Many statements are made about charging from a car alternator and it is difficult to get it right and the biggest problem is understanding the battery type that you have and the thing that most people do not understand is what will you battery accept and this is governed by the battery resistance and the lower the resistance of your battery the higher the charge the battery will accept before the resistance reduces the alternator charge.

Voltage is another thing that is misunderstood and even though you have a 12V battery 12V will not efficiently charge a battery and a battery needs to be charged to 120% give or take the different battery types and the required charge rate the manufacturers specify so assume 14.4V to be a rule of thumb figure but could be from 14.1V to 15.1V

Modern vehicles in Australia are now following the European standards supposedly for fuel conservation and pollution reduction and this is causing alternators to maintain the cranking battery and then lower the voltage to a supply voltage for the normal car operations
and this does not give any consideration for and is detrimental for charging auxiliary batteries.

DC/DC charging can assist in overcoming this situation but to be efficient the charger should be capable of using all the spare amps of your alternator especially if you discharge your auxiliary battery bank down to 50% Depth of Discharge or more because from your depth of discharge to the resistance level of your specific battery it is no good having a 10-20A charge if your alternator is larger as the small amp capacity strangles the output of your alternator until that resistance level in the battery is reached then the DC/DC charger can play catch up at its capacity which is governed by driving time so how long do you want to drive regardless to charge your auxiliary batteries.

Peter D simply answered the DC/DC query but if you do not understand more you may waste money on fitting something that is not as efficient as you would expect .

Peter in his product supply listing showed one from Bainbridge Technology as a BBW1260 and there is no such animal available in the world to date as pictured and the unit as displayed comes currently in only four sizes BB1250,BB122450, BB242430 and BB241230 and the last 2 digits represent the input capacity of the units.

Also the above units are subject to alternator voltage drop conditions and with the 12V they need 13.35 to 13.55 volts to operate efficiently and ramp up to full capacity but with a simple resistor charge they can be modified to accept lower voltage alternator input.

Sterling power has been working to offset the voltage drop alternator which he refers to as RGBF ( Regenerative Breaking Friendly) and has tested a prototype BBW1260A unit which overcome the problem but is yet to become commercially available.

The first unit that has been developed is the BB12100 RGBF DC/DC Charger that is half the size of the one wrongly displayed on the Bainbridge link and if the size and shape of the Sterling AB1280 or AB12130 alternator to Battery Charger and is available in small quantity at the moment through RV Powerstream that has represented Sterling products for a decade now in Australia.

The new BB12100 RGBF also has been built to have more selectable battery types of battery charging algorithms which include Calcium and Lithium and at 100A input current limited it can certainly do what most people hope to get and that is charge your batteries while driving with capacity and efficiency.

Ian

Some people sell boxes

Some sell technology.