I will explain now my phone call has ended.
Basic systems consist of a high power switch and two wires between two batteries mounted close to each other. Many problems arise, you have to switch off before starting the vehicle and back on once the vehicle is running, this is inconvenient and can lead to problems. Forget to switch off before starting and you can damage the aux battery through starting, forget to switch on and the aux battery won't charge. If you switch on and leave it on once the vehicle has stopped you will discharge both batteries potentially.
With two batteries in parallel, of different capacities you will find one may overcharge (smallest capacity) and the largest battery may undercharge.
Blocking diode systems are just that, a diode; a diode only allows current to flow one way, but they have a volt drop across them of around 0.8-103 volts and this often prevents the aux battery from fully charging due to this volt drop as all car charging systems are machine sensed, basically the alternator senses the output and not the battery.
Basic split charging circuits consist of a basic high power relay, this is connected to the main battery and the aux battery, and the other terminals are connected to earth, and one is used to switch the relay on and off. This is achieved by using the feed wire back to the charging light as this switches on at a pre-determined voltage and not current output from the alternator. These are better, but basic, and suffer the same afflictions as the above, they cannot control the charge from the alternator and may overcharge one battery and undercharge another.
Voltage sensed systems are similar in principle to the basic split charge relay, but only work by using their voltage sensing capability, this is achieved by using high power MOFSET's, they can sense both the main and aux batteries power and split the charge according to how much charging each battery actually needs. Basically it biases charge to the main battery and when this hits a pre-determined voltage (generally around 13.2 - 13.5 volts) it switches charge to the aux battery as well as the main engine battery, thus splitting the charge on demand of each batteries requirement so no under/overcharging occurs.
Voltage sensed systems are the one to go for, quality systems from reputable manufacturers are readily available from £50-80 and are easily DIY fitted, many have optional display panels which give the state of charge of each individual battery, the charge rate of each individual battery, and some are programmable with a multitude of functions, and most have the option of LCD screens for even more display information.