Need help with Low voltage protection circuit

[flinx]

I am trying to design a circuit based on this circuit:


I substituted an IRF5305PBF for the Si4425BDY because I wanted a TO-220 package I think I misunderstood the datasheet as I am not sure what voltage turns on the Mosfet.

I am using this TL431ACLPR

I then tried to find a calculator to determine the resistor values for R1 and R2. The resistor values I tried did not work the way I expected.

The source for the original circuit describes it turning off the mosfet at 18.2v and back on at about 19. No matter what combination of resistors I tried (based on the calculators I found) did that.

How would I make the circuit work using the mosfet and TL431 I have?
If not possible with the mosfet what one do I need?
This device would run off of a 5 cell 18650 battery in series.

[budm]

Re: Need help with Low voltage protection circuit

Can you give the link to the original circuit?

[flinx]

Re: Need help with Low voltage protection circuit

Can you give the link to the original circuit?

That is the original circuit.

[redwire]

Re: Need help with Low voltage protection circuit

I see that circuit all over the web, it's another blogger special.

The TL431 always draws some current, it needs to power itself. Usually 0.4-1mA minimum, unless you go to low current TLV431.
So there will always be a voltage drop across R3 4.7k resistor which may be enough to turn on the mosfet a little. You need to add an extra transistor, or changing to N-ch mosfet switching the GND side.
Note the mosfet max VGS is 20VDC so a protective zener and resistor are needed to protect it.
This kind of circuit always incorporates a latch, so it does not chatter on and off because disconnecting load always lets battery voltage go back up, LVD turns off and connects load, voltage sags, LVD trips and voltage goes back up... it oscillates which is not good for some loads to have power on/off like that.

The second circuit with latch taken from http://ka7oei.blogspot.com/2016/08/a...r-12-volt.html

[stj]

Re: Need help with Low voltage protection circuit

i cant see how that original schematic could ever work,
R5 is going to effect your voltage reference, and it will get power from both the load via the load resistance, and the body-diode in the fet

[flinx]

Re: Need help with Low voltage protection circuit

Lets try a different line of inquiry.

I need a device like this:
https://www.ebay.com/itm/16V-Battery...53.m1438.l2649

But I need it to work with a maximum voltage of 22vdc and then cuts off at 18vdc, hysteresis would be nice but is not absolutely necessary.

It needs to handle 15 to 20 amps.

as small as practical.

It does not need to balance batteries or charge anything.

It should not cost much more than the above device.

It would be used in 18v tools and devices that do not have built in battery protection.

If such a device does not exist then I just need a working circuit (using through hole components) and I will handle the rest.

[petehall347]

Re: Need help with Low voltage protection circuit

http://www.diy-electronic-projects.c...ith-Time-Delay
adjust components to suit current and voltage

[eccerr0r]

Re: Need help with Low voltage protection circuit

Main problem you have is that the turn on voltage for the MOSFET is dependent on your load. If you want sharp on/off you will need a higher gain than the circuit you have.

After messing with how to design battery protection, I ended up deciding that you'll end up having to go with the custom ICs to control your FET anyway, and yes they will be SMT. If you don't, you'll be burning a significant amount of power in your nice through hole design, so much so that you'll kill your batteries anyway if you don't choose custom battery protection (like that "U1" component in the example) or more expensive ultra low power/noise amplifiers.

[sam_sam_sam]

Re: Need help with Low voltage protection circuit

This device would run off of a 5 cell 18650 battery in series.

What are trying to power with this battery pack

Depending on what your power requirements are you might not have to reinvent the wheel
If you are just wanting to charge the battery pack

[flinx]

Re: Need help with Low voltage protection circuit

What are trying to power with this battery pack

Depending on what your power requirements are you might not have to reinvent the wheel

anything that require 18 to 21 volts in the category of power tools. but it could also provide power through a buck convert to whatever I might want.

the annoying thing is that there are many devices that do this but they are either not the correct voltage range or cost over $100

[sam_sam_sam]

Re: Need help with Low voltage protection circuit

Are you looking to charge the battery batteries or something else

[sam_sam_sam]

Re: Need help with Low voltage protection circuit

There might be other solution to this problem

[flinx]

Re: Need help with Low voltage protection circuit

Are you looking to charge the battery batteries or something else

No the device would just be there to add over discharge protection when used on devices or with batteries that do not have that.

[sam_sam_sam]

Re: Need help with Low voltage protection circuit

Take a look at this device

https://www.ebay.com/itm/Battery-Mod....c100276.m3476

[flinx]

Re: Need help with Low voltage protection circuit

Take a look at this device

https://www.ebay.com/itm/Battery-Mod....c100276.m3476

I have tried some of those devices, the problems with them are that they are too large to fit inside of tools and devices, they have displays that draw power and cannot be turned off. That one is meant for controlling the charging of a battery not running a device and then cutting off power.

I have some voltage alarms that might be hackable to do what I want with the addition of a relay. I was hoping to use a mosfet to save space. My limited knowledge of them led me to believe they were an instant on/off type O' thing, but I have since learned that is not the case.

I have spent 2+ weeks searching for anything and everything and using any combination of words regarding battery protection, under voltage, low voltage, cut off, etc. I have found many devices that do what I want but they all are just outside of the voltage, amperage, or price range of what I need.

There is one company that makes exactly what I want but I'm not paying $200 for it. I was going to design my own circuit board but I held off till I could test some designs and that's when I found that most if not all of the circuits I could find do not function as the person who posted the information claims, do not work at all, or only sort of work.

I like to keep things as simple and as small as I can, and avoid using things like arduinos to do things that such devices would be overkill.

as usual I have apparently tried to do another project that has turned in to a quest for unobtainium devices.

[sam_sam_sam]

Re: Need help with Low voltage protection circuit

What BMS balancing protection board are you using
Also can you please post what you are looking for that cost $200.00

[flinx]

Re: Need help with Low voltage protection circuit

What BMS balancing protection board are you using
Also can you please post what you are looking for that cost $200.00

I am not using any particular board. If the battery pack has one I do not know what it is, I would also be using 18650 packs that would not have a BMS.

I looked in to BMS, BEC, and UBEC options none of them could handle the amps needed or were the wrong voltages, and I do not want to use the device to charge the batteries, nor do I want to build battery packs.

this is a listing for the board in question there are 3 options.
all 3 are very pricey and the boards are covered to prevent seeing the secret inards.

[flinx]

Re: Need help with Low voltage protection circuit

It is kind of bizarre there are devices used for RC racing cars that have low voltage cutoffs, they make them for 2, 3, 4, 6, 7,and 8 cell lipos, yet for some odd reason they skip 5 cell which would work perfectly for what I need.

[eccerr0r]

Re: Need help with Low voltage protection circuit

So how much time do you expect the low voltage cutoff to occur and when you *actually* want to charge the pack, mind that with this cutoff design you have to bypass it to actually charge it (unless the drop/loss through the body diode is acceptable, which is not in most cases for lithium packs.) Also keep in mind if you charge and put the pack in cold storage for a week, your cutoff eats power and can still kill the battery that way too. I'd say most common off the shelf parts through hole design will end up emptying out a full pack in a month, and after it empties it, will continue to overdischarge the pack, defeating the purpose of the protection.

In any case I still don't get it: most if not all lithium ion packs available out there have overcharge/discharge protection. Even my HFT cheap crap Li-ion pack. This is due for liability purposes, having packs immolate due to overdischarge is not going to be safe.

Or is this a homemade pack? In this case your design is still insufficient to fully protect the pack.

Also if size is a problem, through hole is not small either, especially with the number of components needed to properly do voltage protection.

What *ARE* you trying to do specifically? Pictures would be good.

Okay I have to make it clear of my question: What is your battery pack. What are you powering with the battery pack. How much are you planning to use the device. Are you going to be leaving the device in a drawer for a few weeks when you're not using it.

[flinx]

Re: Need help with Low voltage protection circuit

this device has nothing to do with battery packs in so much as it would just protect them from being over discharged.

The idea is to be able to use battery packs that do not have protection (there are lots of these) or LIPO packs which do not normally have protection.

I cannot provide pictures as I do not have any device at the moment.

I will give you an example:
Black and decker makes a cordless vacuum cleaner called the dust buster (you may have heard of it). It runs on 16.8v (yes I know I need 18 to 21 but this is just an example).
It has a 4 cell 18650 pack inside that dies after about 2 years because the amp draw on it is 11amps continuous when in use.

When it dies the batteries could be replaced with better batteries OR one could install an adapter on the back for a makita or other 14.4v battery if one were to purchase the cheap chinese version they do not have battery protection in them, heck many of them do not have cell balancing.

The issue is that repairing the vacuum with new batteries (of legit quality) is about the cost of just buying a new vacuum.

Were one to be able to use already owned battery packs one could save a lot of money not having to buy a new vacuum cleaner every 2 years.

This is only one example.

I want to be able to take a device that might have bad batteries and refit that device for using other battery packs allowing the continued use of said device for longer.

The problem I keep running in to is that if I did want to do this ONLY for said vacuum there are devices out there and I would not be asking on forums for help.

Because I would like to do this for 18/20v devices I cannot find a reasonably priced solution. This has turned in to a quest. I generally post questions like this on forums when I have exhausted all other avenues (that I can think of) I then try forums for ideas that I may not have thought of OR sometimes I just don't know the name of the device or circuit I need.

Once I get what I am looking for I post all the information, schematics, and what not on a blog or said forum and my youtube channel so others might be able to use the information and thus save themselves much money and rage in the long run.

Please forgive me if my post sounds angry or whatever tone does not translate in text. I do this a lot, I eventually find or make what I want but sometimes it takes me a while to gather the required knowledge and materials.

PS. the devices would not be stored with batteries attached.