CataCluj

I am looking for an Undervoltage Cut-Off (UVCO?) for LiPo (3s) for a non RC application, basically for an elbow heater pad (age, eh?).
I have this USB heater pad that doesn't get warm enough at 5V but does at around the 11-12V from a 3s Lipo.
Problem is, I think I already damaged one battery as by the time I noticed the heating pad wasn't heating anymore the load voltage was... below 5V. It went back above 9V without load but that's not good, eh?
So, I'm looking for the smallest Cut-Off I can get (the whole point is I can carry the battery in my pocket while heating my elbow so the UVCO has to be small).
I see most if not all RC cut-offs connect to the ESC to reduce power; that's not what I need.
On Ali I found some little "12V" Undervoltage Cut-Off boards with somewhat adjustable voltage (hopefully to ~10V) that could work, but use a relay, and that's too bulky. Plus, I think they would oscillate as the voltage goes high without load.
My pad uses 3A so a MOSFET should do, even for much higher currents.
Is there such a thing ready made?
If not, I may design one; it just feels a shame to spend that design effort for a one off, eh? I may just make it on a protoboard.
Thank youse all, preemptively :-)

tedsander

Following - I have a electric glow fuel pump powered by a small LiFe cell. Apparently sometime, the switch got bumped and ran the battery all the way to 0. Not good. It would be great to have a similar cut-off for that application.
Again, bulky would be bad. Current demands may be less, likely, that what your pad requires.


PS - found this, but at 4.5" x 3" still a bit big:

CataCluj

I think I could make them much smaller, maybe just over 1"square, more if mounting holes (heatsink?) were needed, for high current (>10A?), and maybe 5mm thick.
I wonder if adjustability would be important for people; I know many like to measure and adjust everything :-) and I wouldn't mind either, but it would drastically change things: fixed voltage could be purely analog therefore simple/cheap; while adjustable would require microcontroller/interface to avoid potentiometers that can change in time/vibration.

Big Alice

I'm not sure if these are what you're looking for or not. Most are low voltage buzzers but there's some switches in there too.

CataCluj

Thank you Big Alice,
This one seems closest (I only see ones that ship to Canada):
... amazon... (I can't post links yet)/Electronics-Salon-Voltage-Disconnect-Protect-Prolong/dp/B019F3BEIO from CZH-LABS
It seems to have 3 problems:
1. It will likely reconnect again as soon as it disconnects because no-load voltage will likely go above 10.7V
2. It's bigger that I'd like
3. From one reviewer it seems it uses quite a bit of power itself. Probably no biggie as I would likely disconnect it when it trips, but it would be nice not to have to.
I think for up to 5A I can make a really small one, maybe 1.2" square and under 1/4" thick

CataCluj

I made a prototype; should trip around 10.4V.
It just tripped at 10.3V and then no load voltage jumped to 10.6V, then slowly went up to 10.8V. As designed, it did not turn on again. SUCCESS!
The prototype is bigger as I used some through hole components and, of course, the connectors are "extra". YEEEEEEEEEY!
I tried to attach a pic but the forum uses links which I'm not yet allowed to post.

fiyijib

If you find a relay-based undervoltage cutoff module that meets your voltage requirements but is too bulky due to the relay, you can modify it by replacing the relay with a MOSFET. This reduces the size and avoids the oscillation problem you mentioned.

CataCluj

Thank you, but:
I think if they oscillate with a relay (maybe only cheapest would; can't say) they will also oscillate with a FET; you just won't hear it mechanically. It may still damage some loads.
Maybe some can be replaced, some can't, as the output is no longer isolated so it might depend; I do think generally you're right.
Replacing the relay would make it much thinner but same surface area.
Anyway, I made my own; works fine.
Thanks.