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Almost
everyone I speak to who has a battery and/or a 3-way fridge in their
caravan complains of two things.
Their van battery never seem
fully charged when they stop and many need replacing after a year or so.
The fridge never seems to work very well on 12V especially in hot weather,
particularly compared to 240V or gas operation.
Well, the
reason is simple, there isn't enough voltage getting to either of them.
In the case of the battery, it will never charge fully in a typical
installation, which means its
capacity is reduced and the plates are sulphating resulting in premature
failure.
You can run the heaviest cable you like from a standard tow vehicle to the caravan
battery, it still won't fully charge. Heavier cable, hence lower voltage drop will help with the
problem but it will not be total answer.
If a 3-way fridge is running off of the battery, this exacerbates the
problem even further.
In the case of the 3-way fridge, heavier cable will help here also but 10A
with this long cable run (car & caravan) will still result in
significant voltage drop as far as heater efficiency is
concerned.
A volt or two makes a big difference at 12V but is negligible at 240V and
coupled with the fact that the 12V heating element
is often a lower wattage than the the 240V element in many 3-way fridges and
you can see why 240V operation is clearly superior to 12V operation in
practically all cases.
Also see my 3-way fridge page.
So what is my answer ?
Well there are two solutions to the battery charging problem and one to the
fridge problem.
Don't copy this unless you understand all of the
implications in doing so.
To fully charge the caravan battery, it must
be charged at 14.4-14.8V and
then floated at 13.4-13.8V, the 12-13V at 15A coming from the tow vehicle is
never
going to
do the job.
One way around this problem is to use a DC-DC converter charger. These will accept
an input of (typically) 9 to 15V with a 3 step output (Boost 14.4V,
Absorption 14V, Float 13.6V depending on the type of battery to be
charged).
Also see my battery charger page.
If it has an output of 20A, it should happily operate the fridge too.
Typical of these is the Arrid Twin Charge available in Aust and the
DanPower in the UK.
My solution to both problems is to charge my battery with the 240V battery
charger and to use the fridge in 240V mode (not 12V) when mobile.
(See a recently added
diagram on the right. This diagram is intended to show the
general arrangement only, in this case it does not show any detail of the
vehicle wiring which is not simply a direct connection to the
battery. That is a separate subject that I will have to cover soon.)
I simply use the 12V from my tow vehicle run a 12VDC-240VAC inverter
installed in the caravan. The inverter happily puts out the 325W at 240V
required as long as the input doesn't drop below 10V. (175W for the fridge
+ 120W maximum for the batt charger and allowing for 90% inverter
efficiency.)
( I have a more detailed electrical block diagram that should more
clearly describe my setup in response to many emails wanting more
information.
See my battery charging arrangement
and battery charging page 3. )
In my case a modified square wave inverter is perfectly adequate for this
task.
Before you decide to use this approach, you would need to confirm your
battery charger is happy with the inverter waveform.
The fridge element being purely resistive will be just as happy with a 240V RMS square or modified
square wave as it is with a sine wave.
If you are running a domestic compressor
type fridge or freezer the inverter waveform must
be a sine wave.
(Not
a "Modified Sine Wave", "Quasi Sine Wave" or "Pure
Modified Sine Wave" which translated from salesman language means modified square wave, see my Inverter
page. It may
run but efficiency will be lower (higher power consumption) and likely
overheating of the compressor will result in shorter life.
You would also need to confirm that your tow vehicle alternator has sufficient
reserve capacity** for the task
no matter how you charge the battery or run
the fridge
and that all the cabling and plug/sockets are adequately sized
for the task.
In my case the cable run and connectors from vehicle engine bay to caravan
inverter must be able to handle 30A with a voltage drop not exceeding 3V.
With this system, my battery is fully charged when we stop and the fridge is as cold as it is
on mains power, a huge improvement on the way it was ex factory.
Before the change, the vehicle alternator would attempt to charge the
caravan battery, but partially due to the load of the 12V fridge element,
neither the fridge or van battery was receiving adequate voltage and increasing
the cable size would not have achieved this due to limitations with all
standard vehicle alternators/regulators.
**The vehicle
alternator in my case must be able to supply up to 30 Amps continuously
in the high ambient temperature of the engine bay in addition to
normal vehicle requirements (very low in day time with non-electronic
diesel). The alternator manufacturers name rating usually
refers to a relatively short duration intermittent
output. The alternator manufacturer's
temperature/time/output de-rating data is very
hard to come by but I would not be confident of long alternator life unless
the alternator rating was at least double the peak load of the
vehicle + caravan. In my case, I changed the original for a 90A
one. Be aware that at very low RPM, overheating due to
reduced air flow through the alternator can result in alternator
failure.
If you are contemplating
doing anything similar, get independent and qualified assistance, all
electrical work should only be done by suitably qualified persons.
Also see My Battery Charger, Battery, Solar,
Refrigerator and Generator pages.
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