Finally assembled the circuit!
Battery: ultramax slaumxnp7-12h
Relay: kacon k710-2pl
Charger: energizer enc4a-uk
Lights: sycamoreled sy7180nm/ww
V reg: ‎Keensoca6m9t4y0b
Now the testing process. Relay is warm but not hot. After that, see if the boss likes it. Wish me luck.

 

The circuit in post #101 will work quite well, but there are precautions. The first is that the charger voltage must be set to the battery specification for "float charge", and the second precaution is that the charger circuit must not draw current when connected to the battery but not powered.
I created a similar circuit for an elderly friend who used one of the electrically operated recliner chars that also raises up to help the person stand when they require it.
In our area we were suffering rather frequent mains power failures, never associated with fires, local or distant.
So after showing up a couple of times with my power inverter and a long extension cord, the battery backup in a box was created. EXcept my version used diodes instead of a relay.

 

Cheers MrBill, the charger has a maintain charge phase and stops charging after a certain capacity is reached.
No idea how much back-trickle will flow through the charger, but I've got 7AH to play with, powering only 4 2ish watt LEDs.

 

OK, in my application I had one series diode for isolation and several in series to drop the voltage for float charging. Unfortunately I did not get to see the long term results, 8 months later that old friend died, and relatives of his I did not know quickly cleared out his estate. Everything into a huge trash dumpster. Thousands of dollars worth into a landfill.

 

Sorry to hear that MrBill. Relatives, eh, who wants 'em?
Anyhoo, how were the diodes wired, and how did they perform voltage control? Can't say I've ever heard of that application.

 

Does anyone see a problem with running the relay coil for months at a time?

What does the manufacturer say? They should have run the tests, published a datasheet and stand behind what they say. That means, don't buy an unbranded relay from Amazon or eBay.

 

What does the manufacturer say? They should have run the tests, published a datasheet and stand behind what they say. That means, don't buy an unbranded relay from Amazon or eBay.

The main power control relay in most industrial manufacturing machines is on for at least 2 8 hour shifts every day,the demand is for 100% reliability. Never any problems .
But with relays yo do not get what you do not pay for..
A relay operated withing it's ratings will last. But it must be a relay rated fr continuous operation. A horn relay willfailwithin an hour.