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My application is primarily portable ham radio operation. I chose a self-balancing, 4.5 Ah, 12.8V, 4s3p LiFePO4 battery pack, a 28W panel and an MPPT solar charge controller with extremely limited knowledge of batteries and solar charging. My choices were based on power requirements (of course), my desire for MPPT and my need for small, lightweight, highly portable components (my controller weighs only a couple of ounces). I had hoped that in sunlight, the controller would distribute power between the battery and the load. However three factors make this impossible:

  1. The battery pack's on-board PCM disconnects the battery when charging rate reaches 0.02C.
  2. The charge controller passes panel Voc to the battery and load terminals when no battery is present.
  3. My load is not rated to handle panel Voc.

As a work around, I keep the load isolated from panel Voc via a DPDT switch with which I manually connect either the panel or the load to the controller. So, instead of a system in which the controller optimally distributes power, I'm forced to alternately charge and and discharge the battery.

I recently conversed with someone who has been using a DIY LiFePO4 battery pack successfully with my controller model, with none of the problems that I described above. It seems to me that the difference between his setup and mine must be that his battery's PCM doesn't disconnect near the end of CC charging.

My questions are:

  1. Can someone recommend a commercially available, self-balancing LiFePO4 4.5 to 6 Ah battery pack whose PCM has no low charge rate threshold?
  2. Can someone recommend an MPPT charge controller that has load terminals, is super small and lightweight, and that handles a disconnected battery gracefully?
  3. I'm considering building a 4s2p battery pack from 26650, 3.2V, 3300 mAh, 2C LFP cells (1C max charge rate, 2C max discharge rate). Can someone recommend a PCM board that would meet my needs?
  4. If I were to stay with my current controller and battery, would it make sense to place a voltage regulator or some other circuit between the controller and the load to protect the load when the battery disconnects? If so, it should ideally consume little power and only drop the voltage when it exceeds 14.6V. (I'm out of my league with this question.)
  5. What is the likely reason for the battery's disconnect at 0.02C? Wouldn't it make more sense to remain connected and allow the controller to go into CV mode at it's programmed CV voltage of 14.6V?
  6. I realize that I haven't provided much detail and that some of my terminology and concepts might be wrong. What additional information would make my quesions meaningful and answerable?
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    \$\begingroup\$ LiFePO4 is not designed to stay connected, you battery pack will last much longer than your mate's. You either need to design an intelligent switching circuit to do what you want automatically, or change to lead acid, which chemistry is great for floating across a load while charging to a speciifc voltage. \$\endgroup\$ – Neil_UK Oct 27 '17 at 6:23
  • \$\begingroup\$ Thanks for your reply. The battery manufacturer sells an MPPT controller that permits floating, but it's too big and heavy for backpacking. Also, I recently saw a PWRgate unit for ham radio that uses AC power with LFP battery backup and a built-in solar controller; it allows a fully charged LFP battery to remain on standby indefinitely. Do these units reduce battery life? Similarly, would leaving a CC/CV charger connected to an LFP battery in CV mode for an extended period damage the battery? \$\endgroup\$ – dcorsello Oct 27 '17 at 13:34
  • \$\begingroup\$ LiFePO4 is more robust to overcharge than other lithium types, but still benefits from being disconnected at full charge. The 4s configuration is mainly aimed at automotive lead acid replacement, where the 14.4v float charge is tolerable for LiFePO4, assuming the car is not driven 24/7, that it gets decent rest between 14.4v floats. 14.6v sounds excessive. If a solar charger delivers 14.4v all day, every day, it will degrade the battery faster than a charge regime that disconnects properly at full charge. \$\endgroup\$ – Neil_UK Oct 27 '17 at 15:01
  • \$\begingroup\$ 14.6V is the battery manufacturer's CV spec. \$\endgroup\$ – dcorsello Oct 27 '17 at 16:09

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