I'm working on stuff in that market, it's still largely is. DC Power System Design For Telecommunications is still a must read and it doesn't even cover the last 15 years or so of development, notably lithium batteries and high efficiency rectifiers.

I will say that this is a surprisingly deep and complex domain. The amount of flexibility, variety and scalability you see in DC architectures is mind-boogling. They can span from a 3kW system that fits in 2U all the way to multiples of 100kWs that span entire buildings and be powered through any combination of grid, solar and/or gas.

Not to be _that_ guy, but it was technically -48V DC.

Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground.

This reminds me of the early google data centers that directly soldered those massive duracell lantern batteries directly to the motherboards as a primitive battery backup. I'm struggling to google examples of it, this would have been back around 2008, but i have a vivid memory of it.

edit: found it https://www.cnet.com/tech/tech-industry/google-uncloaks-once...

Yeah I always heard that the phone lines carried their own power, and in Florida the phones did keep working when the power went out, but I never knew why.

So the grid was always charging up the lead acid batteries, and the phone lines were always draining them? Or was there some kind of power switching going on where when the grid was available the batteries would just get "topped off" occasionally and were only drained when the power went out?

Interesting, so this is why the phone line still worked when power was out across the whole town.

You need thick cables if you want to power a rack with 48V.

-48V! :-)

I still have a bunch of 48vdc comms gear in my powerplant.