Playing with a 4 way relay board to make a 'Smart' 4 way trailing
socket the other day and also getting a mobility scooter in the back
garden hooked up to a charger indoors, got me thinking about all the
12v batteries I have and the need to keep them all topped up.

You can buy an 8 way CPCO relay board that will probably carry the 12V
5A you might see on yer average car charger and with that connected
up to the 'commons' of all the relays could be remotely switched at
whatever interval was appropriate and even be switched
'interactively'.

Eg, imagine you have 6 batteries hooked up randomly across the 7
outputs (one would be reserved to connect the charger to the group)
and you want them all to be charged till the charger says it's done,
once a month ...

You activate the first relay and read the voltage across the 'common'
(and -ve) to see if a battery is connected or not, then if it is,
power up the charger and wait till the opto sensor you have taped to
the 'charged' LED, tells the system that it's done, to disconnect the
charger and to move onto the next output.

Now I've done most of all the above already, the opto sensor
monitoring the electricity meter, the relay board for the trailing
socket and the reading of various sensors (temperature / humidity /
light) to trigger various events, it would just be the pulling it all
together in Home Assistant might need so doing (for me, a hardware /
electronics guy, not a coder). ;-)

I've actually already got the functionality of the above but only 4
way, and not interactive re charge (just battery presence and time per
output) but this could be far more flexible and you could keep an eye
on the whole lot remotely (and monitor things like the voltage /
charge curves / times historically).

Cheers, T i m

p.s. Whilst each output would be fused in the event of a misconnection
or two relays being pulled or stuck in at once, there is an
'interlock' feature in the relay driver software that should inhibit
more than one of the output relays being pulled in at a time).

p.p.s. You would probably need a decent 'Smart' charger that was very
well protected (against reverse connections or extreme conditions,
like short battery) plus not requiring any buttons to be pressed to
start the process going. Something like an Optimate 2 should do it.