Prevent batteries going below 20% charge

ZoomieFan · 2025-09-30T10:50:09.937Z

I hope I’m posting this in the right section.

I’ve a heated vest. 12v 41Watt. It can be powered by a 12v outlet or with a kinda expensive $120 battery. DIY battery and box would cost far less.
That is if I simply connect it, but then the batteries get drained down to zero.

So my question is, does anyone know of a simple protection circuit that stops draining my 21700’s below 20%?

chops728 · 2025-09-30T11:24:54.050Z

A 3S BMS for Li-ion cells. you have to know what your amp draw is .
https://www.amazon.com/Lithium-Battery-Charger-Module-All/dp/B0FLD71HJT/ref=sr_1_1_sspa?crid=17YMKQ33GR9PL&dib=eyJ2IjoiMSJ9.Wvk7T15JMKx9LnPb2CzBKKyRL07iwZ326pr2WaltVwFeKPloITBaodowrbhYr0j_OxbV75cWLz3ZgUV95D5pLIALQWjYvgZJoNafHdJlDSFVbBfemmJZOGCZ13KEKoIPb06474mmlLp6_qnOrFH0HEr1aG1oxdzjPyBg5o1vGinN21hgC0NTuYxpXCWd06iQ_NVyXl8OrbVgTxwgBnjyFXTl07dWdIy-zr5YnMuFV30.ZwKJkioeJvju19-pGSgoyr6af6QcDkzfbMHgGVc-pb0&dib_tag=se&keywords=3s+bms+20a&qid=1759231451&sprefix=3s+bms%2Caps%2C222&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

Qubeet · 2025-09-30T11:31:32.659Z

chops728:

what your amp draw is

41 watt, so 3.41 amps.
With 12.6V it must be 3.52A, still way below BMS’ limit.

ZoomieFan · 2025-09-30T12:04:53.206Z

Thanks for the quick replies. Looks (almost) what I’m looking for.

Been looking at specs of similar BMS and am wondering what the tolerance of the input voltage is.
4x3.6= 14.4 volt
That’s fine for the jacket because 14.4v is quite common on motor cycle poweroutlet the jacket is designed for.

But what about the BMS?
I found simalar BMSses and when they have input voltage info they mention a narrow range with 12.6 being the higher limit.

Qubeet · 2025-09-30T12:16:58.137Z

3.6 (or 3.7) is a nominal voltage of Li-Ion battery (specifically NMC). Actual voltage changes between 2.5 and 4.2 volts (0% - 100%). So 4s pack will have 16.8 volts which is too much. You should build a 3s pack.

Since the jacket works fine with an automotive voltage range you can look at LiFePO4 batteries. They have a nominal voltage 3.2V and actual range 2.3V-3.65V. So 4s pack will have 9V-14.6V.
LFP batteries require specific BMSes and chargers.

ZoomieFan · 2025-09-30T13:02:28.193Z

Or a 14.4v BMS?

nkon.nl/novat/4s-bms-4mos-ebd02-ab.html

Sklep ABC-RC

BMS 4S 14.8V module with balancer for charging and protecting Li-Ion cells -...

4S electronic system working as a battery protection against over-discharge. This module also acts as a charger for these batteries.

chops728 · 2025-09-30T15:33:51.114Z

The question is, will your vest operate on the higher voltage. 4 cells Li-ion moderate charge over 16v
A 3s / li-ion full charge 12.6v , will the vest run at the lower voltage 9v when almost totally discharged.
Yes LiPo4 cells match up to 12v way better, but they are twice as heavy, and capacity per size is usually half. If you go with Li-ion make sure you use quality everything, don’t want a battery incident with a vest strap on you .

Wingman · 2025-09-30T15:50:05.563Z

chops728:

Yes LiPo4 cells match up to 12v way better, but they are twice as heavy, and capacity per size is usually half.

The limited energy density of the LiFePO4 cells is indeed a shame, making them relatively heavy compared to their capacity.

For this reason it’s a shame that there aren’t many LiMnFePO4 (with the Mn being Manganese), available yet, since they offer (for the biggest part) the advantages of LiFePO4 batteries (safer and higher cycle life than Li-ion), but also with a higher energy density (+20%) than LiFePO4.

LiMnFePO4 generally have nominal voltage of ~3,75V and a fully charged voltage of ~4,20V or up to 4,35V, depending on manufacturers specs. This higher nominal voltage is also the main reason for the higher energy density.

But again: limited availability and I’m not sure if a Li-ion BMS would work for this (given the closely matching voltages) and it might be difficult to get a dedicated LiMnFePO4 BMS at this point. (And obviously they’re still not matching the capacity/energy density of ‘regular’ Li-ion cells.)

ZoomieFan · 2025-10-01T06:34:28.234Z

I must admit I only know the very basics about BMS, and the specs I find are a bit confusing.
The way I understand the specs on Amazon, Ali and scattered over the internet is that the BMS has a rather tight input range, but outputs a stable voltage?

The vest is rated at 41watt, but I don’t know at which voltage. A car/motorcycle battery is 12v, but in reality there’s often a hight voltage on the cigaret lighter.
So is the vest 41 watt at 14v or 12v?
A voltage drop from 14 to 9 will have a serious effect on the claimed 41w of the vest.

I’ve read the replies about 3 vs 4 cells and the type of cells, but now I think that if the output voltage can’t be kept fairly stable, the whole idea is flawed.
Then I would need something that maintains a fairly stable output voltage.

chops728 · 2025-10-01T11:14:16.486Z

A BMS doesn’t ensure a staple voltage output. It just protects the cells from overcharge and over discharge. The only way to get stable voltage output would be to go 4S and use some sort of Buck converter back down to 12v ( head spinning yet ) Most of these devices as the voltage drops the amperage will raise to keep the wattage output close to the same. The only problem I see with the lower end of battery voltage is, will the vest keep you warm enough. All this stuff takes trial and error.

To me if you are not familiar with this stuff, you might want to buy a prebuilt battery pack ready to go. There’s plenty cheaper ones to choose from

ZoomieFan · 2025-10-01T15:06:01.579Z

(post deleted by author)

ZoomieFan · 2025-10-01T15:18:50.315Z

I Googled for a buck convertor.
https://www.tinytronics.nl/en/power/voltage-converters/buck-(step-down)-converters/dc-dc-ajustable-step-down-buck-converter-xl4016-200w
https://www.tinytronics.nl/en/power/voltage-converters/buck-(step-down)-converters/qskj-dc-dc-adjustable-step-down-buck-converter-xl4005-5a

I like the first one. Some overkill for durability.
7 cells fit nicely in the min and max input range (empty and full cells.

The BMS: https://www.tinytronics.nl/en/power/bms-and-chargers/bms/daly-bms-li-ion-li-po-protection-circuit-7s-15a-with-balancing
Connect the cells to a BMS and the BMS to the buck convertor should work, right?