Timed and/or auto Switches
I did send some additional comments, but they are not shown on this thread. Maybe they went somewhere else!
I think the basics has been sorted out and explained to me sufficiently, of how to protect the different chargers and battery sets from interfering with each other.
The next concern is to install switches to sense mains power, which will then connect/disconnect the backup system from the rest of the circuit. To make it free from human or manual intervention, the switches also need to work via timers, which should be adjustable between 0 (or 1 !) to a max of 60 minutes.
My little knowledge of electronics tells me an electronic switch using a transistor or thyristor coupled with a 12V heavy current relay, could form the basis of such a switch. Sensing 220V AC, it switches the timer on (timer circuit powered by 12V DC and not 220V AC) , or the relay directly without any timer. Not sensing any mains power, timer will start its delay, whereafter it will switch off the relay contacts. The idea is to delay switching on the backup battery pack after mains failure, by 10 - 30 minutes, and also delay switching off the main battery backup, 30 - 60 minutes after mains power is restored.
Then the final safety measure will be a second 12V relay after the first one, without any timing. Its sole function is to protect the main backup battery from damage due to over discharging during a prolonged power failure, by disconnecting the mains backup battery from the 3 systems which it is backing up. Immediatiately when the battery voltage falls below a preset limit, it should switch off this second relay, and when the battery power is at a sufficiently high but not yet fully charged level, it should switch back on again.
Searching a bit on the internet, might provide me with suitable circuits, but will probably not tell me how to determine the values of each component. And it may also not be very suitable circuits for my requirements. So I need your experts to provide me with these 3 circuits with component values, please.
The 3 circuits will be a 1) mains sensing circuit including a relay working off 12V DC.
2) An adjustable timer circuit, 0 - 60 minutes, also powered from 12V DC, to go between the sensing circuit and relay in circuit 1 above.
3) A low and high voltage sensing circuit (for a 24V battery pack), to drive a 12V DC relay. The lower limit (I guess about 10,5 x 2 = 21V) switching off the relay contacts, and only switching it on again above a second upper limit (I guess at about 24V, which is still below theoretical max of 13,8 x 2 = 27,6V). Power to drive this circuit also 12V DC. This circuit should include 2 LED's to indicate whether the voltage is below or at the lower limit, or at or above the upper limit.
Our electronics shops will only re-open on 10 January due to the local holiday season, so I cannot even start with the diodes and resettable fuses now!