Charging a battery using the caravan socket

[rojellis]

I have a solar power unit (with large battery) that needs a top-up from my car - rather like a caravan leisure battery.

I would like to do this from my caravan socket on my car. However, I cannot find a connector to buy, and I don’t understand the caravan wiring diagrams. The diagrams appear to show the car connected directly to the caravan battery, but surely that would draw far too many amps (if the battery was flat) and blow the car’s fuse. Would I not need a resistor or regulator in the circuit, to limit the amps to about 25 or 20?

Any info greatly received.

Thanks
RE

 

[ProfJohnL]

Hello rojellis,

And welcome to the forum.

You have posted an intriguing question.

The circumstances you have given us with your large battery and an uncertain level of charge are a bit unknown, so I can't gve a definative answer. But I can't remember if we have ever had a question about blown charging fuses excepte where someone has done something silly. like shorting or reversing a battery.

It is presumed that when a 12V lead acid batteries terminal voltage drops to 12.7V it is effectively discharged. If by any chance your batter is less than this, it is severly discharged and may be damaged.

Alternators peak charging voltages should be no more that 14.5V. the charging potential difference is therefore just 1.8V. If that gave rise to a current of more than 20A that would mean the total resistance of wire and the batteries internal resistance would be less than 1.8V / 20A = 0.09R Which in reality is unlikely.

I don't think you should have a fuse problem, but you may have a battery problem.
.

 

[rojellis]

Thanks for the answer, but I am not sure this allays my fears. Car jump-leads are huge, because the car battery has the potential to deliver 150 amps to a flat battery. So what is to stop that happening via the caravan socket?

Yet this must happen every day in caravaning. Your caravan battery is flat, so you connect it to the car and charge it up. So what is to stop the car battery passing 100 amps down the caravan socket and blowing the fuse??

I looked at the wiring diagrams and was confused. You appear to be joining the car battery to a leisure battery with a 30 amp wire - and yet you do not set this wire glowing or blow the fuse. But I am not sure why.

RE

 

[Icaru5]

Hello Rojellis,

It seems that you may be confusing charging current with the cold cranking current absorbed by a starter motor when trying to start a car’s engine.

To correct your train of thought in terms of electrical theory, it’s not about the current that the power source has the potential to deliver to the load (edit: “potential” is the wrong word to use in this context !), but rather about the current that that the load draws from the power source, and subsequently, as a charging current, a flat battery does not have the potential to draw 150A from a fully charged one !

The figures and calculations that the Prof has given you are in line with typical battery charging scenarios, but the act of jump starting a car, which you have now introduced into the scenario, and hence the purpose and cable size of jump leads is a different thing altogether.

To refer back to your initial question which was regarding the “top up” of the battery that forms part of your solar power unit, I really don’t think you need to be concerned about excessive current flow into the partially depleted battery. Unless your solar power unit has a significant quantity of batteries resulting in an unusually high charging capacity for a solar power unit intended for leisure purposes, the circuits and fuses which form part of a correctly wired vehicles towbar electrics will be more than adequate to deliver the required current, irrespective of the state of charge of the battery.

 

[ericmark]

The 150 amp you talk about is not battery to battery but batteries to starter motor. So battery to battery max voltage is around 12.8 and min 11.8 so one volt connect the two together maybe 10 amp for a few seconds but then down to around 1 amp or less. As donor battery voltage quickly drops and slave battery voltage rises.

So in real terms it is the alternator that pushes the amps into the slave battery so max amps is output of the alternator not much to do with donor battery volts. So yes could be 80 amp. However to get 80A the cables must have a rather low resistance and with a standard regulator set to 13.8 volt again the donor battery volts will soon rise, so within a couple on minutes it will drop.

Much depends on battery size, fork lift battery may take 100 amp for 4 hours, but a 110 Ah caravan battery is only a very small battery as batteries go, and I found using a stage charger starting at 3.8 amp within 5 minutes down to 3 amp, and it will hold that for around 30 hours.

The larger stage chargers use higher voltages 14.4 for VRLA and 14.8 for open cells, so a typical 35 amp charger will charge 10% to 80% 110 Ah battery in 2 1/2 hours but to get the last 20% will take around 6 hours as it is that last 20% which takes the time.

So with a split charging system a DC to DC inverter will even at 10A rating will take around 8 hours to charge from 10% to 80% and then another 6 hours to get last 20% so around 14 hours to fully charge, problem is you may run a narrow boat for 14 hours, but not a car towing a caravan, so it will only get to 80% in the main, and unless you recharge fully once every two weeks, it will lose capacity slowly so soon well under 110 Ah.

So if you boost a battery first from your car, then connect to solar panel it may fully recharge in a day, but unlikely other way around because it takes so long 80% to 100%.

So this one will charge at 10A and for 12 volt up to 50A are available. These will charge faster than a split charging relay or diode, and you select charge rate, but cable length is not critical, but with a split charging relay or diode then it is the length of cable that limits the charge rate.

 

[ProfJohnL]

So in real terms it is the alternator that pushes the amps into the slave battery so max amps is output of the alternator not much to do with donor battery volts. So yes could be 80 amp. However to get 80A the cables must have a rather low resistance and with a standard regulator set to 13.8 volt again the donor battery volts will soon rise, so within a couple on minutes it will drop.....

I really couldn't follow this paragraph, can you try again please?

 

[ericmark]

Yes in real terms it's the alternator that pushes 80A into the slave/domestic battery. With a narrow boat the regulator may be changed for a stage charging regulator on the second alternator or a alternator to battery inverter used so on start up three 160 Ah batteries used for domestic supply can get 160 amp charge for first hour, clearly pushing this through a 13 pin caravan plug and socket is not really an option.

With the old Lucas 18 ACR alternators the Durite rectifier pack had the same size diodes for the warning lamp as main output, so with a machine sensed regulator one could add three smaller diodes for the warning light and field current and use the original warning light supply to charge a second battery, this worked very well, charging the second battery at 35 amp if required.

However using something like 25 mm cables really designed as welding cables, and forklift plug and socket for jump leads, this was how my auto electricians van was wired with moveable link bars so I could jump start 24 volt wagons.

There are two things which limit the current to caravan battery, one is resistance of cables, and other is size of caravan battery. The leisure battery will not accept power as fast as a VRLA used to start modern stop/start cars, and 2.5 mm cable has a resistance of 0.036Ω per meter so 10 meters through car and caravan well more as return wired in caravan not chassis so looking at a 0.4Ω resistance at 13.8 - 12 = 1.8 volts difference between batteries gives a charge rate of 4.5 amp, some alternators have a higher voltage output and a battery could start under 12 volt so at start up you may get 10 amp, until the battery voltage raises a bit.

However in the case of a battery in a car boot, with 16 mm cables much shorter and no 13 pin socket one could get 60 amp, and with a battery connected to a 13 pin plug with just 1 meter of cable sitting at rear of car and the socket has just on meter of cable to prime mover battery in boot then you may get 60 amp, so in this case, we could have a problem.

The DC to DC battery charging inverter is the answer, fitted in the caravan a 20 amp version will overcome volt drop and ensure a 3 hour trip will pump 60 Ah into a 110 Ah battery until it gets to 80% charged state, and will also stop over 20 amp passing through cables.in correct direction, and remove need for any relay or blocking diode in the car to stop reverse current. Even a 10 amp version is likely better than a split charging relay, and not affected by stop/start technology or engine management turning off charging under load, No need to run around with head lights on.

The problem is the control is switched from car to caravan, so a car set to tow a caravan with DC to DC inverter fitted if used with old caravan without the inverter fitted could allow start current to flow caravan to car, so belt and bracer's is fit a schottky blocking diode in the car.

However to date it seems wiring of caravan charging systems has not caught up with today's technology so voltage dependent relays are still fitted in cars instead of the schottky blocking diode so as the engine management switches off alternator caravan battery charging and more important the anti snake devices are switched off.

I am I am sure the same as any other caravanner, I did not want to fit my own tow bar, I got it fitted before I took delivery of the car, As a result not a clue how its wired up. And BS7671 does not stipulate use of voltage dependent relays or schottky blocking diodes so if the installer has not fitted them I can hardly complain. To be on save side however my caravan has a charging fuse, so worst case is that fuse will rupture. I think from memory around 16A so since to date has not blown, clearly less than 16A in either direction flows.

 

[ProfJohnL]

Im sorry Eric,

I have read and re read your comment and futher response and it still does not make sence. Particularly in the context of the OP's question. I need to repost the part of your comment which is directly at odds with the matter in hand.

"So in real terms it is the alternator that pushes the amps into the slave battery so max amps is output of the alternator not much to do with donor battery volts. So yes could be 80 amp. However to get 80A the cables must have a rather low resistance and with a standard regulator set to 13.8 volt again the donor battery volts will soon rise, so within a couple on minutes it will drop....."

You can't have it both ways. Most car alternators have their voltages regulatated to about 14.5 V. This is to protect the cars electrics and electronics for over voltage damage. The output to a battery IS HIGHLY dependant on the standing voltage of the battery connected across the alternator. The battery's standing voltage reduces the potential difference which in turn through basic V=IR calculations reduces the available current to charge the battery. If the alternators output was limited only 13.8V the currents involved would be proportionally smaller.

In your 80A alternator scenario the maximum current to a discharged battery would be (when you take into account the resitances and battery voltage ) in the order of 10A and on a voltage regulated alternator that current would drop as the battery takes more charge and produces a larger standing voltage. The uncertanty for rojellis, arises from the unknown state of the storage battery. But it is unlikley to take enough charge current to blow a 20A charging fuse.

On that basis rojellis, has nothing to worry about.

 

[rojellis]

Thanks for your answers.
I understand now....

RE

 

[Gafferbill]

......I go off grid and I don't have or want a solar panel.
I have the standard 13 pin setup which will charge the caravan leisure battery when towing.

For charging when pitched I use a 600 watt pure sine inverter to produce mains voltage from the car battery.
The inverter is fitted with short leads as close to the car battery as possible when in use and packed away when not required.
I made up a connecting lead to feed this mains supply to the caravan using the normal socket.
I then use the caravan charger to charge the leisure battery.
It is essential to be able to monitor the car battery voltage to make sure you are always able to start the car.
For this I use a simple digital voltage gauge that plugs into the car's cigarette lighter socket.
System has worked well for me over many years and is dependant on having a car battery in good condition.

 

[Anseo]

......I go off grid and I don't have or want a solar panel.
I have the standard 13 pin setup which will charge the caravan leisure battery when towing.

For charging when pitched I use a 600 watt pure sine inverter to produce mains voltage from the car battery.
The inverter is fitted with short leads as close to the car battery as possible when in use and packed away when not required.
I made up a connecting lead to feed this mains supply to the caravan using the normal socket.
I then use the caravan charger to charge the leisure battery.
It is essential to be able to monitor the car battery voltage to make sure you are always able to start the car.
For this I use a simple digital voltage gauge that plugs into the car's cigarette lighter socket.
System has worked well for me over many years and is dependant on having a car battery in good condition.

Really interesting post Bill.

Can you give an indication of what electrical itemsduration you can use and how many days you can stay off grid with your method.

I will assume the car battery is topped up by the car alternator during your trips off site, would love to hear more on this, thanks.

 

[Gafferbill]

Hi John .....I only use the system I described for charging the cararavan battery or powering a 500 watt hair dryer that my wife may use when off grid.
Everything else is powered by 12 volts or gas.
I can stay off grid indefinitely. ....it relies on using your car to recharge it's battery so diesel is required
I have other systems I can show you in September but they are bespoke to my car but may be of interest.

 

[Anseo]

Hi John .....I only use the system I described for charging the cararavan battery or powering a 500 watt hair dryer that my wife may use when off grid.
Everything else is powered by 12 volts or gas.
I can stay off grid indefinitely. ....it relies on using your car to recharge it's battery so diesel is required
I have other systems I can show you in September but they are bespoke to my car but may be of interest.

I am all ears Bill, thank you.

I would love to know the systems you utilise.

See you September

 

[ProfJohnL]

......I go off grid and I don't have or want a solar panel.
I have the standard 13 pin setup which will charge the caravan leisure battery when towing.

For charging when pitched I use a 600 watt pure sine inverter to produce mains voltage from the car battery.
The inverter is fitted with short leads as close to the car battery as possible when in use and packed away when not required.
I made up a connecting lead to feed this mains supply to the caravan using the normal socket.
I then use the caravan charger to charge the leisure battery.
It is essential to be able to monitor the car battery voltage to make sure you are always able to start the car.
For this I use a simple digital voltage gauge that plugs into the car's cigarette lighter socket.
System has worked well for me over many years and is dependant on having a car battery in good condition.

Hello Bill,

It is of course your choice how you do it, but just out of interest do you have a reason or objection to solar panels. Or is it simply with your system you don't need one?

 

[Gafferbill]

................or is it simply with your system you don't need one?

.......I don't need one and the output claims made for a panel are always quoted as the maximum possible.
Rarely achieved in practice as the panel has to be spotlessly clean....not in shadow and angled towards the Sun so that light rays are perpendicular to the surface.
Many people do use solar panels as systems are widely marketed whereas the use of the tow vehicles excellent alternator charging system is strangely overlooked.

I store my caravan at home so have access to EHU and my caravan use is around 100 days / year with around 25 off grid.
The general perception is that off grid is "roughing it".......all I can say is that coupled with the use of gas and using my system to provide a plentiful supply of 12 volts......... then it isn't

There are some very good MH&CC and C&CC local groups that hold well organised rallies in some great locations.....nearly all off grid and often around £10/day.