Caravan battery charging from wheels
 

I recently read that caravan batteries generally only reach about 80% charge from the car. Giving this more consideration, a thought occured to me today. With self-propelled caravans being so common (i.e. a motor), why don't caravans charge their battery from their wheels (i.e. dynamo)?

I realise the motive power for caravan parking aren't designed to do this, and I realise attempting to utilse them in this way would be insanity.

Okay - forget the motors. But why don't they charge from an alternator or something coupled to a wheel? Either by belt of directly?

It's just an idea. But why not?

Morning Canis, ref.charging caravan batteries,does any body use Solar panels? also any boat/barge owners 'on board'? what are the wind driven charging units all about?? could they be utilised for van use?

Sorry,more questions than answers, regards,Keith.

I wonder if that's true or not. There ought to be enough 'headroom' in the car's charging system to top up a caravan's battery whilst the car's on the move. It's no different in reality to a split charge system used in a mobile home,

The energy wouldn't come for 'free'; the additional drag created by the alternator in charging the battery would have to be paid for in diesel/petrol by the car dragging the caravan in the first place; in addition to the cost of the alternator and control gear on the caravan.

Hey Canis

That's not as crazy as you think.

Picture the old hub dynamo's on old pedal cycles.

If the caravan wheel hubs/brake back plates were to incorporate similar.

....but then think about the fact there has to be a electrical link between car and caravan for lighting anyway making separate charging unnecessary.

Unless we make the vehicle to caravan lighting link wireless :lightbulb:...look what you've started:animal-smiley-085:

As for the 80% charge theory, my caravan battery always shows full after being towed. Maybe that's down to the quality of my car charging system

Hello Ksmi,

Yes solar panels are already fitted to many a current caravan model but as a rule they only maintain the battery at its last charged level.

...as for your wind turbine thought. What about harnessing energy from the free spinning vents you see on RSPCA van roofs...

So many ideas but then when I think about how the uk caravan industry is hell bent on building leaky lightweight vans then the less electrics inside the better.

I think it's down to the length of wire, the wire has impedance, so the available voltage at the caravan end is lower. Too low to reach a full charge, although I'll accept that with a large enough battery 80% would be quite useable.

Having said that, the source was an online webpage whose description of ohms law and wire impedance was true but their javascript calculator was wildly inaccurate, and made me look a complete @rse when I quoted it. Their maths might be rubbish, but the idea is sound; Longer wire means more impedance overall, more impedance means less volts. I think. :-/

Haha! You're quite right, energy is not free. But the energy has to be paid for in fuel no matter how it gets there, and you'd be suprised how much drag an alternator working hard puts on an engine. I've no figures on the subject, but you've probably noticed the tick-over speed drop when you couple a flat battery by jump leads.

A lot of people use solar these days. It's still rather expensive to cover a van roof, and out of my lowly budget, but it's certainly cheaper than it used to be. I would imagine on a sunny summer holiday solar would give a significant boost to a battery.

Yes, they do seem to emply a strange construction method, which leads me to my next question; Why don't them make them out of ABS?

Erm, anyone want to go into business with me? I've suddenly become the next Dyson, sort of, HAHA!

KERS for caravans.......

12v systems do suffer volt drop, but is the battery capable of being charged past 80%?

Solar is great and getting cheaper £50 now buys a decent solar panel that will top up a battery and a £200 array would keep a camper happy.

Wind again is great but would need to be set up each time. More safety concerns than solar too. Parking up for a long time would keep a battery topped up if the conditions were right.

Why not just stick a multimeter on the caravan battery and a current clamp around the main live and ground cables?
You'd be able to prove or disprove your theory in minutes.

Hah! If you saw my caravan, you'd understand why this will remain a thought experiment for the time being :-)

I have a motorhome which has a solar panel and a leisure battery. There are lots of caravans i see on my travels that have solar panels either on their roofs,like my motorhome, or portable ones that plug in to the control panel of the van.
They are quite efficent and top up the battery quite quickly to maximum charge.

Whether or not the leisure battery gets charged to more than 80% I think depends on the sophistication of the split-charge system in the car.

One primitive systems which were just a basic relay contact, its possible with the electrical loads in the car that if the nominal battery voltage was lower on the car battery (i.e. it was older or of a different type) then the caravan battery may not reach full charge. I don't think this is anything to do with the resistance in the wiring to the caravan as when nearly fully charged a healthy battery should be drawing less than 1 amp.

On more modern systems however with diode isolation and voltage sensing relays this is no longer an issue.

I personally cannot see any benefit from installing a wheel driven charging system on the caravan, it would be expensive and complex. There is no "live" axle to drive the alternator, so you would need to re-engineer the suspension. You would then need some form of "step up" gearing as alternators start to work in the range above 3,000 rpm. I doubt caravan wheels get much above 300 rpm (if you look on your car engine you will see that the crankshaft pulley is much larger than the alternator pulley).

Why go to all that trouble when it is being towed by a mini power station?

For those using or contemplating solar - the panels are getting better and cheaper BUT you are much better off spending a little less on the panels and getting a proper MPPT solar charge controller. These substantially improve the efficiency and charging capacity of the panels, the life and health of your battery and allow you to combine the panels in series. Most have DC charging inputs so you can still charge off the car on the move.

Super easy to calculate.

14V x 160A = 2240 Watts or 2.24 kW

A Volvo 185hp D5 engine is 136kW

So the alternator running at full output is using 2.24/136 = 1.6% of the maximum engine power

But the engine is not at maximum power all the time, at idle it uses around 4kW to 8kW, so at just above idle the alternator is easily around 20% of the engine load.

Not insignficant at all

And remember the downside of charging a caravan using a trailer-wheel driver generator: the energy to pull the caravan comes from the vehicle engine.
You incurr losses through the vehicle transmission and drive system.
So charging the caravan directly from the car alternator is the most efficient as you don't have the drive train losses being added into the equation.

If the caravan battery is not charging to 100%, consider increasing the diameter of the charge wire coming from the trailer plug to the caravan battery.
The larger the wire, the less voltage drop and thus more charge voltage available.

Thanks, some really informed replies there.

Charging a 12 v flooded cell battery to 100% of it's capacity takes a long time. From 50% capacity to 80% might take several hours. To get it to 100% might well take a further 6 or more hours. As someone said earlier volt drop in the cable from alternator to battery will have an impact but even with no drop it's still a lengthy process.
If anyone really wants to get their head around the issue can I suggest they look on a site such as www.canalworld.net you can go on as a guest. Use the search facility and you will find enough information to keep you reading until doomsday. Like most forums you will find a range of opinions but also many absolute facts

Frank (Boatie) Narrowboat owner with a 700 ah battery bank.

Start by saying I'm not a caravaner (so shoot me if you want I just browse here occasionally to broaden my mind), however I do drive an articulated truck with a big trailer.
Trucks do use a 24v system so dragging up elementary electrical formulae from my school days, twice the voltage means half the current which would be 1/4 of the power lost due to resistance, but with a 40ft long trailer we still get lots of power to the rear lights, I'd suspect that losses due to length of wire is minimal unless the builders have really cheaped out and used very thin wires. What I find is more often a problem with an artic is the quality of the connection of the plug/socket on the front of the trailer, often one can observe that with the taillights and fog lights on adding the hazard flashers will cause the other lights to pulsate due to the earth return not being able to carry all that current, usually on the truck that is down to damage and wear in the plug/socket, on a caravan I would also consider the possibility of corrosion in the plug/socket reducing the efficiency of the connection.

Batteries on truck trailers are not so common but with more and more electical gizmos on the trailer they're not so rare either. The charging methods are perhaps a little behind caravans given that it's not so established. Early trailer batteries were often only charged when the side lights were turned on, in a crude sense it ensured the truck was up and running before putting extra load on the electrical systems but it did lead to one company I worked for having to make a ruling that sidelights were to be kept on at all times as over the summer the batteries for the security systems would go flat leaving the trailer 'insecure'. My present job involves some double decker trailers with a moving deck which is done by an electrical hydraulic system, these are generally powered from mains electric at the hub but use their onboard batteries at the outbase depots, usually there's no problem but if we need to we can take power directly from the tractor unit but that uses what we call an Anderson Lead which has wires like jumper cables and a chunky plug with large electrical contacts.
As others have suggested the 80% charge figure may be more related to the way a battery charges combined with the level of sophistication of the systems in the car and caravan to manage when the battery is actually being charged. Bear in mind that on a car we don't tend to do a deep discharge of the battery very often, usually just enough to start the car, whereas with a caravan one would be discharging the battery all evening to have light, watch telly etc and then need to put back all that charge while driving.

I simplify matters by booking electric hook up on the pitch.
Van comes home for washing/ loading before a trip so gets connected to fully charge battery before trip, electric hook up means battery full at end of stay, wash unload before return to storage means battery full when it goes in and I fortunately have facility to charge in storage occasionally.

Landrover doesn't have S plug and no idea if volvo 13 pin has split charge facility as never tried to charge battery on the run.

Paul.

I have a 100 watt solar panel on my caravan roof that keeps my battery charged up all year round, When on sites and using my van we only use site electric, the battery is used for the motor mover for putting it in and out of storage or on my drive to pack things up and to postion on site.
While in storage the solar panel maintains the leisure battery charge, even through winter, and ensures it is charged for next time I need to hitch up with motor mover, the van is too big and heavy to pull by hand and not enough space to get car to the front to hitch up, and the leisure battery keeps the alarm/tracker battery topped up. Many new caravans now come with solar panel charging as standard.

It's a solution looking for a problem. My leisure battery holds a charge all winter and will still drive a motor-mover no problem. The only charging it gets is when it's hooked up to my car, or via electric hookup, or occasionally from a solar panel when there is no hookup. It simply doesn't need to be charged via the caravan itself. If you have a decent leisure battery charging from the van itself simply isn't required in my experience.

While I agree with most of what you say, last winter in the very cold weather my caravan battery was far less efficient and the load of the movers was evidently making it struggle.
I like to move the van during storage to help prevent pitting of the wheel bearings, just half a revolution makes a difference. Without a reffresher charge my van battery would not of lasted the winter. I have no solar panel and it's a case of self discharge over time, cold weather inefficiency combining to result in a van that will not move on the movers at the end of the winter.

Paul.

Sure, your battery may be less than it was, but charging from the caravan wheels won't change that. I'm fortunate, we now have power points in the storage yard so if I really needed to I could charge from them, though I haven't had to yet.

When daughter acquired her XC70 she had tow hitch and electrics fitted by an independant specialist. Van has twin plugs but much to my annoyance he fitted 13 pin electrics to the car. Discovered this too late and daughter wasn’t of a mind to change things. Later found that there is no charge to the van battery as there is no connection via the 13 pin socket as the wiring simply isn’t there. Another thread somewhere suggests that this is now standard practice and 13 pin does not support charging the van battery. Info. I’ve found seems to show that only pin 10 is switched 12 volt and can be used for fridge or battery Is this correct? Looking at the gauge of the wiring to the socket it doesn’t look heavy enough for either and definitely not to have both by creating an internal connection somewhere in the 13 pin to twin 7 pin adapter, which I had thought of doing.
I fitted the hitch to my 940 myself with twin 7pin plug electrics and there is charge to the van battery.

That may well be the case for aftermarket kits, but the Volvo electric kits definitely do support battery charging. BMW have an issue with not supporting by default on uk-spec cars, but Volvo do the right thing and wire up all 12 of the pins (1 pin is reserved in the ISO spec for 'future requirements').

It certainly is supported but the aftermarket electrics kit (why would you) probably doesn’t.... did the Indy specialist update the car with the towing module so that the brakes etc respond properly while towing?

The cars rated towing capacity is based on the factory towing spec that includes the software updates......