Charging 3rd Battery in Camper/Caravan

Hi gone bush
If you just want to charge it while driving run it from your starter battery just run it through a relay set up so it only charges when the cars motor is running its the easiest and cheapest way my has been good for yrs an still going if not sure i will give you a bref instructions too
pat

Hi Gone Bush

Have a read here...

Caravan Charging

It should give you the answers you need.

If you already have an isolator use the auxiliary side of it via a 50A breaker.

Regards

Derek.

Hi Gone Bush, your dual battery system works by sensing the voltage at the cranking battery.

While the motor is off, the DBS is sensing the battery’s voltage but once the motor is running and no matter how low the cranking battery is, the highest voltage is now going to be the voltage provided by the alternator.

So, usually only a few seconds after the vehicle is started, the voltage at the battery is going to be above the DBS’s cut-in ( turn on ) voltage level.

Some DBS units have a designed cut-in delay that allows the main or cranking battery to get the first initial few seconds of power, this delay is usually no more than about 60 seconds.

The answer to your question is, at no more than 1 minute after you start your vehicle, your auxiliary battery will be getting a charge.

A WORD OF WARNING, I would not take ANY notice of the info supplied in the link above. Like most of Derek’s electrical info, it’s based more on Scotch Mist than anything to do with reality.

In most cases, I just ignore his postings but the problemwith this one is that if you were to take any notice of these diagrams and set up your dual battery system based on his dangerous info, you will probably end up burning your vehicle and / or caravan or camper trailer to the ground.

For a starter, no cable is sold ( by reputable sellers ) based on the current marked on the cable. This current marking has ZERO to do with the way the cable will be used.

The current rating of cable is the maximum current the cable can carry before the insulation starts to melt.

Next, Derek's info states you need a fuse or circuit breaker of between 60 and 100 amps, there is no way you need a circuit breaker bigger than 50 amps.

Automotive circuit breaker and fuses are specifically designed to tolerate about 50% more current than their marked rate for about 1 hour before tripping.

So if you take Derek’s advice and fit a 100 amp circuit breaker to HIS 100 amp cable and you get an overload, say caused by a faulty battery, the cable could have 150 amps passing through it for an hour before the circuit breaker tripped.

Forget the hour, after just a few minutes at that current, the cable would most likely have caught fire.

You need to know how to safely and correctly set up a dual battery system before you give advice to others. These diagrams are neither safe or correct.

The size of the cabling is totally inadequate to be able to charge your batteries in the back of you vehicle, let alone in a caravan or camper trailer.

I’m no fan of Collyn Rivers and many of you know why, but at least he does know what he posts about, not so with Derek. He’ll end up killing someone.

I'm not going to get into an argument here but simply answer your question based on my own knowledge and experience and the advice coincides with the schematics of Derek's diagrams.

If you have a dual battery system, any connection to accessories or additional battery circuits should be connected to the Auxiliary Battery terminals.

This gives the best protection to the primary battery and its sole use (in a dual battery system) of starting the vehicle and keeping it running.
The primary battery is therefore ISOLATED from any other auxiliary electrical circuits and devices. That is the reason for a dual battery system in the first place.

You can also replace "dual battery system" with "multiple battery system" by connecting additional "remote" batteries, be they in the back of a vehicle, or camper trailer, etc. but always from the auxiliary battery circuit, including any additional battery charge controller systems.

Hi Gone Bush

Here are the breaker and fuse trip times for the circuits I use and recommend.

The first one is for auto reset breakers and the 2nd one is for our ANL fuses.





All my circuits are tested prior to publication.

A typical short circuit is what you can expect on any inter battery connection and this would trip a breaker in under 1 second. If you use a ANF fuse you can expect a short circuit to trip the fuse in 0.1 seconds.

Quite fast indeed.

Regards

Derek.

Drivesafe,

I'm a bit perplexed that I have stirred up something here.

Nevertheless, your comments seem mainly related to cable size and fuse/breaker rating.

Could you clarify for me please: would you connect the cables leading to the Anderson plug to the Starter or Auxiliary battery?

Hi Gone Bush

Unless you decide to fit a 2nd isolator it is best to use the Aux side of the existing isolator that you have. (Connect to aux battery)

I would still use a 50A Auto reset breaker and at least 8 B&S wire.

Regards

Derek.

Hi Gone Bush, unfortunately that cable is even thinner the the cable Derek recommends, which is still way to thin to be able to charge a battery in the rear of a vehicle let alone in a caravan or camper trailer.

The very minimum to be able to charge your caravan or camper trailer is 10mm2 but 6B&S would be preferable.

BTW unlike the info in Derek’s article, 6B&S ( 13.5mm2 ) and 16mm2 are actually two totally different sizes of cable, not the same cable size.

Gone Bush, the cable you have WILL charge your batteries BUT if the battery is low, say 25% SoC at the time you start driving, to fully charge the battery you will have to drive for anything from 15 to 20 hours depending on how much power your fridge will draw while your driving, no matter what type of battery you have.

By fitting 6B&S you will lower the drive time, depending on the type of battery, down to between 5 to 8 hours and there is a lot more to it than just drive time verses fully charged batteries.

Derek, why on earth would fit a 50 amp circuit breaker on such a long run of 8B&S cable.

The circuit breaker ( or fuse ) is also supposed to protect the devices connected to the cable not just the cable and it would take a load of 75 amp for an hour before the circuit breaker tripped but more importantly in a perfect cable run with an alternator voltage of 14 volts, you would be down to 12.6 volts at the battery and this voltage drop does not take into account the additional voltage drop that would be imposed on the circuit by other things like the plug between the two vehicle and the caravan or camper trailer, the circuit breakers ( there should be 2 ), the dual battery controller.

In reality, you would be VERY lucky to have 12 volts at the rear battery.

If you were unfortunate to have taken Derek’s suggestion and fitted 8B&S cable, then the absolute maximum size circuit breaker to use is 30 amps other wise you are just wasting your time.

Thanks David, I was hoping you would intervene.

All this sniping actually made me feel hesitant about posting any more questions; I'm sure others may have felt the same.

Asking questions on this Forum and using the information in the response is no different than dealing with my local sparky, home cinema retailer, investment advisor or doctor. I always seek a lot of info before I do something. (Which means I will monitor how my small wiring goes and make a decision on replacing it later on.)

But I would hate to see this Forum degenerate into a slanging match. I have participated in a Forum on Digital Broadcasting and Home Cinema and if you don't agree with the small group of long term members who have taken control of it you are slammed down something shocking.

The Internet is all about getting information. It's up to the individual what they do with it.

Now where are those emails on bleep enlargement.....

Gone Bush,

I have spent 35 years as a Technician/Technical Officer/Engineering Officer working on DC powered equipment. I have the qualifications, but won't list them here, to interpret the information of both Derek and Drivesafe.

Leaving aside personalities, inferences and criticisms, IMHO I would follow the advice given by Drivesafe in the issue of a third battery, and heed his warnings. I have no knowledge of other issues they may have disagreed on in the past, nor do I wish to be embroiled in them.

I have sought and used information from the websites of both of these people.

To me, volt-drop is everything and therefore using the largest cross-sectional area copper cable that is practical has always been my priority. Combining this with the lowest rating fuse or breaker that will not fail under the intended load will give maximum efficiency and protection. In this instance, I would connect to the auxilliary battery or the Aux battery side of the isolator.

No offense intended Derek, just my two-bob's worth.

Cheers,

Russ.

Just a quick thought for all reading this.

Keep in mind manufacturers use the MINIMUM they can get away with in vehicle design. Every cent is calculated in the cost of building. They now each model has a limited lifespan (Think about it this way if they built 200,000 of a model and wasted $1 on each vehicle there costs go up straight away by $200,000) Ever noticed how there is no "play" in wiring looms, linings, carpet etc.... reduces cost over the life of a vehicle.

Now lift your bonnet and have a look at the thickness of cable being used. Notice how thick the high current wiring is? And that is in a very short run (relatively speaking when looking at the topic of running wriing to a caravan/trailer)

Now we all know the longer the cable the higher the resistence and the thicker the cable needs to be to maintain the voltage and not overheat.

Look at your appliances, notice how short the cable is, see what is said about running a welder on an extension cord or your caravan off a standard extension lead this is all common FACT!

And at the risk of facing moderator wroth. Drivesafe has a point, perhaps a little heavy handed but when someone is posting info that someone who might not know better could reasonably use and it pose a serious safety risk then someone SHOULD speak up.

Stuff legal obligations how about moral responsibility? How often do we see pics and posts about vehicle fires? I'd bet that these fires are more often than not caused by non genuine wiring.