My Renault has a cigarette lighter type point in the rear passenger well. It has a 120 watt maximum output. I dont have a plug on the towbar to charge the caravan battery, so I was wondering if I can make use of this to do the same. Making up a cable is not a problem. Any info is gratefully accepted. thanks everyone.
Charging Caravan battery from accesory pointin car
- Thread starter raccoon
- Start date
- Mar 14, 2005
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Hi Racoon,
There are ways of arranging to charge a battery in the boot of a car,
There are necessary safety issues to be considered:-
A battery is a heavy item and it must be secured to prevent it from moving about whilst driving.
Whilst charging it may give of hydrogen gas, so it needs to be in a sealed container that is vented to the outside.
There is more than enough power in a battery to melt the standard wiring used in cars if a short circuit occurs, so any cables must be fused (both positive and negative).
By the same token, the charging currents can be quite high and although the accessory socket may be rated to 120W (10A @12V) the quality of the electrical contact they make is often poor, 10A may well start to heat up the plug and socket enough to melt plastic parts leading to a short circuit and possible fire etc.
Most accessory sockets are only switched by the ignition key. This means that they can be powered up without the engine actually running. This is not good news, as if you connect a spare battery to the socket, it will back feed and keep all the 'accessory' items powered up even when the the key is removed. To prevent this the socket would have to be rewired with a split charge relay which will isolate the battery completely unless the alternator is producing power.
So for the reasons above, an accessory socket is not a suitable choice to charge a battery. The best way is arrange to pick up the caravan charging circuit from your tow-bar socket with a plug and cable brought in through the soft boot lid seal - Don't forget to fuse both positive and negative leads.
There are ways of arranging to charge a battery in the boot of a car,
There are necessary safety issues to be considered:-
A battery is a heavy item and it must be secured to prevent it from moving about whilst driving.
Whilst charging it may give of hydrogen gas, so it needs to be in a sealed container that is vented to the outside.
There is more than enough power in a battery to melt the standard wiring used in cars if a short circuit occurs, so any cables must be fused (both positive and negative).
By the same token, the charging currents can be quite high and although the accessory socket may be rated to 120W (10A @12V) the quality of the electrical contact they make is often poor, 10A may well start to heat up the plug and socket enough to melt plastic parts leading to a short circuit and possible fire etc.
Most accessory sockets are only switched by the ignition key. This means that they can be powered up without the engine actually running. This is not good news, as if you connect a spare battery to the socket, it will back feed and keep all the 'accessory' items powered up even when the the key is removed. To prevent this the socket would have to be rewired with a split charge relay which will isolate the battery completely unless the alternator is producing power.
So for the reasons above, an accessory socket is not a suitable choice to charge a battery. The best way is arrange to pick up the caravan charging circuit from your tow-bar socket with a plug and cable brought in through the soft boot lid seal - Don't forget to fuse both positive and negative leads.
- Mar 10, 2006
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John L
While technically correct, no car or caravan has the neutral/negative leg fused.
In industrial dc applications both legs are fused.
Even the DC aspect of your home house alarm will only see the +ve leg fused. I believe it is due to the voltage being 12v.
The 24v dc systems and above i worked on had both legs fused.
I would run straight from the battery with 25amp cable, fuse it at the battery with a 20amp fuse, put this through the split charger. And fit a 12s socket.
While technically correct, no car or caravan has the neutral/negative leg fused.
In industrial dc applications both legs are fused.
Even the DC aspect of your home house alarm will only see the +ve leg fused. I believe it is due to the voltage being 12v.
The 24v dc systems and above i worked on had both legs fused.
I would run straight from the battery with 25amp cable, fuse it at the battery with a 20amp fuse, put this through the split charger. And fit a 12s socket.
- Mar 14, 2005
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Hello Ray,
On a fixed installation where the leads are not likely to reconnected regularly single sided fusing is usually adequate, but where the leads are frequently reconnected, the chances of making a reversed polarity connection are greater.
There is also the possibility that when the charging system is not being used, the leads may accidentally dangle and short out. By fusing both +/- leads the wiring in the rest of car is protected.
Incidentally back the early 1980's there were some caravan fridge installations that were not properly connected, The 12V system was relying on the gas pipe for the negative return. as a result the 12v wiring on the water heater burnt out as that completed the 12V circuit for the fridges. As a direct result of that and to protect their products Carver included a -ve supply fuse.
Changes to the the NCC codes of practice on caravan wiring has prevent further issue of this nature.
On a fixed installation where the leads are not likely to reconnected regularly single sided fusing is usually adequate, but where the leads are frequently reconnected, the chances of making a reversed polarity connection are greater.
There is also the possibility that when the charging system is not being used, the leads may accidentally dangle and short out. By fusing both +/- leads the wiring in the rest of car is protected.
Incidentally back the early 1980's there were some caravan fridge installations that were not properly connected, The 12V system was relying on the gas pipe for the negative return. as a result the 12v wiring on the water heater burnt out as that completed the 12V circuit for the fridges. As a direct result of that and to protect their products Carver included a -ve supply fuse.
Changes to the the NCC codes of practice on caravan wiring has prevent further issue of this nature.
Hi John and Ray, thanks for your input. The battery is in the caravan. It is in a compartment that has ventilation so its not a problem. I can make a circuit up with a relay to make sure no power is running back to the car from the caravan battery. The cable that I would make up would have the relay sitting in a box in a little cubby hole with the cable running out to the tow bar where the lead from the caravan will plug in.
I know it sounds like a lot of work, but I have the parts and dont want to pay out the
I know it sounds like a lot of work, but I have the parts and dont want to pay out the
- Mar 10, 2006
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Hello John L
What you are saying makes no sense.
If you want to protect against reverse polarity, you would fit a diode across the supply. I did this when i make my burglar alarm in the 80's.
Fusing both legs does not add any extra protection.
Regarding your comments re the carver fridge, if the wiring burnt out , it could only be as a result of the wiring being rated lower than the fuse, obviously using a pipe as a return is not advisable while it is carrying gas.
To get a fault current you need a circuit, will a fuse in one leg only, the fuse will interrupt the circuit.
No circuit = no fault current.
What you are saying makes no sense.
If you want to protect against reverse polarity, you would fit a diode across the supply. I did this when i make my burglar alarm in the 80's.
Fusing both legs does not add any extra protection.
Regarding your comments re the carver fridge, if the wiring burnt out , it could only be as a result of the wiring being rated lower than the fuse, obviously using a pipe as a return is not advisable while it is carrying gas.
To get a fault current you need a circuit, will a fuse in one leg only, the fuse will interrupt the circuit.
No circuit = no fault current.
- Mar 14, 2005
- 19,174
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Hello Ray,
For normal single sided DC supplied systems a fuse in the Positive feed is usually more than sufficient. Reveres polarity protection can be afforded by the use of diode whether as a series element or as a shunt element. And in most cases would be enough for appliance wiring protection, But I consider that lead acid battery charging uses such high currents that belt and braces by fusing both positive and negative battery connection wires is a sensible precaution.
The problem that occurred with the Carver products, came to light when in 1979 a number of caravanners reported that their water heater either blew its fuse as soon as it was turned on or just failed to work at all. On return to the company, the heaters all demonstrated that their negative power supply cables had suffered excessive current, yet the positive supply fuse (1A) remained intact. So excess negative current was flowing - but where from?
Further investigations found that all the affected caravanners also reported that their fridges were not working whilst towing.
The fault was that the fridge's 12V negative supply cable was not being taken directly back to the caravan supplies common negative point, but was being connected to the chassis of the fridge in the mistaken belief that the caravan chassis was a reliable 12V negative. The result was that the negative current was using the gas pipe as conductor. The gas pipe being copper was generally an excellent conductor and it connected the water heater, and the water heater completed the negative circuit via its supply cable.
Following this Carver fitter a 1a fuse to the negative as well as the positive supply cables primarily to protect their equipment.
As subsequent change in the NCC's codes of practice ensured that proper negative returns were made for all appliances, and the neither the gas pipe or the chassis of a caravan can be used for the primary circuits. Though both should be attached to the continuous protective conductor for mains electrical safety.
For normal single sided DC supplied systems a fuse in the Positive feed is usually more than sufficient. Reveres polarity protection can be afforded by the use of diode whether as a series element or as a shunt element. And in most cases would be enough for appliance wiring protection, But I consider that lead acid battery charging uses such high currents that belt and braces by fusing both positive and negative battery connection wires is a sensible precaution.
The problem that occurred with the Carver products, came to light when in 1979 a number of caravanners reported that their water heater either blew its fuse as soon as it was turned on or just failed to work at all. On return to the company, the heaters all demonstrated that their negative power supply cables had suffered excessive current, yet the positive supply fuse (1A) remained intact. So excess negative current was flowing - but where from?
Further investigations found that all the affected caravanners also reported that their fridges were not working whilst towing.
The fault was that the fridge's 12V negative supply cable was not being taken directly back to the caravan supplies common negative point, but was being connected to the chassis of the fridge in the mistaken belief that the caravan chassis was a reliable 12V negative. The result was that the negative current was using the gas pipe as conductor. The gas pipe being copper was generally an excellent conductor and it connected the water heater, and the water heater completed the negative circuit via its supply cable.
Following this Carver fitter a 1a fuse to the negative as well as the positive supply cables primarily to protect their equipment.
As subsequent change in the NCC's codes of practice ensured that proper negative returns were made for all appliances, and the neither the gas pipe or the chassis of a caravan can be used for the primary circuits. Though both should be attached to the continuous protective conductor for mains electrical safety.
- Mar 10, 2006
- 3,274
- 47
- 20,685
John l
If you prefer you could use a bridge rectifier for reverse polarity protection, something i always do when i convert the caravan smoke alarm to run of the van battery.
I do that also because it drops the voltage down by 2x0.6 volts, depending on whether or not the diodes are silicon.
For that reason i would not use a series diode in a charging circuit. The 0.6 volt drop is the last thing you want in a charging circuit. Also the rating of the diode would be high, increasing the cost, and also a heat sink would be required.
So in practice it's a none starter.
While your history lesson makes good reading, it is a result of how things were done back in the day.
Now in the present day, there is not to my knowledge a car that is wired with double fusing, not even at the battery, nor again to my knowledge a caravan.
All neutrals common to the road lights for example are connected together inside the caravan, the fridge common is unique to the fridge supply.
Maybe carver double fuse inside he appliances, i don't know.
What i do know is putting fuses in negative legs were an electrician would not expect to find one is poor practice.
And for the diy job raccoon is about to install, completely unnecessary.
If you prefer you could use a bridge rectifier for reverse polarity protection, something i always do when i convert the caravan smoke alarm to run of the van battery.
I do that also because it drops the voltage down by 2x0.6 volts, depending on whether or not the diodes are silicon.
For that reason i would not use a series diode in a charging circuit. The 0.6 volt drop is the last thing you want in a charging circuit. Also the rating of the diode would be high, increasing the cost, and also a heat sink would be required.
So in practice it's a none starter.
While your history lesson makes good reading, it is a result of how things were done back in the day.
Now in the present day, there is not to my knowledge a car that is wired with double fusing, not even at the battery, nor again to my knowledge a caravan.
All neutrals common to the road lights for example are connected together inside the caravan, the fridge common is unique to the fridge supply.
Maybe carver double fuse inside he appliances, i don't know.
What i do know is putting fuses in negative legs were an electrician would not expect to find one is poor practice.
And for the diy job raccoon is about to install, completely unnecessary.
Gentlemen, Gentlemen,
I am just going to leave it alone now. I am going to change the car in the near future anyway. The battery is only to run the water pump and awning light every now and then. So it does last a while. So thanks for your input anyway.
cheers
I am just going to leave it alone now. I am going to change the car in the near future anyway. The battery is only to run the water pump and awning light every now and then. So it does last a while. So thanks for your input anyway.
cheers
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