Thread: DC-DC charger- which one?

Originally Posted by RoverLander

"Here is a LOADED VOLTAGE TABLE, he correct one to use. This is the only table relevant for RV use, like in your D4"


Please provide a source for this chart and how it works for variable loads on the battery. I cant imagine how a load chart would work unless the load is specified.

It uses a time/draw weighted average.

very roughly...

if you're drawing 300 A (thats unrealistic) then the time steps would be very short intervals lets just invent a battery on the spot that takes 10 seconds between each step. when you see that voltage under that load then it tells you where you are (roughly) on your state of discharge under load.

the same battery with only a 30A load might run 100 seconds between each step (not realistic itd be longer due to Peukert's law) with a 3 amp load youd get 1000 seconds between intervls.

you generally dont use a loaded chart to work out the condition of the battery its more an on the fly calculator look up chart for how long you have untill you hit the nominal "flat" voltage. if you then let the battery rest with no load and no charge put on it it will "recover" a little, how much depends on the battery build and condition.

Originally Posted by RoverLander

I think we are saying the same thing. I was saying that any device connected directly to the negative terminal of the battery can not be measured by the shunt on the negative battery cable. Once you move the device's negative cable to the body then the current drawn by the device goes through the shunt and is therefore observed by the car and taken into account in the cars electrical energy management system. My point is that the devices current is visible to the car so the car can do its normal power management.

RoverLander, I am being 100% serious here, go and study how electricity works.

Your statement is exactly the opposite of what happens.

You are thinking that all negative current has to go to the cranking battery.

This is not the case while the motor is running.

While the motor is running ALL POSITIVE POWER ( current ) comes from the alternator, but for every POSITIVE ( + ) amp that comes from the alternator, there must be an equal number of NEGATIVE ( - ) amps returning.

The NEGATIVE ( - ) amps DO NOT go to the negative terminal of the cranking battery.

If the cranking battery is partially discharged, from starting the motor, and it is pull 10 amps to recharge it, 10 positive ( + ) amps will come from the alternator to the cranking battery's positive ( + ) terminal.

At the very same time, 10 negative ( - ) amps will leave the cranking batteries negative terminal and travel via the cranking battery's negative lead and the battery monitor, to the body of the D4 and then via the motor to the body of the alternator, which is the equivalent of the negative terminal of a battery.

Now lets say you have your sound system cranked up and it is drawing 20 amps.

The 20 ( + ) amps come from the alternator, via the main cable junction at the cranking battery but NOT THROUGH the cranking battery, this 20 ( + ) amps goes to the fuse box, the fuse and then the via the wire loom to the entertainment system.

Because you entertainment system is drawing 20 ( + ) amps, there needs to be 20 ( - ) amps returning to the alternator. So 20 ( - ) amps travels via the body and motor back to the alternator.

The entertainment system's negative ( - ) current never goes anywhere near the cranking battery's negative lead or terminal or the monitor.

So there is no way the D4 knows anything about the 20 amps ( - ) or ( + ), the entertainment system is using.

Originally Posted by Blknight.aus

you generally dont use a loaded chart to work out the condition of the battery its more an on the fly calculator look up chart.

Spot on the money!!!!!!!

Originally Posted by RoverLander

[I]"Still on topic and no disparaging remarks fro me....

No.. Just underlying sarcasm masking a blatant refusal to acknowledge the information presented.


We've presented the data, provided reasoning...

Enjoy your Dc-Dc device and all its limitations...

For those with MY14 onwards D4s... If your VIN falls between 698741-726591 then ask for TSB LTB00667v2 to be carried out...

Pop quiz:

Which direction is the flow of electrons in a DC system?

Simplest way I can put it is with a water analogy (sorry, this crap pic is the best I could do quickly):

Dam = Alternator
water = electricity
Height of water in dam = Voltage
Water Pressure = Current (Higher voltage = higher current)
Toilet = Battery (Cistern actually)
Water pipe = Cable
Two tanks + boy with bucket = DC-DC charger

Scenario one:

• The Dam supplies water directly to each of the three toilets.
• The Dam level is high so the supply requirement of each toilet can be easily met.
• Each toilet will only fill at a rate at which it is capable of and will slow rate of fill as it nears full state of charge.
• In this scenario the toilets fill as quickly as the toilets will allow
• There is no wasted water due to a lack of inefficiencies in the system meaning that there is spare water to supply other things like the kitchen.

Scenario 2:

• The Dam supplies water directly to the main toilet in the house but the other two toilets are supplied by a hard working lad bucketing water from one tank to another.
• The lad, while doing his best is slower than the number flushes in toilet 2 & 3 require.
• This means they never really get to fill up in time before the next flush
• He also spills a lot of water which means he uses a lot more than he would have had the pipe just been connected in the first place.
• Because he has spilled so much water, when there is a drought on there is not enough to wash the dishes in the kitchen (accessories)

All toilets will not over charge, they will stop taking water when they are full.

Originally Posted by Tombie

Pop quiz:

Which direction is the flow of electrons in a DC system?

is that an open to everyone question or do you want someone specific to answer the question.

Originally Posted by Blknight.aus

is that an open to everyone question or do you want someone specific to answer the question.

I would expect you to know it...