Caravan Batteries

Absolutely true...Or use solar panels and a regulator.

Reasoning is this: The length of cable from the car to the caravan battery causes voltage drop. Also many caravans have a protection diode fitted so the caravan battery won't discharge back in to the car. this all means that you wil probably lose at least .6 volts (Diode alone) to 1-1.5 volts (Added loss in wiring to Diode) which means that the voltage at the Caravan battery will, in all likelihood, be between 13.4v and 12.9v (Assuming the alternator produces 14.4v). This is not enough to fully charge the caravan battery.

Peter

I'm very interested in this thread too.

OK - given that you are charging a caravan battery with a charger coming off an inverter - how big has this charger got to be??????

An isolator will give you a fair few amps more than your average "affordable" battery charger won't it? I have a battery charger that I paid a bit for a while ago - and off 240V it can only muster 8 amps... which isn't going to re-charge my 105A/H van battery withing a day's driving....

Right on the money Chrispy, for a start, to be able to get the correct current and voltage to the van to run the inverter properly, you will have to install huge cables from the front of the tow vehicle to the inverter in the caravan or camper trailer.

Even then as Chrispy has pointed out, unless you have a large battery charger, it is still going to take forever to charge the battery.

As for the diode in the caravan, I think you will find this is more to do with a given type of battery isolator and not the specific installation of a diode in a caravan, haven’t come across that one.

Forget the inverter + battery charger idea. For a fraction of the cost of a large battery charger, you can install heavy cables in the first place and this alone will dramatically speed up the charging of the rear battery and the sooner a battery is charged the more likely it is going to be fully charge while you travel from one camp site to the next.

Cheers.

Hi Ray ,
I have set up two systems same idea as drivesafe and they work fine .
Run heavy cables from aux. batt thru' 50 amp andersons to same heavy cable to ct batts.
Important : when at campsite , keep aux. hooked up to ct . if you need to park parallel to ct I use extn. cable with andersons from front of 4by - this keeps aux. & ct batts.even charge .
: aux. & ct batts must be similar ie all wets , gels or AGM to facilitate more efficient recharging - I use 2- 12v x 120ahr.in ct & 100ahr optima as aux.
I get nil voltage drop when rested or charging - runs 80 litre engel & lights no probs.

CYA John

Get a twin charge unit from the 12volt shop and you will have no worries about voltage loss.
see this site
www.allabout12volt.com.au

The unit is called an...... ARID twin charge, it is purpose built to charge a van battery from the vehicle.

Hi Ray,

There is a considerable amount of material on the INTERNET that says a Deep Cycle Battery can not be fully charged from a vehicle alternator. They do not put out sufficient voltage nor the amps in a fashion suited to a deep cycle battery.

Also there is the issue that a deep cycle battery needs a three stage charger designed for the Deep Cycle style of battery.

The inverter, battery charger set up is quite a common approach to fully charging a deep cycle battery in a van or CT.

You will probably have both of these items in any case, so it could be just a case of using existing gear and adding some decent cabling.

Cheers

Magnus

The inverter/charger idea DOES work well, very well.
BUT you need a large pure sinewave inverter, larger than you'd think due to an inductive load.
However, IF you have a large enough inverter hooked up to a three stage charger it works a treat.

Ray
Have run 8mm cables from car to caravan via 50amp anderson plug to two 40amp gel batteries in parallel. Have solidnoid in car for cable when engine is running only.
Have solidnoid in caravan to isolate battery charging if needed to. Use caravan fridge on 12volt during day when travelling. I do not leave car charging all day when travelling let (Electrolux) fridge run on battery only for about two hrs then charge for about same. Been using this set up for about four years. Travelled to west every year from Kimba SA. In January and October mainly. Never had problem with 8mm cable. First trip only had 4mm cable and did not work, nearly wrecked batteries.

used same setup with camper trailer to central aust last year but with sealed lead acid battery and charging all time vehicle engine running with no problems. (Engle Fridge)
Cheers
Charlie

just interested in when hooking up your camper back to main vechile how long it would be before the camper battery starts to drain the main vechiles battery to a level where you wouldnt be able to start main vechile. assumming you havent got an automatic solonoid.

If your ct has an absorbtion type fridge, electrolix or dometic then the battery drain is around 15 amps just for the fridge. I use a charger setup on an inverter, If you know a sparky he can put a capacitor across the 240V output and this will increase the 12V output due to power factor improvement. The inverter does not have to be sine wave in this situation. Charging from the car even with large cables will not work really well if you have one of those power hungry fridges. It works with an engel or waeco OK.

No , just direct hook up to aux. with a simple redarc iso between engine batt which is also AGM .
If one battery say aux. has a higher charge than the other 2 , the alt. decreases charge rate thus extending recharge time - thus the reason to keep the use even on all batts .
When ct left at campsite while you go "4bing" for the day the aux. will be at a higher charge than the ct so on return, connect andersons to allow the batts to equalise overnight .
I forgot to mention earlier , the 4by has a 60 litre engel running as well .
As back up , which has not been needed yet , I have a Yamaha 1000 inverter gen set powering a 3 stage 40 amp True charge batt charger hard wired .
Remember inverters loose approx.20% - you cannot get 240v out of 12v for free .
CYA John

Use an Arrid Twin Charge from the 12v shop as Dodger suggested
They work fine
Neil

It is possible that the current interest in inverter/three-stage battery charger setups follows my Tech Notes article that was published The Wanderer (the CMCA's monthly journal) some half year ago. I have also described and suggested it on several Forums.

It is primarily of benefit in motorhomes that have 24-volt vehicle electrics and 12-volt house electrics, but is well worth considering otherwise.

Before eloborating, I'd like to comment on a number of issues raised in this thread.

That conventional lead acid batteries rarely charge beyond 70% in vehicles is not a myth. Whilst is true that 13.8 volts will eventually bring a battery close to full charge the time required is virtually weeks. Several battery makers state that caravan batteries are typically unlikely to exceed 65%.

The 70% voltage limitation is deliberate as it safeguards batteries against overcharging on very long trips or non-stop usage. Starting is not compromised as the system is designed accordingly.

In practice (as opposed to theory) most car batteries do charge to approx 70%. They discharge to about 67.5% whilst starting. Because they fall to below 9 so volts whilst doing so (and there is thus a huge diffetential between battery and alternator voltage), the energy used is typically replaced by the alternator within a one-two minutes. Whilst the charging limitationm does not affect starting, any parallel charged battery has the same limitation forced upon it.

Most conventional paralleling charging methods work reasonably well but are usually a compromise if disparate batteries are involved - as their charge acceptance characteristics are almost certainly different. Rotronics up-market device gets around this - but is very costly.

The inverter/charging system is most economic where an inverter and/or mains battery charger is needed anyway, but is worth considering generally.

It advantage is that no modifications need to be made to the vehicle (the inverter is just another appliance) and that the charger can be set to provide the optimum charging regime for the battery.

It needs neither humungous inverters or heavy cabling. The charger needs not be huge as long as it is a three-stage unit that provides constant current charging.

Charging a battery at (say) a constant 20 amps is far better than most alternator/regulators can do - so a 20-25 amp charger is adequate unless you have a heap of battery capacity. Driving the charger requires (at most) a 500-watt inverter, connected by cables capable of carrying about 27.5 amps.

I have one minor reservation re using this technique with a trailer. And that is the legality of running a low voltage (240-volts ac) cable between towing vehicle and trailer. There appears to be nothing in AS/NZS 3000:2000 or 30001:2001 that covers this. Whilst I am a professional engineer with electrical engineering experience I am not a licensed electrician - and those I have discussed this with are as equally unclear.

I have no hesitation in suggesting it otherwise. An increasing number of motorhome owners are using it with every success.

I will arrange to post the CMCA feature on my own website asap.
Collyn Rivers

Ray

It sounds great - particularly the 18 sq mm cable!

Incidentally that is the approach I suggest for people with fifth wheelers (ie running the final metre or two at 12 volts).
Collyn Rivers

Nick

AGMs have very low internal impedance. This provides their happy characteristic of charging quickly and very close to 100% from much lower charging voltages than are needed for (particularly) deep-cycle batteries. They will accept voltage between 13.8 and 14.6 or so.

Their operational voltages are also a tad higher than conventional batteries.

In my new book (The Camper Trailer Book) that I am just about to print, I quote off-load rested voltages as 100% charge - 12.8 volts, 75% charge 12.6 volts, and 50% charge as 12.3 volts (ie all about 0.1 volt higher).

As AGMs reduce in price I believe increasingly they are the way to go.

Re having all AGMs or no AGMs, I am partially responsible for this belief having stated this in many magazine articles and in early editions of my books. I did so because that's what their makers warned.

Since then however, Redarc and other similar voltage sensing relays have gained wide acceptance. As long as one of these delays paralleling until the starter battery reaches about 13.6 or so volts, there seem no major problems in parallel charging and thousands of people now do so.

One further reservation is that the charge acceptance is such that there may be a risk of overloading an alternator if attempting to charge too large an AGM battery bank.

There is next to no risk of charging an AGM at too high a rate - they will withstand substantial input currents.
Trust this helps
Collyn Rivers

Collyn

Thanks for clarifying that.

I have searched for and found many comparison charts of rested voltage vs. charge % and they seem to vary quite a bit. I was never sure which one to use. Yours seems on the pessimistic side, but you may well be right. It is never easy to know the true AGM battery charge status because of all the variables which affect the voltage reading. It is especially difficult to know what’s going on when you are actually using the auxiliary battery as you don’t get the chance to rest it. I haven’t been able to find any “rules of thumb”, however inaccurate, to help interpret the state of charge voltage readings you get while say a 3 amp fridge is drawing power at the same time. Do you have any comments, or do I keep wondering what the readings really mean?

Thanks
Nick

My opinion on charging aux batteries is simple and based on some experience;

Use the largest Deep Cycle battery bank you can fit in a given area

Use Large cable to the aux batteries, both pos and neg leads

Use solar panel/s with a good solar regulator

Reason is;
solar works when you are not driving
solar gives high voltage

yes, is only my opinion :-)

Hi,
When I first asked the question I did not know that it would cause so much contaversy. Sorry if I have cause any of you any trouble

For the benefit of anybody considering using the inverter/charger set up, you may be interested to know that this has been done but is considered by Workcover in NSW as an unsuitable work practice and have apparently notified a number of organisations informing them of their displeasure relating to this form of operation.

Note this specifically relates to work place operations but there must be some question to the safety of the sort of operation.

Cheers

Ray , what you have discovered is 12v systems are a HUGE topic of debate (like Hiclones , Nissan V Yota etc etc).

Ask a question and get 100 different ideas , some right , some wrong , some that WILL work , but not necessarily , the best way etc etc.

It isn’t easy to find information on using battery voltage DURING DISCHARGE to indicate remaining charge, but here is a compilation of what I’ve found. As you can see from the Open-circuit voltages on the left from various sources and battery types, there is quite some variation for battery types.

0x 0y 0z 0w 3 4.5 22 90 270 340 Amps
100% 12.70 12.66 12.80 12.80 12.5 12.8 12.5 12.2 11.6 10.4
80% 12.42 12.49 12.64 12.55 10.2
60% 12.20 12.32 12.48 12.30 9.9
50% 12.06 12.24 12.40 12.18 12.4 12.2 12.0 11.8 11.3 9.4
40% 11.90 12.16 12.32 12.06 9.5
20% 11.58 12.06 11.96 11.82 8.9
0% 10.50 11.89 11.80 11.60 10.6 10.8 10.4 9.5 9.4 6.0

Sources

0x - unspecified battery www.windsun.com/Batteries/Battery_FAQ.htm#AGM,%20or%20Absorbed%20Glass%20Mat%20Batteries

0y - Low Maintenance (Calcium Lead) Battery - www.uuhome.de/william.darden/carfaq4.htm
unspecified battery http://www.batteryuniversity.com/partone-13.htm and www.ibsa.com/www_2001/content/faqs/tech_talk/maintenance/faq_tech_maint.htm

0z - Maintenance Free (Calcium-Calcium) or VRLA (AGM) - www.uuhome.de/william.darden/carfaq4.htm

0w - Odyssey Staring Battery Handbook.

4.5A to 270Amp - Absorbed Power N70-size 90AHr AGM deep cycle battery - http://www.absorbedpower.com/battery/pdf/AP-GT12-90C-01Sep05.pdf

3A and 340A - 60 AHr unspecified battery - www.cyb.com.au/automotive/batt_facts/undbatt_2.htm

What an intertesting thread. I find these type of non-agressive deabates very thought provoking. I have been reviewing battery use on my own van and have found the many points raised here full of information.

Great input from all.....thank you