Li-on 18650 4V simply 100mA balancer based on Zener diode ?

[szan]

Hello,
What do you think about such simply Li-on 18650 4V balancer made of Zener diode
in current limited charger to something like around 100mA by using two NPN transistors
powered from 5V in this circuit simulation ?
I'd like to use 10W PV solar panel as power source.
Any advantages of using eg. LM317 as current limit instead of those NPN transistors, which can be sometimes better since,
I can choose them rated with higher collector voltage than fixed LM317 input voltage maximum ?
What you think about such li-on cell balancer in this configuration with charge current limited to aound 100mA only?
It looks good in circuit simulation...

Regards.

[szan]

It time to test real circuit during balancing based on 1 Ohm 1% resistor and Zener diode started at ~4.1V 18650 Li-on Panasonic:



Everything looks as expected in circuit simulation - power losses in 1W rated Zener diode are around 0.3W

Sat 20 Nov 2021 23:19:42 CET  3.99Vz @ 72mA  0.283W  4.062V
...
Sun 21 Nov 2021 00:20:07 CET  After 1 hour... not too much changed - discharge current is around 71mA ...
...
Sun 21 Nov 2021 01:33:48 CET  3.96Vz @ 67mA  0.265W  4.027V
Sun 21 Nov 2021 08:30:00 CET  3.86Vz @ 49mA  3.91V
Sun 21 Nov 2021 09:30:00 CET  3.85Vz @ 47mA  3.90V
...
Sun 21 Nov 2021 13:40:00 CET  3.80Vz @ 40mA  3.84V
Sun 21 Nov 2021 17:53:00 CET  3.77Vz @ 37mA  3.81V
Sun 21 Nov 2021 18:49:00 CET  3.74Vz @ 35mA  3.80V
...
Sun 21 Nov 2021 19:40:00 CET  3.746Vz @ 34mA  3.78V
Sun 21 Nov 2021 20:28:00 CET  3.74Vz @ 35mA  3.77V
...
Sun 21 Nov 2021 21:34:00 CET  3.59Vz @ 31mA  3.76V

24h:
Sun 21 Nov 2021 23:31:00 CET  3.72Vz @ 30mA  3.75V

48h:
Mon 22 Nov 2021 23:12:12 CET  3.56Vz @ 21mA 3.58V
ed 24 Nov 2021 16:55:00 CET  3.39Vz @ 13mA  3.40Vb
Thu 25 Nov 2021 18:49:00 CET  3.29Vz @ 10mA  3.30Vb

End of test day 5:
Fri 26 Nov 2021 16:28:00 CET  2.99Vz @  4mA  3.00Vb

[szan]

Let us see battery voltage when balance current drops below 10mA to around 1mA ... it should be around 4V

Nope, unfortunately experimental data follows BZX85C?V?  Vz vs current  datasheet figures, which shows 20mA current @ ~3Vz 



Those Vz va I figures also confirm measured ~4Vz @ 80mA which is nice it fits with BZX85C?V? datasheet, but it is not Vz we were looking for at low balancing currents 

We need much sharper turn on/off so maybe TL431 will do the job better? 

[Siwastaja]

Yeah, can't do that with a zener, need something much sharper.

TL431-based design is fine.

[szan]

TL431-based design is fine.

Let this circuit run for 24h since only 2h left
...
Sun 21 Nov 2021 21:34:00 CET  3.59Vz @ 31mA  3.76V  22h
...
UPDATE:

24h: From 18650 battery voltage: 3.99V -> 3.75V
Sun 21 Nov 2021 23:31:00 CET  3.72Vz @ 30mA  3.75V

-..
However, ~3.8V single cell battery voltage after close to 24h small balancing current discharge is not so bad - bad news is it will probably discharge to slightly above 2V when we keep this circuit running a few days more...
 
I was looking for something dead cheap, but maybe it will be cheaper buy in higher volume more sophisticated IC to put on top of Li-on 18650 battery as separator.

[szan]

Let this circuit run for 24h since only 2h left
...
Sun 21 Nov 2021 21:34:00 CET  3.59Vz @ 31mA  3.76V  22h

We can wait more - how long it will take to discharge to those 3V, then close to 2.5V 

[szan]

Yeah, can't do that with a zener, need something much sharper.

Idea of using Zener diode and resistor as balancer looks not so bad when we take into account
that we can charge a long string of connected 18650 Li-on cells in series to a voltage determined just by adjusting charge current up to around 60mA so this is cut-off current @ 25*C for those battery cells.

Below are stats from 2 days discharging and it is not so bad that after such long time we still have usable ~3.6V charged battery discharged at a rat eof small LED 20mA 

00h:
Sat 20 Nov 2021 23:19:42 CET  3.99Vz @ 72mA  0.283W  4.062V
24h:
Sun 21 Nov 2021 23:31:00 CET  3.72Vz @ 30mA  3.75V
48h:
Mon 22 Nov 2021 23:12:12 CET  3.56Vz @ 21mA  3.58V

In reality, depending on application this battery pack can be charged during every day by free energy comming from solar PV panels and those small discharge currents can be used for example to keep this battery pack above 0*C temperatures at cold winter days 

It could be nice to have some kind of very cheap MPU with integrated temperature sensor to turn off/on this balancer/heater circuit based on battery temperature and its voltage.

My copper pipe based powerbank looks like this and it designed to fit into bike saddle aluminium pipe 


 

[thm_w]

Hey similar ideas here, I use 3x18650's stuck into my seatpost, I like the copper pipe though. 
I would recommend just buying a simple 3S balance BMS from aliexpress, understanding how it works, and testing it out. Maybe modifying it if it does not do exactly what you want. You can also get the 1S BMS's that strap directly across a cell, but the 3S *should* be able to shut off the whole pack if one cell is low.

Although, it looks like you are using 6S+? Not sure if they have any in the slim form factor, I only see a 5S balance BMS thats narrow width enough to fit beside a cell. Also may depend on your seatpost diameter, I have 27 and 31mm OD.

[szan]

I use 3x18650's stuck into my seatpost, I like the copper pipe though. 

I have also 3S Li-on 18650 cells spot welded to create 3x4V~12V power bank  and I'm able easy to fit into 25cm long copper pipe



BTW: It looks like (cheap 6 pin) Attiny10 is small enough to fit together on small PCB inside 3D printed  ABS cell separators with: Zener diode, 2 x resistors and NPN transistor to create such more sophisticated Li-on cell balancer with something like constant current
Power consumption of Attiny10 in IDLE mode is very low and we should be able sense MPU Vcc (battery) voltage  to turn on/off balancer circuit by using small NPN transistor   

 

[thm_w]

It will fit, and could be fun to do, although for me I rather spend the $2: https://www.aliexpress.com/item/32827726118.html and use the attiny for flashing LEDs or turn signal.

In reality it won't need to balance too much, especially if you start with closely matched cells. Do you have a charger that can tell you the mAh rating of the cell?

[szan]

Although, it looks like you are using 6S+?

Yep,
I've already thanks to EEVBlog forum found quite nice 30V N-channel mosfet:
AO3400A
with  RDS(ON) (at VGS = 2.5V) < 48mOhm   

Attiny10 will drive already tested balancer made of: Zener diode + resistor when needed based on 18650 battery voltage - all I needed was low VGS n-channel mosfet.
We can put this really tiny MPU in sleep/idle mode and setup watchdog to do the job and turn on/off balancer.

BTW: I'm seriously thinking about something like 1-wire interface in the future with custom protocol to output from each such 18650 cells its measured voltage and temperature of this copper pipe for additional thermal protection - charged could then decide what to do based on stats of all battery pack cells ...
 
Lets try design tiny PCB 18mm in diameter since we want fit everything into those copper pipes 

[szan]

In reality it won't need to balance too much, especially if you start with closely matched cells.

Maybe, but I'd like to keep those 18650 Li-on cells at voltages ~3.9Vbat to extend its lifespan,
so when we have switchable balancer we can easy keep each cell close to such voltages
while cells are connected in series.

[szan]

End of test day 5:
Fri 26 Nov 2021 16:28:00 CET  2.99Vz @  4mA  3.00Vb

So we finished discharge test with Panasonic Li-on 18650 battery voltage ~3.00V,
while Zenner voltage was about 2.99Vz and discharge current dropped to as low as ~4mA range.

It is not so bad assuming that we had very simple balancer made of zenner diode and 1 Ohm rezistor in series on battery cell and it can be usable in some cases.

4V charged 18650 battery was discharged within 5days to ~3V.

Normal use case can be that we recharge this battery pack at least 2-3 times a week, so it might work even without MPU based controll with additional N-channel mosfet 

[szan]

Now it is time to charge string of 18650 Panasonic Li-on batteries by using solar PV free energy 20W  0.465m x 0.350 m with additional controller dedicated to those solar panels: Input voltage up to 48V output 12V or 24V  10Amax, so  we can install few such pannels and combine even more input solar PV power - eg.: three such panels in parallel at different orientations to expected sun position to be able get more input direct light power than in the case we had only one solar PV panel exposed to lets say highest position on the sky around 11:00 AM - 13:00 PM