Thanks Eric, I pretty much understand the ground thing now
Except... I see you have labelled the positive terminal for voltage in - but I am unsure about where the negative input point would be.
Also, the "aux out" you have labelled "+Vout" is not actually the primary output. The designer of the circuit said
"There is an auxiliary output, that provides about 20V at no load (depending on input voltage), and comes down as the load increases. I included this for charging 12V, 4Ah NiCd packs, which require just a limited current but not a limited voltage for charging." I will be using the charger to charge SLA batteries and would not be utilising J3 so I presume the +Vout for this would be J2. I have the same question about this though as above: I can see the positive output point but where would the negative output point be?
This is the webpage for the charger: ludens.cl/Electron/12Vcharger/12Vcharger.html