Li 18650 battery protection -against discharge - circuit wanted.

[Epicurean]

I want to make some 12v fans out of computer box fans with 3 18650 Li batteries to use for ventilation over the bed at night. One concern is that Li batteries don't like to be completely discharged. Is there some simple way of preventing this so that they run for a while when I go to bed but shut off when the state of charge drops to, say, 30% or similar? I don't have 12 v in the cabin and don't want to run an extension cord. I'm familiar with, but not expert at, electronics. I thought about zener diodes but am not sure the battery voltage drop off is right. Perhaps a simple timer switch that could be programmed for a couple of hours. Are these available battery operated? I know the AC versions are common. Also, must be adult-programmable. (A countdown timer is best. I never could figure out how to program my energy saving thermostat switch.)

[Epicurean]

I found Aliexpress and Shopee have cheap countdown timers which I believe will work. But I'm still curious if there's a way to use SOC etc. to shut it off.

[evm1024]

There are many small BMS that would shut them down for you.

Take a look at this: https://www.batteryspace.com/pcbprot...limited-1.aspx

Just the first one that caught my eye. in the search

[smac999]

run a 12v wire from dc panel to bedroom.... install USB and cig plug.

[sailing_gal]

You could get a voltage over/under protection module. They are programmable and easy to wire. I ordered one awhile ago from China, under $10.

[Avionics_Joe]

We hang these over the bead, they last all night on low. They also have a light, the only issue is one you have to support the charging cord when charging to get it to work after about a year of use.

https://www.amazon.com/Portable-Batt...dDbGljaz10cnVl

[ikanode]

Search for 12V low voltage disconnect (LVD) on Amazon and eBay. It's a trade-off between price and delivery time (US shipper vs China.

A timer won't work very well since the time to discharge depends on how full the batteries are and how much capacity they lose as they age.

I've used this one:
https://amazon.com/Onyehn-Protection...r_1_6?dchild=1

[Suddy]

Checkout this website. It mentions "protected" Li 18650 batteries.

https://commonsensehome.com/18650-ba...8650_Batteries


They have built in protection against overcharging and over discharging. You can find them on the internet by searching for "protected Li 18650 battery".

[Epicurean]

evm1024
That might work although the one you linked to has a over discharge protection voltage of 2.30v. (That must be per cell.) I suspect that's lower than I want to go. I can't imagine there will be much ventilation when they get that low. I think if I can get the time I'll try something like that.
sailing_gal
I think the over/under protection module is the same thing as what evm1024 suggested, right?
Avionics_Joe
That would be the perfect solution....if I were in the US. Unfortunately, to ship to Malaysia they charge an additional $50+. Maybe I'll get someone to hand carry a couple. But the borders are closed. When I get back!!
ikanode
I think in principle you're right. But in practice to get me started I'll probably do a timer and see what the capacity is after use and how often I need to charge them.
Suddy
When it says "protected" I think it means against overheating and is a safety mechanism. I'm more concerned with cell longevity.
Thanks to all for your suggestions and info!

[evm1024]

Originally Posted by Epicurean

evm1024
That might work although the one you linked to has a over discharge protection voltage of 2.30v. (That must be per cell.) I suspect that's lower than I want to go. I can't imagine there will be much ventilation when they get that low. I think if I can get the time I'll try something like that.
sailing_gal
I think the over/under protection module is the same thing as what evm1024 suggested, right?
Avionics_Joe
That would be the perfect solution....if I were in the US. Unfortunately, to ship to Malaysia they charge an additional $50+. Maybe I'll get someone to hand carry a couple. But the borders are closed. When I get back!!
ikanode
I think in principle you're right. But in practice to get me started I'll probably do a timer and see what the capacity is after use and how often I need to charge them.
Suddy
When it says "protected" I think it means against overheating and is a safety mechanism. I'm more concerned with cell longevity.
Thanks to all for your suggestions and info!

Yes, I noticed that they have a number of different models with differing current levels. You would need to use the lowest max current model so as to get low voltage protection at low current draws.

The price is right and might actually work.

[Suddy]

Originally Posted by Epicurean

evm1024
That might work although the one you linked to has a over discharge protection voltage of 2.30v. (That must be per cell.) I suspect that's lower than I want to go. I can't imagine there will be much ventilation when they get that low. I think if I can get the time I'll try something like that.
sailing_gal
I think the over/under protection module is the same thing as what evm1024 suggested, right?
Avionics_Joe
That would be the perfect solution....if I were in the US. Unfortunately, to ship to Malaysia they charge an additional $50+. Maybe I'll get someone to hand carry a couple. But the borders are closed. When I get back!!
ikanode
I think in principle you're right. But in practice to get me started I'll probably do a timer and see what the capacity is after use and how often I need to charge them.
Suddy
When it says "protected" I think it means against overheating and is a safety mechanism. I'm more concerned with cell longevity.
Thanks to all for your suggestions and info!

I second EVM1024 and Avionics_Joe. To get 12 volts you'll need 4 x 18650 cells in series (3.7 Volts per cell nominal). The recommendation is to discharge each cell to no less than 3.0 volts. Which would mean a shutoff when your "battery" (of 4 x 18650 cells) reaches 12.0 volts. A fully charged "battery" should read about 14.8 V - no load. You may need to regulate the voltage if your fans have a problem above 12 volts.

[Epicurean]

I hope to use the computer box fans because they're quiet but I haven't found one "windy" enough yet. What I have are 0.3A. They seem to run fine on 4 18650's.
Aliexpress has a lot similar to what Avionics_Joe mentioned. But the batteries can't be changed and I've had problems with cheap 18650's from China. And I wonder how windy a 3.7 v fan can be. Maybe I'll switch to the truck fans with the metal grills that rust out. At least they put out breeze.