DC converter for car altenator

[mrinalmani]

A person involved in distributing DC-DC converters in South Africa gave me a brief description of the converter they have been selling and the functioning of that converter. However I am not fully sure about the very purpose of such a device. Please help me understand what this is. I have never seen such a thing before. Below is a part of the conversation with him, and also a link to the devices in use...

"...We use them here to charge a second battery in the back of a 4x4
deep cycle
when there is voltage drop....so we dont have to use to thick cables
so we get the required voltage at the second battery"

"...New vehicles like Toyota Revo they have a smart alternator
which drops the voltage to make the vechile more "green"
so the traditional solenoids which senses voltage to open and close dont work anymore"

https://www.hcdpelectronics.co.za/index.php?page=product_categories&catID=14

**broken link removed**

What I understand is that these people are using an additional lead acid battery apart from the one used for ignition. But for what joy?
All this is very unclear.

Please help
Thanks

 

[dick_freebird]

One thing a DC-DC converter with a wide input range
can do, is make a steady 13.6V charging voltage from
a (say) 10V sagged IGN feed. So the camping battery
can be charged from the vehicle system voltage even
though that voltage would not charge (but discharge)
the battery itself. And if this were the case then the
starting battery would be marginally charged if at all
while the engine runs, so having a spare (charged)
would be right handy.

Then, so would a better alternator.