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.