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Solar charger for 2S LiPo. Protection circuit?

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ppTRN:
Hi everyone,
I am designing a circuit that needs to be supplied with 6V, so I am going for a dual LiPo cell (7.4V) and a buck converter using MC33063.
I would like for the batteires to be charger using a small solar panel, but I am struggling finding an adaguate protection circuit.

You can find attached the schematic that I intend to follow and I have a few questions.

1. Should I use a linear regulator? I ask this because I do not know how the buck will behave in absence of drained current when the battery is charged. Maybe I use it anyway with a resistor that drains a few mA? This should not be a problem since it does not waste battery but only solar power.

2. Is applying 7.4V directly to the battery the correct way of charging it?

3. The solar panel will output a current not greater than 500mA. Will the battery drain so much current that will cause a massive voltage drop on the panel? (I actually took a look to the typical V-A curves of a solar panel and the voltage is pretty stable up until the MPPT point)

4. I would love a protection IC just like DW01A, but for two cells. I found a solution online that uses one DW01A for each cell, but I do not really trust it.

5. Nothing bad should happend when the voltage delivered by the solar panel is below 8V, or low enough not to allow the regulator to properly wors, right? I also placed a diode to avoid any chance of current flowing from the batteries to the regulator (I will take into account its voltage drop when designing the regulator output voltage).

6. Is this whole solution to trow away because I am not taking into account cell unbalancement? I would buy a pre made 2 cell lipo, not connecting two in series myself.

this is my first time dealing with batteries, I did my reaserches but if you have any suggestions please feel free to share.

thank you very much!

tunk:
The 7.4V is the nominal voltage.
Fully charged they are 8.4-8.7V (see cell specs).
Fully discharged they are ~6.0V.

Edit: For a typical discharge behaviour, see e.g.:
https://lygte-info.dk/review/batteries2012/Molicel%20INR18650-P28A%202800mAh%20%28Gray%29%20UK.html

SiliconWizard:

--- Quote from: ppTRN on February 25, 2024, 05:58:28 pm ---2. Is applying 7.4V directly to the battery the correct way of charging it?

--- End quote ---

No. At least not until it has already reached close to this voltage.

Peabody:
You should use a proper charging IC that will go through the constant current, constant voltage, algorithm, and then terminate charging when the batteries are fully charged.  The TP5100 will do that for either 1S or 2S, but doesn't do balancing, and the TP5100 modules I've seen don't have any protection.  Anyway, I don't know what protection you would use for 2S.

An alternative might be to use a single battery, or two in parallel, with a regular TP4056/DW01 module, then add an MT3608 boost converter to get the 6V.

How much current will your circuit draw?  Will it sleep from time to time at much lower current?

Using a linear charger like the TP4056, the panel will provide as much current as it can at just above the battery voltage.  The charger won't draw the panel voltage below the battery voltage.  It can only draw current to charge the battery when the panel voltage is above the battery voltage.

The panel should probably be a 5V panel.  Maybe 6V.  But not higher.  With a linear charger, higher voltage just dissipates more heat when the sun is bright.  Actually, opinions vary on this.

sleemanj:
I would go the boost route with parallel cells, it's a lot simpler, you can just grab a protected TP4056 board, and a suitable boost converter, and that's it.

If you go series, then you'll need two protection circuits one for each cell, and either a specific 2S charger, or a CC/CV supply configured appropriatly for the charge requirements, and then add your own buck converter...


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