Re: 12S question - now with some facts for a change

Dougal wrote:

|| Richard Brookman wrote:
||| OK, I've had a good look today and most of what I posted earlier is
||| probably nonsense. I blame the Merlot.
|||
||| I've checked out the installation, and the 12S wiring is through a
||| PCT relay (ZR1220 Volton Combi) and looks very professional.
||| Reading the PCT instructions, the Swift wiring diagram and the John
||| Wickersham book, all should be working correctly. The permanent
||| feed from pin 4 now supplies caravan lights when the car isn't
||| running, and switches to charging the caravan battery when the
||| fridge circuit (pin 6) is energised via a relay in the van. Pin 6
||| supplies 14.4v to the fridge when the engine is running. So far so
||| good.
|||
||| I'm putting down the poor performance of the fridge to very hot
||| weather - after 7 hours and about 300 miles, the ice was starting
||| to melt, but nothing was ruined. Is this normal, or have I still
||| got a problem here? (With the previous car, on similar journeys in
||| the UK, the fridge contents were colder at the end of the trip than
||| at the start.)
|||
||| However, I don't think I am getting enough juice to charge the
||| caravan battery from the car. Van battery, rested at least 24hrs,
||| was showing 12.6v across the terminals. With the car attached and
||| at a fast idle, the voltage across the battery rose to 13.25v. My
||| understanding is that a battery needs over 14v to charge properly,
||| and that anything below 13.8 is a float charge. Questions:
|||
||| 1. Am I right in thinking that 13.25v will never charge my caravan
||| battery adequately?
||| 2. Could this be explained by the voltage drop across the length of
||| wire, connectors etc - in which case, do all caravans do this?
||| 3. Could I rectify it by jumping the wire from the car relay to pin
||| 2 (14.4v switched, currently going to "spare" on the caravan plug)
||| onto pin 4, or will this b*gger things up and cause my car to
||| explode?
||| 4. Similarly, could I fit a split-charge relay off the alternator
||| and take this back to pin 4 to get a direct charge from the
||| alternator to the caravan battery?
|||
||| Sorry for the long post. Any help appreciated.
||
|| In general you're thinking along the right lines. Voltage drop across
|| the long cables is probably at the root of your problem. This
|| includes not only the feed but also the return cable(s).
||
|| Have as many earth connections through the 12S as you can to reduce
|| voltage drop/heating and to provide valuable redundancy. You have at
|| least one spare pin that you could employ.
||
|| I think that you'll need to understand the sizing of the cables at
|| the 'car' end of your installation to get to the bottom of it.
|| Specifically look at the input cable to your ZR1220. The usual
|| recommendation (44/0.3)will give you about 3 sq.mm, 35/0.3 gives a
|| measly 2 sq.mm which I'd suggest is not up to the job.
||
|| You seem to recognise that your installation is 'wasting' the supply
|| to pin 2. This is usually the caravan battery charge supply.
|| Investigate at the caravan end how to use the pin 2 output for
|| battery charging (this might mean abandoning the relay which is
|| energised from pin 6). If you don't need to use the car battery for
|| caravan lighting it's easy.
||
|| Don't connect the pin 2 output to pin 4 at the ZR1220 output - if I
|| understand what you are describing - you will not gain from having
|| another connection path through the 12S. In fact you'll make things
|| worse - more current (if used) through the same cable/pin means more
|| voltage drop. Make use of the pin 2 feed as I mention above.
||
|| The ZR1220 is effectively providing the equivalent of a split charge
|| relay feed from the alternator and providing it to pin 2. (It's
|| probably not fully equivalent and depends on from where the ZR1220
|| feed comes.)
||
|| Here's hoping that I haven't further muddied the waters!

Not at all. Thanks for a very helpful reply.

You say: "Investigate at the caravan end how to use the pin 2 output for
battery charging (this might mean abandoning the relay which is energised
from pin 6). If you don't need to use the car battery for caravan lighting
it's easy."

So if I took the pin 2 supply where it enters the caravan (currently not
used) and took it direct to the battery + terminal, that would supply the
battery with a full charge when the engine is running. That would still
leave pin 4 to supply the caravan lighting. I'm not sure if the pin 2
supply gets as far as the van but I could easily rig it up.


Mucho appreciated.

--
Rich
==============================

I don't approve of signatures, so I don't have one.


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