24v bus connected to 12v caravan Q's

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Aaron Menzies

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Mar 23, 2007
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Hi

We have just bought a 22' bus and a 27' caravan (the bus is to tow the caravan, we'll be about 50' long!!!) to go for a one year holiday!  8)

But... I have a problem that I have found no answer to! :)  The problem is that the bus is 24v and the caravan is 12v, how do we make them compatible???  We've got a 110w solar panel, two 75Ah 12v batteries in and a small inverter for running a laptop, cell phone charges etc...  I would also like the caravan batteries to get charged when the bus is running, if possible.  I will convert the light bulbs to LED lights.  All cooking and heating will be done by gas, so power consumption won't be too high.  I have set up up a large solar system for a cottage so I do have a bit of experience with solar/electrical stuff.

Any ideas?  We'll be leaving in 4 weeks so I'm looking forward to sorting it out as soon as possible!  :)

Thanks so much for your help!
Aaron
 
You can get a voltage reducer (step down transformer) to reduce the 24V to 12V. But are you sure the bus has no 12V power at all? Most of them use 24V only for engine starting (on diesels) and use 12V elsewhere to power standard automotive lighting and instruments.

Here's one example of a 24/12 voltage reducer.
 
Reducing voltage is easy, increasing it is hard. (I have been known to design power supplies)

Though a good high current regulator might be costly, I suspect as do others one may be there already.

I have designed several lower capacity power supplies that usually start with 18 volts and cut it down to 12, 18 is chosen out of convience as opposed to anything else, (I got a deal on transformers that when rectified and filtered put out 18 volts)

Of course that was years ago  I could just as easy start with 24 or 36 and cut it down, the regulator board is the same, only one part changes with higher voltages (You need to waste more power at higher starting voltages so you need better pass trasnistors and heat sinks)
 
Hi again,

Yes - all the bus is 24v, the two starter batteries are wired in series so all lights and everything are 24v.

I thought about the reduces, but it wouldn't help with battery charging, and I don't now quite how I could wire the break lights etc, through the reducer.  Another thought was to change all the lights on the caravan to 24v, but then we'd have to change the boat and trailer...

Another crazy thing, is that the bus used to be for passenger/wheelchair, so all the wheelchair stuff has been removed (before we got it) but a few cables still poke through the floor and they have a reeding of 5v on them! so I think there is a converter wired to the battery!   now I have to find it so I can get rid of it.

Thanks again,
Aaron  :)
 
Oh, forot about the lights, I can tell you how to do that easily as well, It is really very simple to do

First the reducer,  This is basically the electronics section of a 3-stage charger (No transformer, rectifier or filter, just the regulator part) that goes to your battery, nothing else.

Then the standard method of controling lights on the towed is diodes, we are not going to use diodes

We are going to use relays instead

24 volt relays with the lines from the coach going to the relay 24 volt dc coil and the contacts simply hooked up across the switches so when you turn on your tail lights the relay closes and the tail lights on the towed come on

Turn and stop work the same way
 
Hey John,  that's a great idea!  I never thought of that one  :D     Now I just have to find a heavy battery charger that runs on 24v (if there is such a thing)!

 
Vector makes a line of "Smart Charger" automotive battery chargers.  I do not know what the internal voltages are but if you can find something like a Progressive Dynamics Intella-power converter with a dead transformer (open primary or secondary) that would be ideal
 
John,
I have a perfectly good PD-9055 that I would be willing to send Aaron for just the cost of shipping, however I don't think this is a job  for someone not fairly well experienced in electronics (no offense, Aaron). In that vein, I would be happy to make the necessary modifications as per your instructions, and ship the completed job off to him post haste. If you would be so kind as to supply me with them, I'll get at it immediately. Oh, it's a 109142 Revision "C" circuit board.
 
I don't have the schmatic for the PD series and PD's are perhaps not the best choice (They do some special stuff there in Marshall where they are made) but the idea is to inject the 12 volts ahead of the regulator board

I think a vector 20 amp charger might be a better choice for mod.  But I've not seen one of those either.

As I said, I tend to make my own from scratch when i need to do that kind of thing

 
Hi Karl

That is really good of you to be so willing to help!  If that is possible, and you have the time ? that would so appreciated! 

I live in New Zealand though ? but would be really happy to pay for shipping to here.
 
Karl said:
I thought we were trying to go from 24V to 12V. Not correct? But you did say the PD would be ideal, and that's what I have. :)
Click to expand...

Sorry typo, I meant to say inject 24 volts ahead of the regulator.

Converters have basically two sections, conversion and regulation, at least in the better versions.  (Some cheap ones try to do conversion and regulation in one stage)

Conversion involves a transfoemer or switching power supply to bring the 120 volts down to something reasonable, then a rectifier to convert to DC and perhaps a filter capacitor  THIS IS THE point where you inject the 24 volts)

Progressive Dynamics uses, or used, some advance technology in this area. 

And since I've never looked at the service manual, I don't know.  If the converter is a transfoemer to rectifier to filter to regulator and control board design, then you insert 24 volt at the filter stage, optionally remove the transfoemer (or leave it, so the unit can be used in it's "normal" mode) it is that simple.  If it is a switching type.. Then it's gonna depend on things I can not tell w/o seeing the schematic
 
We got the problem sorted out by using a 24v inverter and a 12v 25A battery charger.

Here are some photos of the rotatable tiltable solar panel stand and the ladder we made for getting up to change the angle of the panels,

 

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