the boards which I will refine a bit next week and upload.

    When this is done, you could order those boards, solder them and add them between your valuable rechargeable batteries and your devices.

    I'll put up all the needed files shortly!

  • 03 – cutting costs

    Jan05/01/2019 at 10:20 1 comment

    What is the aim?

    This log is all about: How can we make the whole board cheaper while keeping all the features and not killing its safety functions. This all refers to the early v0.2 version which is by no means optimized yet!

    Ways to cut costs

    I try to think of mass production here (Disclaimer: I AM NO PROFESSIONAL, it's just what I would do to keep it cheap).

    Let's say we want to make a thousand of those boards to sell them. A few thoughts come to my mind then:

    1. less parts = cheaper
    2. substituting expensive parts with widely available "china brands" = saving costs
    3. using jellybean parts and same footprints
    4. smaller boards = cheaper
    5. boards need to come fully assembled
    6. [...]

    Let's get into more details here

    The above points will be anatomized in greater detail here.

    1 – using less parts

    This is a no-brainer, right? It's not as easy as it sounds. Most parts are there for a reason, so we need to look at every part and ask ourselves: do we really need it?

    Here are a few parts which can be substituted by other brands but can not be left out altogether:

    • Microcontroller: needs to be there for all the main functions and hackability
    • MOSFET: our main-switch
    • input and output connectors
    • a voltage regulator: we need at least a low-Iq 5V rail for our logic-level MOSFET
    • a few caps at the various inputs of the chips for stability

    So, if all those parts are needed, which parts could we possibly cut from the list? A decision in this category usually has some effects on other parts of the circuit.

    1.1 - choosing another voltage regulator

    The TPS70950 is a nice LDO voltage regulator with an EN-pin and <0.1µA quiescent current in shutdown mode. Neat! But this comes at a cost: €0.3395 per piece buying 1000.

    No we need to ask: what OFF-current draw can we accept? From other circuits I know we can get ATMEGAs and ATTINYs down to around 2µA total using a system load switch. Say, we would accept 10µA total OFF-current. This doesn't drain an already "empty" cell too much. So we have 8µA for other parts. A hint from @Simon Merrett was using an HT7550-1 from Holtek. These draw max. 3µA idling (with no load), having no EN-pin though.

    Fits our bill perfectly! Costs €0.075/pc buying 1000. Here's what I mean with changes down the line when substituting components:

    voltage regulator in v0.1

    Looking at the circuit above we can use the HT7550-1 instead:

    The 1µF caps can stay in place, no need for 10µF in our circuit, cause it uses only little current.

    Changes to the circuit are:

    • the EN-button needs to be tied to one of the µC's interrupt pins
    • ZD1 can be left out as the 5V rail will be always on. While not exactly expensive it's another saving!
    • R3 can be omitted as well

    Total savings: Around 87%. or 265€ per 1000 boards.

    1.2 - choosing another MOSFET

    With MOSFETs it's a bit more like "pay less get less". Considerations are:

    • Switchable current 8A (10A) max.
    • max. input voltage 30V
    • no mounted heatsink
    • small PCB area

    So, we need a type with a maximum drain to source voltage of > 30V to keep the MOSFET from destruction, that's why I chose 40V (the v0.1 one has 30V max.).

    Next consideration is the resistance RDSon at a gate voltage of 4.5V. Datasheet values are mostly for a MOSFET silicon temperature of 25°C. So we need to de-rate it with the given value of the datasheet to be sure it is proper low resistance even when hot.

    With little board-space we need to take the heat-up into account which is around 50°C per W of "wasted" current.

    A quick look at Digikey presented me the SiSA72DN.

    On-resistance is around 0.007 ohm at a die-temperature of 125°C and 4.5V gate-voltage.

    Fits our needs perfectly and has a very small footprint...

    Read more »

  • 01 – boards v0.2 ordered for tests

    Jan04/27/2019 at 11:20 10 comments

    Update May 15th, 2019

    Just soldered the first V0.2 board on a hot steel plate:

    It works, burned the bootloader (for fuses) and the blink sketch. Runs just fine. Next I'll try to read the three pots and switching the MOSFET on/off.

    Edit: Everything seems to work so far. Will post an update soon to keep you posted!

    Read more »

View all 3 project logs

Enjoy this project?



Simon Merrett wrote 04/28/2019 at 10:38 point

Hey @Jan I know this is a preliminary version but is there a schematic you are happy to share? 

What regulators are you using? 

Also, what MOSFET do you envisage using and at what price? Maybe we can find something with equivalent RdsOn and logic level driving? 

What about a "Pro" version which adds JST balance connectors and has the ability to monitor each cell individually? 

Ref the pots, what about fixed resistors and pin header bridge jumpers? Just move the bridge jumper across the correct pair of pins for your configuration. Not as compact as pots but doesn't require a screw driver / flat tip to switch. Dip switch would be the same but I don't think it's cheaper than the pots. 

To shrink the board, can you put the FET and connectors on the bottom? 

For the microcontroller, how about the new ATtiny range (404, 814 etc)? Given the low speed requirements for the processing, they have some fantastically low current options when running on their internal 32kHz oscillator. 

  Are you sure? yes | no

Jan wrote 04/30/2019 at 16:55 point

Hi @Simon Merrett

Let me address everything individually:

1) schematics are in the files section

2) the regulator is a TPS70950 which works up to max. 30V input

3) you're right with the MOSFET. I changed it to an ultra low Rds_on one (1.1mΩ @ 4.5V gate) so I can ditch the driver

4) a pro version is not planned yet. I have to take sume things as granted, which in this case includes propperly balanced/charged battery packs

5) I used pots because the user can set the setting quickly and individually without the need to fling a soldering iron or something.

6) all alternatives to tiny pots seem to use more board space :(

7) board shrinking will be done in the next iteration!

8) the micro will be changed to some easy to program Attiny.

I appreciate your very eye-opening and competent help so far!

  Are you sure? yes | no

Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates