12v Regulator, charging circuit

[tenmen10]

I have attached a 12v charging socket that I found on youtube - I am having problems with 12v lead acid batterys, over a period of trickle charging they are degrading and will not hold a charge.

The trickle charger is a 12v 500ma and the powercells range from 12v7ah - 12v22ah

Firstly I want to fuse the unit to stop overload - I assumed this circuit would do it for me with the cut off, what I cant understand is the use of Diodes. With 12v Battery I assume Im look to charge to about 12.4 what Diodes would I need, also how does the circuit actually work - does it know when the Battery reaches 12.4 the cutoff kicks in.....I don't want it constantly kicking in when connected to a 12v charge that surges to 15v

Sorry for so many questions

[pebe]

The circuit you show requires the input supply to be 18V to 22V. So why are you only using a 12v supply?

In any event it won’t work. The two 1N4001 diodes are added to the circuit to increase the 12V zener voltage to 13.25V, so as long as the supply volts is more than about 14V the 2N4400 will conduct and the relay will be energised. But as you suggest, there is no way that the circuit can know when to stop charging.

The NO, NC and Common contacts on the relay are not marked, so it is not possible to say what is supposed to be happening there. On the face of it, there appears to be a direct path from input to output via the relay contacts, with no current limiter. Running that from a 18V-22V supply would be disastrous!