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I'm currently trying to design an Uninterruptible Power Supply for my microcontroller (Raspberry Pi 3 Model B). I've read from the documentation that the microcontroller requires 5V and the amperage drawn by it will be dependent on the peripherals I use.

Under normal conditions the microcontroller will be powered with the 5V microUSB that it comes with but in the event of a power outage or the cord getting pulled out I want to use rechargeable Lithium-Ion Batteries as a backup power supply. I'd also like the circuit to be capable of charging the rechargeable lithium-ion batteries, with a way of preventing overcharging to the lithium-ion batteries.

I also want my microcontroller to be portable so that way I can use it outside. So I wanted to use a solar panel to provide power to the circuit, but in times where the solar panel is not providing enough power I wanted the rechargeable lithium-Ion batteries to supply the necessary power to the microcontroller. I'd also like the solar panel to recharge the lithium-ion batteries when the solar panel has excess power to give out that isn't necessary to the microcontroller. Again, the circuit needs to be capable of preventing overcharging to the lithium-ion batteries.

I've looked around and seen some others doing something similar but I haven't found anything that gives me a concrete explanation or schematic that I understand.

My attempt: (Note the 5V is the microUSB or Solar Panel and the 4V supply is the rechargeable lithium-ion batteries.) I understand the values aren't correct and that a DC-DC Boost converter might be required... I also need something in place to make sure that when the batteries are fully charged that reverse polarity does not occur, correct?

Any help is appreciated!

This is when the 5VmicroUSB or the Solar panel is connected, notice it is 'charging' the Rechargeable Lithium-Ion Batteries (Idk how to model rechargeable lithium-ion batteries )

This is when the 5V microUSB is disconnect, and then the lithium-ion batteries immediately start providing power to the microcontroller

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  • \$\begingroup\$ The Raspberry Pi 3 can't operate at 4V. Minimum specified supply voltage is 4.75. Undervoltage lock-out tends to happen at 4.5-4.6V. \$\endgroup\$ – Enric Blanco Mar 26 '17 at 22:32
  • \$\begingroup\$ Yea I knew the values were wrong, I was just trying to provide an idea of what I was trying to do. But knowing the minimum supply voltage is useful, thank you! Any other advice is appreciated! \$\endgroup\$ – HappyTurtle Mar 26 '17 at 22:52
  • \$\begingroup\$ Look for "power management ICs" and/or "battery charger ICs" at the websites of the usual suspects (TI, Linear, Maxim, Microchip, etc.). They have some solutions that could match your needs. \$\endgroup\$ – Enric Blanco Mar 26 '17 at 23:20
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Your attempt doesn't even begin to meet the requirements of each component in the system, which are:-

  1. Raspberry Pi model B: Regulated 5V DC supply at up to 1.7 Amps.

  2. Lithium-ion cell: maximum voltage 4.20V. Maximum charge current ? (depends on battery capacity and 'C' rating). Minimum voltage under load 3.0V.

  3. Solar panel: efficient transfer of energy to battery and/or load (unless you don't mind using an oversized panel).

To run the Pi off a single Li-ion cell you absolutely need a voltage booster. This booster must supply up to 1.7A at 5V, so it may draw up to ~3.3A from the battery (assuming 85% converter efficiency).

To run the Pi off the solar panel without draining the battery, the panel must supply up to 10W to the converter. In practice this means you probably want at least a 20W panel.

A voltage booster can only increase (not decrease) voltage, so if the panel puts out more than 5v you need a buck converter to drop it down to 5V or less. To charge the battery you must drop the panel voltage to 4.2V and apply current limiting.

But solar panels automatically limit current according to light intensity, so your circuit also has to work properly when the panel cannot supply the set charge current. When this happens you need to throttle the charging current back to prevent the panel voltage from dropping below its maximum power point. Converters that do this are called MPPT (Maximum Power Point Tracking).

To avoid damaging the battery you need some way to shut it off before becoming completely drained, and probably some warning so the Pi can be shut down while it still has power. When switched off your circuit must draw negligible current from the battery.

Finally, you want sufficient battery capacity to run the Pi for a reasonable period of time. The battery capacity you chose will effect the maximum charging current and how it can be applied.

If the battery can take more current than the Pi draws then you could just hook the charger and booster straight to it without steering diodes. Then the battery will 'float' at up to 4.2V, while any excess current is absorbed by the booster (if it needs it). If the battery can only handle low charge current then it must be isolated from the output while being charged, and you need a bypass to route excess panel current to the booster. Your attempt embodies the basic concept of this configuration, but does nothing to address the other requirements.

Bottom line is you cannot create a practical solar-powered UPS from two diodes and a resistor!

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