DC to DC Chargers

A Better Way To Charge

Although we ordered our truck chassis from Volvo with a large 240 amp alternator for charging not only the engine batteries, but the house batteries as well, it turns out it really didn’t help us in the long run.

You see, what the truck salesman didn’t tell us (probably because he didn’t know himself) was that the electrical current generated by the alternator is actually throttled. Known as “smart alternators”, these modern marvels of technology are constantly monitoring the charge level of the engine start batteries, and the amount of energy the truck chassis needs to operate, and then adjusts its output to match the need.

Of course in theory this sounds great, as it’s necessary in order to meet the ever more restrictive emission standards being imposed by countries around the world. But there’s a problem. Most RV and expedition truck batteries are located towards the rear of the vehicle. The long run of wire from the front of the truck where the alternator is, to the house batteries in the rear, causes a voltage drop in the line that’s significant enough to prevent the house batteries from receiving enough energy to charge. The alternator reads the level of the engine start batteries and puts out the correct amount of voltage and amperage to charge them. If the alternator were to put out a higher voltage to overcome the line drop at the rear, the increase could cause damage to the engine start batteries. If the house batteries were mounted close to the alternator like the engine batteries, it might be a different story. But generally this isn’t the case.

To solve this problem, a new technology was developed over the last few years that is catching on in a big way. Identified as DC to DC chargers, these devices are able to trick the smart alternators into outputting more power than they normally would. The way these new chargers work is by pulling power from the truck engine batteries. Then they increase the voltage enough to overcome the line drop before they send the charging power down the long wires to the house batteries. When they do this they deplete the engine batteries, and this depletion is sensed by the smart alternator which dishes out more power to try and keep the engine batteries fully charged. So in essence, the alternator is now doing the job you originally sized it for…charging both banks of batteries at the same time.

The other thing the DC-DC chargers do is to adapt their output charge characteristics to suit the type of batteries that were chosen for the house (Flooded, Gel, AGM, LIon, Nicad, etc), since they may be different than the start batteries. In the case of our lithium ion batteries, they also adjust their output to provide an initial heavy “bulk charge”, then the moderate “absorption charge” and finally the maintaining “float charge”. This varied charge level helps to keep the batteries in top condition which in turn leads to longer life. Something that is much appreciated considering some of these large lithium ion batteries can cost as much as $5,000 each.

For our truck, the Mastervolt DC-DC charger outputs 50 amps of power, and two chargers can be run in parallel providing 100 amps of total charging power. It makes a huge difference, especially in the winter months when more power is consumed by the house for heating and indoor living, and there is insufficient sunlight, especially in the northern/southern latitudes, for the solar panels to provide any energy. Before the addition of the DC-DC chargers there wasn’t enough charge power, even when driving for many hours, to fully charge the house batteries. Now that isn’t an issue.