The basics, the principle is to charge the house batteries via the engines alternator while driving, the idea can also double as a battery charger rather than carrying a generator for the few times you might need to use one.
After about '65 and before around '80 most car charging systems put out roughly 14.4v at the battery, the battery had caps you took off and refilled the water and if you forgot the battery boiled dry and failed. Up until this time most starter motors required a well charged battery to turn the engine over, if the charge got down a bit the engine was very sluggish to turn over. 14.4v is the ideal voltage to recharge lea acid batteries, these include flooded cell like starter batteries, low maintenance seal type batteries, AGM batteries and Gel batteries are all types of lead acid battery. From around 1980 onwards vehicle manufactures started to use different starter motors and electric that would operate at a lower voltage, all the way down to 10v, that way they could lower the charging voltage and use sealed type batteries, so the battery voltage dropped to 14v or less, this reduced the need for constant battery top ups and reduced the number of battery boil dry failures.... and every one was happy.
The next change came only around 5 yrs ago, the so called smart alternator, the voltage would be around 14v still but for only around 30 mins after starting, then it dropped back to 13v, supposed to be a fuel saving move but there were a number of reasons behind the idea which I won't bore you with now.
All this reducing of charging voltages severely effects the recharging of house batteries and the longer the cable the greater the problem. Cable lengths for DC voltage are measured by the full current path, the red cable from the start battery to the house battery and the black cable all the way back to the start battery, this can be a 15 mtr or longer round trip and voltage drop occurs depending on the thickness of the conductor and the number of amps trying to flow through it. The greater the current draw (amps) the greater the voltage drop, so although the alternator may be rated at 120 amps it is restricted to 14v or less, even 20 amps through 6 B&S cable (13.5mmsq copper conductor) through a 15 mtr cable run would be only 13.6v at the house battery, not enough voltage to charge it at all, just enough voltage to stop it getting any further discharged so useless for recharging the house battery.
This opened up a market for DC to DC chargers, they operate by boosting the voltage at the house battery end back up to 14.4v, the ideal lead acid battery charging voltage. They can do this even if the voltage coming in drops down to 10v so you can rely on the fact that there would be enough voltage available at the house battery to charge it at what ever amps the particular model DC to DC charger offers.
How did I go, too much information in one go? Should I make them shorter or add more information?
T1 Terry
