Yet another DC-DC charger discussion

[Ranga]

When planning on running very thick (almost welding) cable from my cranking battery to charge an auxiliary battery about 4m away, this is a response I got from someone who has always seemed knowledgable on the subject:


"... it is a technical impossibility ... to charge a caravan battery from the vehicle alternator - UNLESS you are using a DC/DC charger. The cable from the vehicle should be used to direct power the fridge only when travelling.

Anybody that tells you they can direct charge a second battery at six metres plus distance from a modern vehicle alternator is dreaming. An alternator regulator can only sense the battery which is closest - which is the main starter battery. The only charge you can get into the caravan battery is the potential voltage difference between the starter battery and caravan battery - 2A generally and 4A if you are lucky. You may even find that lead from the vehicle on the caravan has a diode in the line which drops the voltage .6V."

I really hate that I can't get my head around why DC charger's are either essential or redundant!

[rover-56]

It has always been my understanding that the problem is the vehicle alternators regulator - it wants to hold the start battery at 13.8v, while the deep cycle battery to get 100% charge needs 14.0 to 14.5v, so the frig battery is never fully charged.
The exception is AGM batteries, which are happy at 13.8v.

That's my take on it, experts on here will soon tell us if I am wrong.....:p

Collyn Rivers tells the theory of battery charging a lot better than me though.

Terry

[Mick_Marsh]

Ranga, he does have a point. If there is so much volt drop in the cable the second battery may not charge. In that case, you need something to bring the voltage up.
With the cable you are running, though, I don't think you will be getting much volt drop in the cable.
Are you running a dual battery system such as one of those trackside units?
I thought they managed charging voltages.

Actually, thinking about it further, I might not be quite right.

As the voltage drops at the second battery, the current flow will drop, reducing the voltage drop in the cable, increasing the voltage at the battery. Something like that. Just thinking here.
The net result is the battery will charge. Just at a slower rate than the battery on the shorter cable.

But, I wouldn't worry about it if your using thick cable.

Oh, and give DC/DC chargers a miss.

[Ranga]

Are you running a dual battery system such as one of those trackside units?

Yep, a Traxide SC80

[THE BOOGER]

Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years:)

[Mick_Marsh]

Yep, a Traxide SC80
Use that then.
Doesn't that have small charging cables for the second battery?
You only need small cables to trickle charge a battery. The reason we have thick cables is for the cranking amps. What comes out of the alternator is, what, 2.5 sq mm, 4 sq mm, 6 sq mm cable.

[Ranga]

Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years:)

That's what I thought - why do we need DC to DC chargers all of a sudden? Perhaps modern alternators and/or auxiliary batteries are different these days?

[Mick_Marsh]

Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years:)
Yes, because there is not much volt drop.
In my line of work, we deal with longer lengths of cable. If you were trying to charge the battery 15km away with 1.5 sq mm cable, I'd say you'd have a bit of a problem.

[Mick_Marsh]

That's what I thought - why do we need DC to DC chargers all of a sudden? Perhaps modern alternators and/or auxiliary batteries are different these days?
The ones I have seen have all sort of electronics in. They pulse charge at varying rates. Much like some of the 240vac chargers I have. Probably un-neccessary for your application.

[THE BOOGER]

Some things he says dont make sense I had a piranha db setup in my d1 no diode I have a supercheap db Setup in the s111 no diode and a sidwinder db140 in the d1 now still no diode if you have an isolator why would you have a diode that drops the voltage? All 3 setups have charged the batteries to full charge on 6 plus hrs of driving no problems. The way i see it the average dc dc charger at 20 amps will put a lot less power back into the batteries than an alternator at up to 140 amps for the sidwinder or 80 for the sc80 over the same time:)

The amps to the battery are only limited by the wire and alt output provided the voltage is over about 12.8 the batteries will charge

[Mick_Marsh]

Sif you have an isolator why would you have a diode that drops the voltage?
The diode he mentioned I assumed was so the battery would charge but not discharge into the system i.e. isolate the feed.

[Vern]

Ranga, I have an sc80 in our amarok, it charges a battery about 4m away, then charges another 2 batteries another 4m away. It works great. This guy is talking BS. I'm running 13.5mm2 cable, has less than .1v drop at its final destination.:)

[Tombie]

The bloke "a flowing body of water" is proof that you don't have to be technically capable or correct to have your scribblings published...

I'm sure Drivesafe will wander in here soon...

Take a Deep Breath Tim!

[Hastykiwi]

God knows this has been done to death, but then I always settle in with a bowl of popcorn and a beer when these threads come up.:)

The charging of the aux batteries with varying length of cable, is simply a function of time. Of course they will charge, it might just take longer, if, there is some significant loss on the cable. As for DC-DC, if it is supposed to charge the batteries quicker than the alternator, from a low soc, how are you supposed to get the current there if, all other things being equal, there is major loss on this long cable. Its bollocks. Helpful hint though, if you want Tim to stay sane, I wouldn't mention the name Collyn Rivers. :o

[drivesafe]

This guy is talking BS.

Vern, did you get that in one.

Ranga ( and everyone else ) for a starter, who ever was giving you the “ADVICE” ( read B/S ), once a motor is running and the alternator is providing power to the vehicle ( this can be a few minutes after the motor is started ) the alternator CAN NOT SENSE THE STATE OF THE CRANKING BATTERY.

The only voltage that is being “SENSED” is the alternator’s own voltage and this will usually be much high than ANY battery’s voltage.

Next, in this day and age, very few caravans will have a blocking diode fitted, these went out with the ark.

Now for this crap about voltage drop.

It does not matter if you have voltage drop.

If your batteries are low, then you are going to get some form of voltage drop, and this includes when using a DC/DC device which will actually have a greater voltage drop than your alternator will, if the batteries are low.

AND NOW TO MY PET HATE.

I hate the way DC/DC devices are marketed. NOTE I do not hate DC/DC devices, I hate the way they are sold.

Ranga your “ADVISER” was partially correct when he stated “....it is a technical impossibility to charge a caravan battery “ but the correct statement is “It is a technical impossibility to FULLY CHARGE LOW batteries while driving.

And this means you are unlikely to fully charge low batteries while driving, whether you are using your alternator or a DC/DC device.

The reason has nothing to do with the method you use to charge them, they will not be fully charged because it is highly unlikely you will drive long enough to allow the batteries to reach a fully charged state, because low batteries can take up to 10 hours of charging to get them up to a fully charged state.

But if your batteries are low, you are going to replace FAR MORE used capacity using your alternator, than any DC/DC device can replace in the average drive time.

Again Ranga, your “ADVISER” must have been using speaker size wire if he was only getting 2 to 4 amps going into caravan house batteries.

I have measure as much as 63 amps going into 3 low house batteries down 10 metres of 6B&S twin ( 13.5mm2 x 2 ) cable.

Ranga, there are plenty of snake oil pedlars prepared to say anything to try to convince you that you need something you really don’t need.

[Vern]

Vern, did you get that in one.
Yep, smarter than I look:D

[THE BOOGER]

The diode he mentioned I assumed was so the battery would charge but not discharge into the system i.e. isolate the feed.

I understand what the diode will do its just any half decent DB setup will isolate the battery ss a diode has nothing to do:)

[justinc]

And just to add a bit MORE here, I have an SC80 mounted in the back of my 110, with both batteries, at the furthest LHR corner of my vehicle. At least 3 to 4 M of cable between the alternator and the starter and aux batteries. At IDLE I get 14.2V at the alternator, the batteries inside the vehicle AND the camper trailer battery on the drawbar connected through the anderson plug. I am running welding wire cables from the Starter pos terminal which is directly connected to the alternator with 3B&S cable down to the rear of the vehicle. The earth cable is returned straight to the engine block at a starter motor bolt, and all body earthing from the transmission to the chassis and the body.

Never had any issues with this system unless I am continually driving 2 km to work and back for a week in winter, headlamps and demister fan etc so after day 6 I am sluggish to turn over on cold start. Cue either a good run into town or a night on the ctek :)


JC