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Forum Index : Solar : 12V Solar charger MPPT for 18650 (2s, 4p) with 3.3V and 5V output

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LRAT
Newbie

Joined: 15/06/2021
Location: Australia
Posts: 4
Posted: 06:41am 21 Sep 2023
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Howdy,

I am replacing my current weather station with something smaller.
The brain of the weather station is going to be an ESP32 as I want to use the WiFi capabilities.
The project will be charged by a 10W, 12V solar panel. I live in Melbourne and we can have many days of overcast conditions, so, I'll need redundancy in battery capacity.
I did the sums and I probably need 4x 18650 Li-ion cells in parallel. But because I also need a stable 5V output for my sensors I probably need to double the batteries so that I can put them in series to achieve 8.4V (If that makes sense).
I will also need protection against over- and under-voltage.
Anyway, I did my research and I think the following is going to be my best option:

The 12V solar panel will be connected to a CN3795 MPPT Li-ion charge control module.
I will put a Schottky diode to prevent drainage overnight.
I then will connect two Mini360 switching power regulators. One will be set to 3.3V and the other to 5V with common ground

I am not an electronics specialist and I would like to receive some constructive feedback if my thinking process is in the right direction.
I've been looking for off-the-shelf MPPT chargers but I can't find anything for multiple 18650 cells in parallel. Most chargers are for lead-acid battery technology but I want to switch to Li-ion batteries as they are common, cheap and long-lasting (When treated correctly).

Also, I've been told that it's possible to buy 18650 Li-ion batteries with inbuilt BMS; Is that correct? If yes, where do I find these? I don't want to skimp on quality and reliability so brands like Samsung or Panasonic would be the best.

Anyway, I hope somebody can give me a nudge in the right direction. Many thanks in advance for your help/advice!
Cheers,

Luke
 
Revlac

Guru

Joined: 31/12/2016
Location: Australia
Posts: 881
Posted: 08:46am 22 Sep 2023
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Hi Luke, Welcome to the Forum.

You can parallel 2 or more 18650 cells, you still only need to balance it as one cell, Actually all it needs in over and under voltage protection.

If you want 2 in series, there is 2S balances available, 2S2P should be fine.
I don't know of any 18650's with built in BMS, its unlikely it would fit in the 65mm size constraint.
The CN3795 is a neat little unit.
I'm not an Expert either.
Cheers Aaron
Off The Grid
 
LRAT
Newbie

Joined: 15/06/2021
Location: Australia
Posts: 4
Posted: 04:51am 24 Sep 2023
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Thanks Aaron.
Yes, I was also surprised to hear about 18650 with inbuild BMS.
I've never seen anything like it either.
It looks like that I will look at building a BMS for each 2S2P battery set.
Good to see your endorsement for the CN3795. I will give it a try.
Thanks for your response!
Much appreciated!
Luke
 
pd--
Senior Member

Joined: 11/12/2020
Location: Australia
Posts: 118
Posted: 05:13am 24 Sep 2023
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You can get protected cells that have a over/under voltage cutout built into the  cell.
https://www.aubattery.com.au/product/panasonic-18650b-3400mah-protected-li-ion-battery/

in the base of the cell is a pcb with a monitoring IC and fet's
it will disconnect the cell if under or over voltage

There designed for applications that have no builtin smarts ie torch.
 
Revlac

Guru

Joined: 31/12/2016
Location: Australia
Posts: 881
Posted: 05:18am 24 Sep 2023
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First I have seen, how well will it work in series?
Cheers Aaron
Off The Grid
 
LRAT
Newbie

Joined: 15/06/2021
Location: Australia
Posts: 4
Posted: 05:19am 24 Sep 2023
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Hi pd,
That's a great recommendation!
And the price is not bad either.
Thanks for the link and info!
 
pd--
Senior Member

Joined: 11/12/2020
Location: Australia
Posts: 118
Posted: 05:56am 24 Sep 2023
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  Quote   how well will it work in series?

I have one torch that takes 2 in series.
it just turns off when one cell goes lo voltage
and its the cell that is turning off as the torch just gets dimmer & dimmer with other cells

I have 6 cells that i have been using in dumb devices for ~3 years now
they just work.
 
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