Charging caravan battery with no mains.

[ericmark]

I want to charge a leisure battery from the car to power items in the caravan. Although only two towing cars in the family there are many more cars of people who may want to use the caravan so it would be an advantage to be able to fit the charger in a range of cars the cig lighter socket would be favourite. The unit I found is rather expensive at £100 approx, and it is not really designed for the cig lighter socket. It has two live feeds from donor battery one for power and other sensing battery voltage, and one negative ground which means it needs a really good ground to ensure as it switches on the volt drop will not turn it straight off again.

Clearly if the battery became flat one could have two crocodile clips and run the car at fast tick over to re-charge battery. However you could just as easy use jumper leads with same results. The wiring diagram is shown below.



There is clearly a balance between cost of unit and performance and 12A does seem to be reasonable if one leaves the caravan to visit local attractions likely one will have engine running for an hour so that's 12AH which should be enough to keep water pump and one light working assuming fully charged to start with for a month. I use a solar panel to stop self discharge problems but to get one large enough to keep battery topped up when parked under trees is rather expensive. At the moment the solar panel is direct vie internal diode with no regulations at 0.1A not a problem but as the size increases then one also needs a PWM or MPPT regulator and this also has problems with battery charging when the battery is also used.

I tried charging batteries on a narrow boat and realised there is a problem when the battery is used as the battery voltage does not only raise and fall due to charge rate but also due to usage and with the heater on the current did not drop below the 5A needed to switch the charger into float and the battery voltage was held high until the timer switched to float and then would under charge until it reached the reset voltage and restarted the cycle.

So only way was to have two batteries one in use float charge only and one on standby on stage charge each day swapping battery being used. OK with narrow boat and 4 x 160AH batteries that was OK but don't really want two batteries in the caravan.

Although as I have said running car engine with jump leads between car and caravan battery will to start with put a reasonable charge into the slave battery but this will quickly reduce and only with a stage charger of some type will a battery be charged in a reasonable time.

My thoughts were this is not a new problem why re-invent the wheel there must be products already on the market. However my thoughts then turn to the narrow boat again. Traction batteries used in fork lifts, mobility scooters, golf trolleys etc will last for around 7 years however listen to narrow boat owners and batteries were only lasting around 2 years clearly they were sulphating due to not being fully charged where the charger was foxed due to using items off the battery while being charged.

Sterling did do some really fancy systems with a pulse charge which would work with a battery being used and charged at the same time but the price was something else.

There was an attitude we have always done it this way we know best and narrow boat owners would talk a lot about how to care for batteries but in real terms as shown with battery life they abused the batteries and killed them. Likely I will get same from caravan users although since at some time of the year the caravan will likely return home and battery charged plus removing a 60AH battery and charging is some what easier than with a 160AH battery so owners will tend to remove battery for charging in the winter.

However on this forum we have some thinkers who do not just say that's how we have always done it so that's the way to go but think outside the box.

So this year caravan was on a site with hook-up so no problem however next year likely we will be looking at a cheaper site without a hook-up and the family will take it in turns using the caravan so looking for comments on what is available and cost effective ways to maintain a battery in a usable state.

What was wrong with gas lights and foot or hand operated water pump? Years ago the battery was not required it just made life easy. Today no battery means no lights, no water, and no heat even the fridge and cooker use the battery to light the gas.

 

[bernardgreen]

What was wrong with gas lights and foot or hand operated water pump? Years ago the battery was not required it just made life easy. Today no battery means no lights, no water, and no heat even the fridge and cooker use the battery to light the gas.

People have it too easy these days.

The problem I see with charging from a car that is not normally used with the caravan is a lot of temporary connections being made and removed with the inherent risks if someone does it wrong.

There are small chargers powered by propane gas and using non flame fuel cell technology ( sorry I could not see any maker's name on them ).

 

[ban-all-sheds]

it would be an advantage to be able to fit the charger in a range of cars the cig lighter socket would be favourite.

Be aware that many of the "cigarette lighter sockets" in cars these days are nothing of the sort. They are physically the same (as it's a de-facto standard) but are actually 12V outlets designed to be used to power small accessories, USB charger converters etc, and cannot deliver anywhere near the amount of current you'd need to charge a battery.

And even if the socket was originally fitted by the maker with a lighter plug rather than a plastic blank, I'd be wary of the ability of it, and the wiring to it, to safely cope with high currents for extended periods - the design point for a lighter, remember, is for 10 amps or so for just a few seconds.

 

[JohnW2]

Be aware that many of the "cigarette lighter sockets" in cars these days are nothing of the sort. They are physically the same (as it's a de-facto standard) but are actually 12V outlets designed to be used to power small accessories, USB charger converters etc, and cannot deliver anywhere near the amount of current you'd need to charge a battery.

Indeed, and I've been told (don't know if it's true) that some of these sockets are now protected by pretty modestly-rated fuses.

Kind Regards, John

 

[stillp]

Be aware that many of the "cigarette lighter sockets" in cars these days are nothing of the sort. They are physically the same (as it's a de-facto standard) but are actually 12V outlets designed to be used to power small accessories, USB charger converters etc, and cannot deliver anywhere near the amount of current you'd need to charge a battery.

And even if the socket was originally fitted by the maker with a lighter plug rather than a plastic blank, I'd be wary of the ability of it, and the wiring to it, to safely cope with high currents for extended periods - the design point for a lighter, remember, is for 10 amps or so for just a few seconds.

Interesting point. I'm off to check the current requirement for my cool box.

 

[ericmark]

There is a second reason for wanting to charge from the car. My mother in a wheel chair has three electric ones two have reasonably big batteries and have around a 15 mile range but the light to transport one is pedestrian controlled with a 20AH battery. I would also like to charge that battery in the car. Also would help with RAYNET exercises.

My cig lighter is fused to 16A the same with my wife's car and yes since you would need to climb in the boot to access one of them clearly a power supply which she uses for the phone.

£90 for a charger is a little steep and I would think getting an 8A model would be good enough and as you say dropping from 12A to 8A would be more in keeping with the supply to a cig socket.

However the problem is the cut off point and the one I found clearly is designed to be hard wired I have run a 150W inverter for an extended time however I have never monitored how much power it used. In the main to power a PC. I had thought of just plugging a small battery charger into the inverter. Clearly that would not be a stage battery charger.

I did wonder before returning to caravanning why it needed a 40AH battery (A721.55.3.2) now I see why. I do note regulations state max volt drop of 0.3 volt but think my caravan was manufactured before that was a regulation. How they can give a volt drop when there is no current rating I do not know.

Assuming a charge time of 1 hour i.e. half hour from caravan and a 40AH battery the calculation given in the regs (A721.525) says I need a 4A charging current. Can't really see how that is enough but that it would seem is the charging current used to work out volt drop.

What has gone through my mind is the PWM or MPPT regulator are basic switch more power supplies and I wonder if they can be used to transfer power from another battery as well as from a solar panel or wind charger? The problem would be to limit the power taken from the donor battery. I also note with a MPPT regulator the load is taken from the regulator not direct from battery so it could charge without the load upsetting the sequence.