Temperature-dependent voltage regulator for battery, cut-off switch, and state of charge reading?

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Joined Jun 10, 2020

I would like to use a ZJIVNV 180 W 12 V power adapter, which has a secondary connector for battery charge—13.8 V 1.5 A in this mode—and discharge, in conjunction with a Yuasa SW280 absorbed glass mat (AGM), valve-regulated lead-acid (VRLA) battery whose capacity is 7-9 Ah and whose float charge, according to the documentation, is 13.6 V. Please do recommend a better-suited, less environmentally toxic battery type, if applicable.

The purpose is to assemble an uninterruptible power supply for a small network-attached storage server based on a Pine64 ROCKPro64 single-board computer (SBC) whose power draw should be less than 50 W during normal operation and less than 100 W at maximum load—the battery itself should draw 13.8 V * 1.5 A ≈ 21 W in charge mode.

The identified challenge is that not only is there an excess float charge voltage of 0.2 V, but also the battery requires that it be decreased by 3 mV for every degree Celsius above 20° C due to greater reactivity (and, therefore, that it be increased by 3 mV for every degree Celsius below 20° C due to lower reactivity).

Furthermore, considering that, to my knowledge, the monthly self-discharge rate of a typical AGM VRLA battery is only 2% at 20° C (which is doubled for every 10° C temperature increase), and that limiting the float charge extends the lifespan of the battery, I believe I also need a kind of voltage-controlled switch, smart or otherwise, that cuts off charge when the battery state of charge is 99%-100% and resumes charge when the state of charge falls below, say, 95%. The same—or another—switch should cut off the load-driven discharge when the state of charge falls below, say, 40%.

It has come to my attention that, beside the inefficient linear regulators, a temperature-dependent voltage regulator could be of the switching kind—a less-than-ideal solution given that the UPS would be placed near a Wi-Fi router—or hybrid.

Lastly, it may be necessary that voltage and, therefore, battery state of charge reading be transmitted to the SBC when it is powered by the battery in discharge mode so that the SBC may safely and automatically shut off as the state of charge approaches the set lower limit.

This is the outline of the project. I have almost no knowledge in this area, therefore, guidance on the required or recommended study material in order to accomplish it as well as on the needed electronic components and design would be much appreciated.

Thank you.
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