18V Solar Panel : Best way to charge 3.7V li-ion cells ?

WhiteEagle

New Member
Joined
Jan 31, 2022
Messages
7
Hi all,

first message here, thank you a ton for sharing your precious knowledge.

I have a little 150W 18V solar panel (foldable Dokio), and I want to use it to charge lithium-ion 3.7V cells (mostly 18650) during daylight. I am wondering what would be the best way to do so, with good efficiency, and as little hardware as possible.

The most naive way would be to do so :
PV Panel -> PV Charge Controller -> 12V Buffer Battery -> Li-Ion Charger -> Li-Ion Cells

However, this arrangement would require a 12V Buffer Battery, and 2 charge controllers around.
If it is technically doable, I would like to stick to as few parts as possible. Again, my main goal is to recharge 18650 Li-Cells from the PV panel during daylight, and this intermediate 12V buffer battery feels redundant.

I already own good a quality 4-cells Li-Ion charger, and the PV Charge Controller that is included in the Dokio package (probably a cheap chinese PWM based current limiter). The manual of this Charge Controllers says something like "Don't connect anything to the 12V output before connecting the 12V buffer battery". Well ...

I could certainly use a 3S arrangement of 18650 Li-Ion cells as the 12V buffer battery, and the charge controller would charge them. I would also use, further, a Li-Ion cells charger to charge more cells.

But in this case, if I understand correctly, the 12V PV Charge Controller will need to charge the 3S cells directly, as a 12V buffer. There is no way it could charge them through a BMS so that cells remains balanced and in-spec. Therefore, I am concerned that the cells allocated to the 12V buffer battery would not be easy to manage and swap to be used in my tools, lamps etc.

I could also put a dedicated 12V buffer battery (Lead/Acid or LiFePo or LiIon), dedicated because the cells used there would not be extracted to be used in tools. Again, since I just want to charge my 18650 cells during daylight, this intermediate redundant battery feels suboptimal for my use case.

In your opinion, what would be the smartest way to charge my Li-Ion cells during daylight in the most direct and efficient way, with as little hardware as possible.

Thank you all
 
Last edited:

WhiteEagle

New Member
Joined
Jan 31, 2022
Messages
7
For reference, in case someone find this thread while searching for Dokio Charge Controller tips, here is an interesting finding.
This charge controller drains my phone battery when plugged into the controller USB port, even with the panel in daylight.
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,497
This is a unfinished project so maybe you can tell me what’s wrong, but I thought all you needed to charge LiIon 18650s was a:

Solar Panel—>SCC (set to Lion voltage specs)—>BMS/18650 cells

The BMS for 18650s are extremely cheap.

The setup I planned is no different than a normal solar charger except the voltage set to 18650 specs. In your case you’d get a 3S BMS. In my case its an unfinished project for a 2S2P 18650 battery charger. My thoughts on this had been a 100 watt panel and my Victron 15 amp SCC which can limit amp output in 1 amp increments. Although I have not seen the specs on a 18650, I suspect I would limit charging output to 1-3 amps.

My panels tends to output between 18 and 20 volts, and for the Victron SCC, its only a matter if it will reach 5 volts over the charging voltage to activate the SCC. I’ve not had a problem with lead or lithium phosphate 12 volt batteries, but never went so far as to get the specs for a 18650 cell.
 
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