4 18650 cell in series with chargers

Hey everyone, I am trying to build a battery powered water valve, using a solenoid, which is 12V activated.
For powering it, I thought on using 4 18650 batteries connected in series and a step down to get always 12V. Now, the issue is on how can I recharge those 4 batteries...

My initial impulse (Primarily because I have tons of these) is using 4x 03962A modules (1pcs Micro USB 5V 1A 18650 TP4056 Lithium Battery Charger Module Charging Board With Protection Dual Functions 1A Li ion|Integrated Circuits| - AliExpress) and wire them in series, connecting each of them with one 18650 cell, so a basic diagram would look like this (Forgive me the bad quality drawing):

Now, does this sound reasonable? do you have a better suggestion on how to approach this?

Thank you all !

1 Like

You need a BMS Li-ion 18650 Battery Charger that is designed for at least 4 cells. It should cost $4-$10.

This is a wiring example.
That's what I would research.

You can make one from discrete components too, but this is so much easier.

For your idea you need 4 isolated 5V power supplies.

do you have a better suggestion

See post # 2 from @er_name_not_found.
Choose such module with balancing feature and select 18650 batteries of the same type and with the same capacity.

You could, instead, use a single cell and boost converter. Do you know how much current the solenoid draws at 12V? Do you know how long you need the solenoid to work on a charge?

Generally, a multi-cell charger uses a charging voltage of 4.2V per cell across all cells, and switch a resistor across cells that reach full charge early. A set of voltage dividers lets you check the voltage at each cell to get the voltage on each cell.

Wow... didn't know such modules existed... I presume P+ and P- are where my 5V source will connect to, How do I connect this to my 12v Step-down ?

Yeah, this was my first attempt, however doesn't seem to be enough... Using a 12V PSU I am measuring 0.47A

I don't know how much time I need the solenoid to be opened, I am trying to make an automatic watering system for some plants, so I am thinking 15 - 20 minutes a day

Depends on the model, but essentially this forms one big battery so P+/- are the input and output of the battery. Just think of them as the poles of a single battery cell. Check the specs of different models to find one with the correct inputs and output voltages.

Further, this steps up the voltage for you so all you should need is a 12V linear regulator with enough amps for the solenoid.

You guys rock. Thanks a lot!!

So figure half an amp at 12V which means 6 Watts. 20 minutes is 1/3 hour so about 2 Wh/day plus whatever the static draw of the electronics is.

For a single cell (3.7 V 2000 mAh = 7.4 Wh) that would give you maybe three days on a charge.

How long does the battery need to last between charges?

Well, I am planning on putting some solar panels to keep them charged, these are 5V 220ma cells. That's about 1.1Wh. If I step it up to 12V for the charger, I guess I should still be able to get 1Wh out of it, right? (with the loss due to the step up in the middle...)

So, 2 - 3 hrs of sun a day should be enough to keep them charged...

What do you think?

I doubt you need to worry about getting exactly 12V, a solenoid will work perfectly well with quite a tolerance of voltage, so don't bother with the converter to get exactly 12V.