I'm building a system that is physically located far from an outlet and can be periodically disconnected from external power supply.
So, I took the naive approach to put a 12V lead acid battery and plug it in parallel to Arduino Mega with Ethernet shield while AC charger is connected by a 30m wire. I know that I'm shortening the battery life due to bad cycle for such battery types, but I don't care here.
This approach didn't work at all, Arduino (or W5100) somehow manages to drive voltage down to 7.5 volts and completely to discharge the battery instead of maintaining charge.
I tried different types of power supplies, from transformer based to computer PSU with voltage ranging 12-14V -- nothing worked for me.
I only can use one pair of wires and I'm pretty much despair about the solution to that. Right now I'm trying to use car charger and power the whole thing from USB; seems to work but you know, it's ugly and unreliable. And I don't know how Ethernet behaves with that low voltage.
TL;DR. I'm certainly doing something wrong and missing something. What's the correct way to split single 12V power supply to charge a lead acid battery and powering Arduino simultaneously without any interference between them?
Thanks a lot!