Thread: DC-DC charger- which one?

Originally Posted by SimmAus

9 pages in and no one has mentioned installing a Flux Capacitor...worked well in the DeLorean ;-)

Disclaimer: This might not be educational nor factual.

Tried 1 a few years ago but flux capacitor kept interferring with the poleriser and the hyclone

Hi again Silence, you and your mate Peter, haven't the foggiest idea how DC/DC devices actually work, but before giving you an education on how BAD they are in your situation, maybe some history on alternators is called for.

Up until about 10 to 15 years ago, the standard alternator size varied from 45 to about 80 amps, the most common being 55 amp.

The output current of an alternator depends on the RPM the alternator is being spun at, which is based on the motor's RPM, and while cruising down the highway, the motor's RPMs will be high enough to turn the alternator at a speed that allows the alternator to produce it's maximum output current at it's MAXIMUM VOLTAGE.

But get into bumper to bumper traffic and the motors RPMs drop to such a low rev that the alternator can only produce a small percentage of it's total capacity.

So in slow or stopped traffic conditions during the day time, the alternator was flat out producing enough current to meet the basic demands of the vehicle.

At night and during winter, with headlights on, heaters and air conditioners going and probably the sound system cranked up, and the alternator has no hope of producing enough current to power everything.

At this point you would think you would overload the alternator.

But not so, because alternators have a natural way of compensating for current loads that are greater than their maximum output capability, the alternators current level, even when reduced by low RPMs, remains the same but the alternators output VOLTAGE drops.

As I have already pointed out earlier in this thread, it is also the natural operation of most DC appliances to lower their current demand as the voltage drops.

So eventually you get to a lower voltage but the current demand on the alternator levels out.

The problem is that if the voltage has to drop too low to get the current demand down to the alternators maximum capacity at low revs, you now have a situation where the voltage drops below the voltage level of the cranking battery and now the vehicles current demands own are being met by both the alternator and the cranking battery.

So RoverLander, that document you posted up is nothing new and this has been happening for at least the last 60 or 70 years, since alternators have been used in the automotive industry.

Now to present day and D4 operations.

Alternator sizing is still based on what a vehicle needs, when in traffic or idle situations.

The only difference is that vehicle power consumption is now hugh and so we now have alternators with hugh output capacities. A D3 with 140 amps and D4 with 180 amps.

But even these high current alternator can still only provide enough energy at idle or in traffic to maintain the vehicles own energy requirements.

So you still have situation where the vehicles energy requirements will exceed the maximum available current output of the alternator and just as has been the case for the last 60 to 70 years, if the current demands are too high and the alternators voltage level is pulled down to the cranking battery's voltage level, the cranking battery will have to make up the short fall.

Now throw a DC/DC device into the system and you have real current load problems.

As also posted in this thread, DC/DC devices are one of the few DC devices that actually increase their current draw as input voltage drops.

So at the very time when you need to reduce the current load in your vehicle, here is a DC/DC device drawing more and more current as it forces the vehicles voltage even lower.

With the alternator already at full output capacity, this hugh increase in current demand has to come from the only source available to provided it, your cranking battery.

In your case RoverLander, you may be correct, your operating voltage of 12.2v may very well be that of the cranking battery, all because your DC/DC device is giving your cranking battery a hammering. Thats a good setup mate.

With one of my systems fitted and if you end up in a traffic situation, the voltage will still drop, but all the batteries in the system will draw less and less current as the voltage drops.

If the load is still greater than what the alternator is producing, say you might by out off roading at night with heaps of lights on.

With my system, while you will still have to draw power from the batteries, the additional current load is now spread out over all the batteries, which means your cranking battery is never going to be worked as hard as it would be in a D4 without a dual battery system and would be miles less current drawn off the cranking battery is it has to be in your.

An additional benefit with my setup over a DC/DC setup, where the cranking battery would get a caning in such situations, whereas with my system, the cranking battery will have had the least amount of energy drawn from it, so when you do get back to decent speeds, the cranking battery in one of my setups will be fully charged in a fraction of the time needed to charge the cranking battery in your setup.

So why on earth would any clear thinking person want to put a DC/DC setup in a D4?????