The main body of the UPS provides power management and voltage conversion only, relying on external power sources to supply between 7 and 32V DC. This gives me a lot more flexibility to expand the batteries as I salvage them from other UPS's, or do maintenance work on the power supplies.

Source 1 is a generic open frame switching power supply rated for 350W 24VDC, they are readily available online for $30 - $50. I've 3D printed a terminal cover to make it safe, and replaced the stock noisy fan with a quieter external one. The quality is not great so if it dies I have a spare, and due to the powerpole plugs I can swap it out quickly. The voltage has been adjusted up to approx 25.4V to better load share with the solar.

Source 2 is a Tracer 2215BN solar controller with 20A of MPPT goodness, fed from a 250W solar panel on my roof. The load terminals follow it's own battery bank voltages from approx 25 - 29V.

Source 3 is a bank of 12V batteries, maintained on a float charge. I'm currently using a spare car battery - I realise they are not designed for deep discharge but it was sitting around unused, and even at 30% DoD I can get around 2 hours runtime.

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The power flow though the system works like this:

-- Source 1 and 2 are connected to the primary path in the two load sharing controllers, and when their voltages are close internal resistances in the system allow the controllers to smoothly share power between them. Source 3 (the batteries) connects to the secondary path on both load sharing controllers - this provides twice the current capacity which is needed to provide the same power at half of the voltage. Whichever voltage is higher is then fed through to a central busbar.

-- The three main DC-DC converters then take their input from the main busbar and are adjusted to provide 12V output. I have wired up three banks of 12 x mini tamiya connectors as outputs, and they are ordered in terms of priority - cameras / security, home automation, and media / entertainment. It's important to note that the converters are buck/boost, so continue to supply stable 12V even when the emergency battery is down around 10.5V minimum.

-- An additional DC-DC converter (step down only) also takes it's feed from the main busbar and outputs 13.7V back to the emergency batteries in order to float charge them. For the time being it's current limited to 1A.

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Finally, an arduino monitors all of the source voltages, and measures the current consumed by each DC-DC controller. From there I calculate the power consumption and do coulomb counting to track remaining charge in the backup batteries. The DC-DC Converters also have enable pins, so if there is a power outage and the batteries start getting low, the arduino can do a staged shutdown of the output segments. The arduino also connects via a USB-TTL adapter to my central internet router, and periodically transmit a status string. The router dumps this string to a file and shares it with other PC's via HTTP, so that simple scripts can then be used for automated shutdowns.