Charging a third battery in the boot via external Anderson plug

You only get a significant voltage drop when the demanded amps are high. As it gets towards the end of charge the difference of voltage will be less and so are the amps, therefore the battery will get to very close to fully charged. It is only within the vehicle and not to the rear of a camper trailer.

There is no substitute for heavy wire though.

1. A DCDC charger is no substitute for adequate wire size.
Watts in equals watts out - there's no free lunch.
Whilst it will boost the voltage at the other end, it will require more current from the input to do so, and this will be at the expense of even greater voltage drop, which will then require ever more current, and so on.
The eventual result, if the wire is too small, is a piddling charge to the battery, or even that the DCDC device ceases to proceed.

2. Whilst it's true that the voltage drop will decrease with load amps, the problem is that you need lots of amps in the early charge stage to get the battery to that low load point.
The reality is that you may just not drive far enough to achieve that point of charge, and the lower the charge voltage or current, the longer that drive will need to be.

Brian
I agree, so with heavy cable which allows the likely amp flow which can be delivered from the alternator, it will charge the battery because there isn't a lot of voltage drop at all.
The DC from the alternator charges the battery with DC ie, natural DC/DC charging I reckon.

It all works like that in my setup.

Except that the alternator regulator is looking at the terminal voltage of the crank battery, which will rise more rapidly (due to voltage drop no matter how small) than a more remote battery and will taper its output current to suit that rising crank battery voltage.
This means that the remote battery does not get as large a charge current as it might otherwise receive.
A DCDC charger, on the other hand, is seen by the regulator as a load, just like the headlights etc. and so it will boost its output to accommodate that load.
The difference is caused by the relative voltage depression that is produced at the regulator sense point.
The remote battery will be at most a couple of volts lower than the crank battery, this will only slowly depress the crank battery terminal voltage as it sends charge to the remote battery (again limited by wire capacity).
The alternator regulator will quickly replace that charge and will frequently ramp down as the crank regains its set voltage.
The regulator is a constant voltage device, its job is to simply keep the system voltage at a set level (within its capability) irrespective of the load.
The DCDC charger in boost mode is a constant current device, and so it sends a constant set current to the remote battery irrespective of the supply voltage, again within the capability of the device to do so.
This constant current draw causes the alternator regulator sense voltage to become, and remain, depressed, so it increases and maintains its output to compensate.
If you have a close look at an alternator charging a second battery you will see that it may put out a nice high charge current for a maximum of a few minutes, but will very quickly fall to a fraction of its starting point, this is due to the voltage sensing of the regulator, and is the reason why a DCDC charger will always do a better job.
Sorry it's so long winded, I get carried away BIT.

Alternators were ever only designed to charge a starting battery to an acceptable level (not a full charge level) and are temperature compensated as most starter batteries are in the engine bay..... hotter the engine bay - hotter the battery - low the charge rate.

A DC-DC charger will charge an auxiliary battery to near 100% SOC, and will provide the correct voltage and limit inrush current to a more acceptable safer level plus lowering the load off the alternator..... alternators all have a duty cycle, so if it's a 70amp alternator it doesn't mean it can deliver 70amps constant...... good way of destroying alternators.

All the vehicles we fit DC-DC chargers to old and new; we see a good improvement on the auxiliary battery charge rate of 1-1.5volts over the starter battery.

No matter what; there is no substitute for battery cable size even with a DC-DC charger.

Brian you will come across many who don't think DC-DC charging is needed and is a gimmick. if it was fluid; voltage would be pressure and current; the quantity. Higher pressure over comes resistance and the quantity can flow.

Bantam, I think #6 is 4.1mm diam.

Number 6 (or 6 guage) Battery and Start is about 13.5mm2
6mm (nominal) automotive is about 4.5mm2

that is according to the automotive section of my Tycab catalogue.

cheers

And we should not get the battery and starter thing confused with B&S that cable is measured in.......B&S does not stand for battery and starter but Brown and Sharpe who set the standards for AWG (American wire gauge) specifications early last century.

Yes there is some confusion.
In general automotive cables not for battery and starter use are simply refeered in imperial terms to as "such" guage.....with the inherant confusion between american and british guages....which more or less agree to a point but then diverge.

correctly or not.
cables for battery and start use are refeered to as such guage B&S.
And the catalogues have the words "Battery and Start" at the top of a different table of cables than for other use.

Battery and start cables in general do not come in much choice of colour and may be coarser strabded than the other cables.

Then we get the whole metricated guaged cables....cables made from metric strandings that approximate guage sizes but are neither, clean even guage size nor, clean even metricated size.

OH and the stupidity that is refeering to a cable by its insulation diameter rather than its metalic cross sectional area.....an insulation diameter that due to improvements in plasitcs is nothing like accurate.

so we have a 6mm ( insulation diameter ) cable that is in fact 4.15mm nominal OD, is 4.59mm2 metalic crossectional area and is near, but not right on 10 1/2 AWG

This is all an infernal irritation.

For goodness sakes people discuss cable, like the real electricians and the engineers do ..........only in metalic crosssectional area and in metric......you have to convert to crosssectional area to do any calculations anyway.

cheers