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I have a portable solar panel capable of delivering 21W at 5V. I would like to be able to charge my 12V LiFePO4 battery with this panel. My AC-powered charge controller is rated at 14.4V 4A which I assume is used for rapid charging (CC) before switching to trickle charging (according to the manual).

After boosting my 5V supply to 14.4V my current is obviously limited and not going to make it to 4A. My question is whether this low-current charging would hurt the cells.

Tangential to this, does anybody recommend a boost converter or charge controller for this task?

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  • \$\begingroup\$ There's no such thing as trickle charging with Lithium batteries. You start off with CC, then switch to CV and then stop. \$\endgroup\$ – Finbarr Apr 1 at 14:30
  • \$\begingroup\$ Okay, but can the constant current be a low current like 300mA or does it need to be several amps like these chargers provide? \$\endgroup\$ – 2mac Apr 1 at 15:07
  • \$\begingroup\$ It'll work, but it'll obviously take over 13 times as long as at 4A. Can you get enough hours of full sunlight? \$\endgroup\$ – Finbarr Apr 1 at 15:16
  • \$\begingroup\$ It needs voltage regulation and cutoff with cut-in control \$\endgroup\$ – Sunnyskyguy EE75 Apr 1 at 15:16
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Managing Li-Ion-based (and the particular LiFePO4) batteries requires some more sophistication than just "boosting to 14.4V". The charging requires CC stage, CV stage, and proper cut-off. While it is not exactly a rocket science, some electronics is required to do it properly, and portable solar-powered uninterruptible supplies have a good focus in power industry. Googling for [solar-powered UPS] gives about 25 million hits, and device cost is usually at $50 and way up, this is not a $5 project.

But theoretically you have a 60W OEM charger. I assume that your battery (which you prefer to hide from public) uses 0.5C charging rate, and therefore likely has 8AH capacity and gets charged in 2-3 hours. Your solar source has 20W in a good sunny day, or about 4A at 5V, so even at usual up-conversion practices it could deliver 1 - 1.2A of current at 14.4V. Which is not that bad, and can't be considered as "low-current". So your concern about hurting the battery is not well founded, but it will take 3 times longer, and the average daytime might be not enough to fully charge your battery.

Practically, if you have a limited solar source, the battery should be sized properly to utilize the daytime energy. More, using a solar source with fixed 5V converter is very suboptimal for 14.4 V output, so you better consider re-designing the solar panel to make a dedicated 4S batter charger. Or go to AliBaba/eBay/Amazon and get the whole professionally-designed controller for $39.

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Or go to upscale professionals for $550. Other options are available.

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