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What Is The Charge Rate Of The Caravan Battery Via Car?

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I have searched a number of forums for a clear answer on this, but I cannot find one. Presumably it is dependent on a number of factors, but if somebody could give a guide that would be great.


I know that if I connect my caravan to my car with the grey 12S cable, it will charge the caravan battery, and power the fridge AS LONG AS THE CAR IS RUNNING. I'm guessing when the car is not running, neither occurs.


I am planning on doing some trips without electric hookup, and I have no idea how much of a solution charging via the car will be. I am aware it isn't a particularly efficient solution, but how many amps is typically put into the leisure battery, compared to a standard charger, when running the car?


Most chargers seem to range from around 4amps to 8amps, and some can be as much as 12 amps, but these are less common. Any more than that seems to be very expensive.


I can run an 8amp charger off my car, using a 150W inverter (with the engine running to avid flattening the car battery) but I have no idea if this is worthwhile, compared to simply plugging into the car 12S


Can anybody offer me any steer?


Thanks in advance,

Edited by garethdphillips
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The charge rate to the caravan battery will depend on the cars alternator output voltage and current, with a smart charging alternator this can be variable.


Assuming then that you are getting sufficient volts from the car, then the wiring inside the car and caravan to the caravan battery has to be of a sufficient size to avoid excessive voltage drop, assuming the wiring has been chosen wisely then some voltage will reach the caravans battery, I usually see over 14 volts across mine if the alternator is giving out say 14. 7 volts for a figure, and the caravan battery isn't taking too much current.


Mine never does take a lot because its fully charged prior at home or on site. So the charge only has to put back whatever the motor mover has taken out.

Edited by xtrailman
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This is something that I will be checking on Wednesday when the van goes in for its first service. Once I am hooked up I will check the charging rate with the car engine running, idle and stopped. Hopefully I will be able to give some figures and info then.


In the meantime I expect someone will come along shortly with all the information. :)

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and :welcome: to Caravan Talk


Unless otherwise stated all posts are my personal opinion 

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I have tried many times to charge two batteries from a single alternator. The point is at about 12. 8 volt the battery will take nearly no charge at all. But at 13. 8 volt it will charge reasonable well. At 14. 8 volts it could over charge it the battery is within 90% of being fully charged. Between these we have just 2 volts. 6mm cable will lose 7. 3mV/A/M so at 12A and 4 meters is 0. 35 volts when we only have 2 volts to start with as we add cable length to get power into the caravan very quickly you get to the point where the charge rate drops to a couple of amps and a 120Ah battery would take 60 hours to charge and we don't drive for that long.


So to extend the time before it fails split charging does work but it is not enough to keep the battery fully charged. However reduce the cable length and there is more of a chance so fit the battery in the boot and now you may have some success. Also of course it is being charged as you go for a day out without the caravan. Of course assuming no special energy saving built into the car.


The same problem exists with boats and now you can buy special 12v to 12v chargers which take the power and turn it into AC transform it up and then turn it back to DC to charge the battery.


The idea of an inverter powering a battery charger is really the same thing but two big draw backs. With a simulated sin wave inverter the wave form is clipped to a point where likely the charger will fail to work unless it is also a switch mode charger and with true sin wave it will overload the FET's that give the peak voltage and burn out the inverter this has already been tried and failed.


So next has to be cost a 12v to 12v inverter is not cheap and it may mean flat car battery if it takes too much. Durite and Sterling both make the units. So next is using a generator and from these pages and what has happened it would seem this is the best option.


Years ago with gas lights and foot operated water pump using a battery charged from the car worked but today we are just asking too much. Clearly LED lights and not too many and no central heating fans or motor mover then you may scrape by with solar panels but most vans now draw too much. Read http://www. caravantalk. co. uk/community/topic/90570-ring-rinv1200p-pure-sine-wave-inverter/ and see what has already been tried and learn from others mistakes.

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Years ago with gas lights and foot operated water pump using a battery charged from the car worked but today we are just asking too much. Clearly LED lights and not too many and no central heating fans or motor mover then you may scrape by with solar panels but most vans now draw too much. Read http://www. caravanta. ..-wave-inverter/ and see what has already been tried and learn from others mistakes.



We seem to "scrape by" fine thanks with just a solar panel thanks. The OP in your link doesn't mention what size panel he is using which does make a difference. Sometimes there seems to be a lot of theorising and not so much practical experience.


The OP hit the nail on the head when he said " Presumably it is dependent on a number of factors"

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Funny that I'm facing the same challenges as you right now. I've been here in Hope Valley for the last three weeks with no EHU. The thread ericmark linked to above is about my failed attempts at installing a proper inverter in my car. To be fair, the solution might well be sound but the quality of the inverter from Ring Automotive is suspect in my opinion.

I would suggest there are four options.

1. Towing electrics. I have no experience of the 12N/12S setup but I'm guessing they function the same as the 13-pin, which means that whilst you can charge your battery, the habitation relay will disconnect all other electrics so you can't 'live' in the van. A variation would be to connect jump leads from car battery to caravan battery, which would allow you to live in the van. ericmark has highlighted the voltage drop issue and with the variability of charging voltages it's difficult to say with any confidence just how efficient this charging method would be.

2. Alternative energy sources. Solar, wind turbine, hydrogen fuel cell all have their place but as the OP mentioned using a car, I'll not discuss these further.

3. Car connected inverter. There are two methods to use an inverter:
3i. Power a charger. Connecting the inverter output to a standard battery charger would give you a known charging time and, if it's a smart charger (stage charger) then this will likely be the quickest method of charging. A full charge starting at 8A would likely take 3-5 hours.
3ii. Power the EHU. Connecting the inverter output to your EHU hookup basically gives you a portable EHU. The 'vans charger will charge the battery and you can use the mains sockets to power up to the maximum output limit of the inverter.

This is the method I have chosen.

Right now I have one of those backup battery packs in the car, now it's actually a bit rubbish (long story) but it does have an inbuilt 300W modified sine wave inverter. Every couple of days I 'top up' by connecting the inverter to my EHU via the standard EHU cable. I've found that my geek toys (laptop, bluetooth speaker, camera charger etc) are actually quite a drain so when the inverter is connected I also charge a couple of Goal Zero Sherpa 50 battery packs. This then powers my geek stuff leaving the 'van battery just for pumps etc. I usually 'top up' for about two hours and this usually takes my battery from around 30% to 80% charged. Of course, leaving your car idling for two hours isn't a good thing. ..

Btw, a 150W inverter just isn’t going to cut it. Your van’s power supply / charger is likely rated at 20A, at 12V that’s 240W, then add losses in the mix…My 300W inverter often cuts out when the battery is really flat – I have to switch everything else off until it’s charged a bit.

I attempted to ‘upgrade’ my inverter to a 1. 2KW pure sine wave inverter and you can read all about that messy chapter in my life in the thread linked above.

4. Leisure generator. This would undoubtedly be the simplest and most reliable option and would avoid idling your car for long periods. Just start it, connect the output to your EHU. Depending on the output capacity, you would very likely have a full EHU service. I've heard good things about the Hyundai suitcase generators e. g. HY3000SEI 2. 8KW genset.

My dream setup (if I could ever afford it) would be a ‘van installed 1. 8KW inverter, with the battery charged by 500W solar panels during the day and a hydrogen fuel cell at night. Any spare solar output would produce hydrogen for the fuel cell via electrolysis of water. I really dislike fossil fuels. ..

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Thanks all, for the detailed replies.


I like the idea of running the caravan EHU, I had already considered this, as it will obviously be far easier to have the option of using the caravans 240v sockets, rather than trailing cables. To be honest I hadnt thought about turning the ZIG charger on (it's al old 1993 caravan) as I assumed this would be only a slow charge compared to my heavy duty charger, which I believe can go up to 12amps.


the 150W inverter appears to run the 12amp charger OK, it has low and high (which I think are 8 and 12) but on either setting the inverter doesn't appear to complain. The battery was near full though (the battery I was charging), I guess I will see a different behaviour if the charge is low? - but then I have the option of setting the charger to low?


So could anybody tell me which is best in order of speed of charge?


I'm guessing:


heavy duty charger via inverter (if 150W is OK)

Caravan ZIG (if the 150W inverter will run it, unsure what the rating on the old units is)

12S cable from car

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In the end you've got to suck-it and see. Get yourself a 20-25amp meter and put it in series with the van battery and try all these options out. A 150watt inverter might be ok if the van battery isn't discharged but try charging a flat one and see whether it complains or dies


If charging from the car via the 12s don't forget that some of what you are hoping to get into the battery will also be running the fridge if it's on - most fridges will be taking 8-10 amps, you will also be charging the car battery and powering any other electrical devices whilst you are driving, so it doesn't leave all that much spare for the van battery.


Try putting a multimeter across the battery (on the 20 VOLT range) and measuring the violtage when the car is plugged in and the engine is running at about 2000rpm (driving speed), you need to measure about 13. 5 volts or more to get any sort of charge into the battery unless it's nearly flat

Edited by matelodave

2018 S-Max Titanium 2. 0 Tdci (177. 54bhp,180ps,132kw) Powershift + 2015 Unicorn III Cadz, Ventura Marlin porch awning

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The charge rate will also depend on the sophistication of the caravan's charge controller.

The Schaudt CSV 409 A unit I have has a quoted towing vehicle input charging rate of 8 Amps.

It is sophisticated in that it actually lifts any voltage supplied from the car [i recall from 11 VDC ?] to present power to the caravan's battery on a smart charging routine at up to 14. 3 VDC.


I am not up to speed with the more sophisticated offering now fitted to British built vans.


I used to transfer power on site from my car to caravan's battery with a small inverter in the car, over a 230V AC extension lead to a 3. 6 Amp CTEK charger coupled directly to the van's battery. It was managed by me logging the time so I could be confident of the car starting. It works and transmitting as 230 VAC minimises losses whilst the CTEK's smart charging optimises the charge level. In those days the TV we had was piggybacked off the AC; running one off DC with a 14. 3 VDC charge might upset TVs not specifically designed for automotive use.

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I am not familiar with these ZIG chargers, I’m guessing they function similarly to modern units in that they provide what is essentially a 13. 8V power supply that supplies DC services and with whatever is left charges the battery up to a specified current limit (mine is 20A). If that is the case, you might find it charges a flat battery more quickly early on than a smart charger kicking out a staged 8 or so amps but as the battery charges, its voltage increases and because the standard charger is voltage limited to 13. 8V, the charging current quickly reduces. That’s why generally speaking a smart charger charges faster but does not push more or less energy into your battery than your caravan’s float charger given time.

As matelodave says, unless you’re planning to go high-tech and start measuring things, probably best to suck-it-and-see. Personally, I like the convenience of plugging my inverter into the EHU because I can then use the ‘van sockets for others things and charge the leisure battery.

If your priority is the fastest method of charging your leisure battery then a smart charger (staged charger) with the biggest output you can afford and is supported by your inverter would be number one option. For example, Sterling Power do a 20A smart charger, I would suggest this would require at least a 300W inverter. CTEK do a 25A smart charger. A 25A bulk charge would take your battery (assuming 110AH) from flat to 80% charged in around three hours.


Number 2 would be powering your EHU (and, therefore, charger). Whilst this method might not be fastest, for me it adds convenience.


Number 3 would be towing electrics.

I’ve not considered the genny in this list.


For info only, a standard smart charger (staged charger) operates in three main stages. Stage 1: Bulk charge, constant current (e. g. 8A), voltage sensing. Stage 2: Equalisation charge, at 14. 2V switches to constant voltage, current sensing, reducing charging current. Stage 3: Float charge, at 2A switches to 13. 8V float charge. Smart chargers dump as much energy as possible as quickly as possible into the battery whilst also protecting from gassing (generally considered to be 2. 4V per cell, or 14. 4V overall).

NB there can be more stages (e. g. CTEK chargers have 8) but these are bells and whistles built on the above three fundamental stages.

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Just had a thought, there is a hybrid Option 3 (Car connected inverter), which I am currently testing. You can plug the inverter from the car into the EHU, then disconnect the battery via its fuse, then plug your stage charger into one of the internal mains outlets. This allows you to make use of all your internal mains sockets AND have full DC services AND charge the battery as quickly as possible using a smart charger! Sweet!


It seems to be working so far, it's already in Stage 2 after just 30 mins. Mind you, it was about 50% charged to start with.


p. s. It's important to pull the battery fuse because the additional loads from internal DC devices (e. g pumps) will confuse the stage charger (lower the voltage it is sensing from the battery) and divert power away from charging the battery.

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To shave a little of the faffing around off you could fit an amperor charger like this one http://www. amperorassociates. co. uk/c-battery_chargers/BatteryC-3_25A. html . rovertug and I have both replaced our bailey standard chargers with this unit, it has a 25a 3 stage charger and a float mode, all I have to do is flick a switch to go between the two modes - no plugging it into the mains and trailing leads to the battery.


All seems highly Eco unfriendly to me all this hanging around in fields with 3 litre cars running to charge our batteries!


Given the price of a decent inverter - (tictag you'll find out how much they are when you finally buy one! LOL, couldn't help meself) the smart money has to be on an lpg genny, honda eu10i or the 20i for keen microwavers!

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Yeah, you're right. It's a stop-gap for me. Not good for the car, either! Caroline has a moan when I actually go somewhere about the DPF* being full. Smoke trailing out of a 3-year old Jag is not a good look!


I'm being irrational: if I buy the genny then fossil fuels have won! But will I ever afford my preferred green system? Maybe I should start being practical and less idealistic (I'm sure I wouldn't have said that prior to turning 40! ;))


My hybrid option 3 test went OK'ish. The bulk charge was pretty quick but stage 2 just went on and on stuck at 2. 6A, like something was actually drawing 2. 6A i. e. a load. To test I actually disconnected the battery completely from the PDU but still remained at 2. 6A. Maybe just a charging peculiarity of my leisure battery. You're right, replacing the 'vans charger with a stage charger would be more practical.


*DPF: Diesel Particulates Filter

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Yeah I'd love to go down the solar / inverter / fuel cell route but the cost is too much, given that the genny might consume 0. 25 to 0. 4 L per hour of lpg depending on load and only be used for say 2-3 hours depending on what the solar can stuff in I think it makes for a middle aged practical compromise on the wish list.

Anyway I'd better shut up cos I seem to have swerved off topic as usual.

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From what I remember the Zig unit was one of the first dedicated caravan chargers. But we have moved on since then. The idea of non fossil fuels with a caravan is really a non starter. Unless your doing a Fred Dibnah and going to tow the van with a traction engine then the electric cars are not really going to tow a caravan far enough to be really worth having a caravan in the first place.

I was against the whole idea of a caravan I felt for two we could stay in guest houses for less money than using a large tow car and caravan but I was over ruled and back to caravan was forced upon me.

Compared with the tow car any fuel used with a generator is just not worth counting.

I have looked at many methods to get free power including building my own wind charger but step one has to be reduce power used so rip out all the electric lights and back to gas.

To to be honest the old caravan had a real problem and that was heating. Although we had a gas fire the early models without electric were non flue and the caravan got soaking wet. Yes we went away for a week in the winter but after all bedding had to be stripped off and taken in the house to dry.

Narrow boat faired a little better that had a Rayburn with a flue but the amount of wood it burnt was silly. The same applies to wood burners in the house to run efficient they have to burn at a set rate. Too low of a burn rate and particular emissions and tar problems too high and inefficient and so only way to use in an ecological manner is with a massive heat store.


So the options are either use sites with electric hookup or use a generator or go super ecological and guest houses. Like using a bus guest house is heated anyway so guests have no real effect on the fuel bill.

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