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The electrical systems in many US RVs, particularly
fifth-wheelers, operate differently from general practice. Except whilst
driving, they tend to rely almost totally on 110-volts mains (Americans
call this ‘utility’)
power or an inbuilt 110-volt generator.
A Queensland auto electrician recently sought my help in sorting out
one of these units. The owner had attempted to use his just-imported
fifth-wheeler for overnight stays away from mains power. He complained
that the back-up battery flattened inside three hours. Worse: there appeared
to be no facility to recharge it from the vehicle alternator yet it took
around 24-hours to recharge (from mains power) using the inbuilt charger.
Fortunately there was a big three-way fridge that switched automatically
to gas when the engine was not running/or mains power was not available.
The electrician said, “the system seems different from anything
I’ve worked on before”. He was to use more or less his own
terms: “tossed by the $#@%^$ thing”. The customer was far
from pacified when the electrician passed on my comments to the effect
that: the system probably was working as intended but relied almost entirely
on a constant source of mains power to do so. Here’s why.
Local RVs (indeed most outside the USA) either have 240-volt equipment
running from the mains supply (when available), or via a battery bank
and inverter when mains power is not available. Or their electrical systems
run from 12-24 volts drawn from a battery bank that is charged from various
sources (eg, mains charging, solar, generator) depending on intended
usage. Some use a combination of the above.
American RVs on the other hand (and particularly fifth-wheelers) tend
to use either big three-way fridges, or electric-only fridges that rely
totally on their 110-volt utility power or inbuilt 110-volt generators.
The remaining systems, and nearly always the lighting, are usually 12-volt
but the power to run these is derived directly from the 110-volt supply
via a so-called ‘converter’. In other words, the whole
shebang normally relies on 110-volts mains power being available whilst
at rest.
There is a nominal battery back-up – but the 12-volt load is not
floated across this battery. Instead, the battery is switched in (automatically)
only if 110-volts is not available. Then, for reasons that will become
clear, it is good for only a very short time (2-4 hours is typical).
It is essentially an emergency-only system.
When imported into Australia, the importer may simply add a 240-volt
to 110-volt transformer. The fifth-wheeler in question was just like
that and also retained the original 110-volt wiring and US-made appliances.
But whilst somehow registered, it lacked the mandatory RCD and double-pole
circuit breaker/s and switches required by Australian Standard AS/NZS
3001:2001. But that’s a different issue.
The typical US ‘converter’ (Figure 1) is a totally unregulated
110-volt ac to 12-volt dc power supply: (i.e., it is a basic step-down
transformer and a bridge rectifier). The main section of the unit produces
20-30 amps at 12.8-13.0 volts and this runs all of the RV’s 12-volt
devices.
A second section of the converter produces 3-4 amps (at 13.8 or so volts).
This is intended to trickle charge the small (in this case 75 Ah) back-up
battery. A relay (shown in Figure 1, and usually part of the converter)
automatically connects the 12-volt electrical system to the back-up battery
if 110-volts is not available.
This system is cheap and simple and, as long as mains power is available,
it works reasonably well – particularly in the USA where free camping
(called ‘boon-docking’ over there) is very rare. There
can be no serious criticism as long as the vendor does not purport the
system to be other than it is. It is intended for typical US usage and,
whilst not perfect (see below), if used accordingly it works well enough.
But that usage does not even remotely extend to bush camping – unless
one reverts to candles. In normal usage, the only real problem is that
the 12-volt system is unlikely to have any spare capacity. The ‘converter’ is
scaled only to power the originally fitted 12-volt bits. Because the
voltage is unregulated, as load increases the voltage inevitably falls.
Turning on extra lights, or extra anything, reduces the brightness of
lights already on. (This happens also to battery systems but to a less
extent.) If the load exceeds the converter’s capacity, a circuit
breaker usually cuts all power until the load is reduced. Because of
the above, it is often not possible to add any further 12-volt loads
unless existing consumption is reduced.
The current draw of many of these US RV imports is typically high. This
is mainly because 110-volts power being assumed to be freely available,
little attempt is made to save energy. Half a dozen or more 20-50 watt
incandescent globes (i.e. 2-4 amps each) are common. The draw may thus
be 20-30 amps for lighting alone – and that is likely to be the
maximum capacity of the converter.
Unless the load is drastically reduced, the ‘battery-back-up system
is, at best, only marginally effective. Even if originally fully charged,
the typical 75-100 Ah battery is good only for a few hours. Worse, the
converter’s charging output is far too low to bring the battery
up to charge in less than a full 24-hours. Conversely, and ironically,
it is also too high if connected permanently, leaving the battery at
risk. Indeed the battery in the almost new vehicle in question was all
but dead.
This part of the system (to me at least) makes little sense. In fifth-wheelers
at least there is usually no provision for charging by the towing vehicle’s
alternator and the converter’s charger output is next to useless.
It is unclear just how users are expected to charge the battery in any
realistic usage – except by adding a proper and separate battery
charger. However, as one RV importer told me: ‘Most users in the
USA don’t even know there is a battery there’.
Whilst these systems work to a point it is hard to understand the rationale
behind them (they are rarely used outside the USA). Even American companies
query their use.The authoritative US company ‘Ample Power’ comments:“Despite
their widespread usage, we think that the RV converter is one of the
worst electrical ideas. We’d be interested in knowing what arguments
led to its acceptance as part of an RV electrical system”.
Changing these systems is inevitably costly: so first see if you can
live with what you have – and/or use a quiet back-up generator.
The time between battery charges can be reduced by using fewer 10-20
watt halogen globes – or 12-volt fluro's. Battery size can usefully
be increased, but this is pointless unless you scrap the battery charging
side of the converter – and add a bigger three-stage charger. But
none of the above will enable one to spend more than a night or so away
from supplementary power.
If you do want to spend extended time away from mains power, the only
realistic solution is to scrap the converter and more or less start over
again. The existing 12- volt wiring should be fine. You will need to
install a decent-sized battery bank and this may need structural work,
as the existing battery housing is likely to be too small. You must of
course also decide how you are going to charge that battery bank: via
the vehicle alternator, solar, a suitable generator – and/or a
high quality three-stage battery charger – but this topic is hugely
too big to cover in articles like this. In brief one needs to install
a system of a conventional type.
There is one further approach and I mention it only because an RV importer
recommended it (allegedly on the basis that ‘it was American -
and therefore best’). Apparently, a few US companies are cashing
in on the failings of existing converters and are supplying up-market
replacements. But these appear to have gone from inadequate to massive
overkill. The Freedom 2000 unit that the importer suggested claims no
less than 130 amps output from its inbuilt battery charger – over
50 times that of the original unit! And requiring at least 650 Ah battery
capacity to even begin to justify it.
These new converters typically cost US$1000-$1500 upwards (in America),
plus about A$400 more for shipping, another whack for import duty, plus
GST on top of the lot. They weigh much the same as some larger bits of
the Titanic – and still need a serious back-up battery bank, plus
some way of feeding that battery charger with close to 3 kW at 240 volts.
Definitely not recommended – unless you truly want to go along
with the original electrical practice.
Details on all this sort of stuff - plus a great deal more – are
published in my books. These are obtainable directly from the Head Office
of the CMCA – and also at CMCA National Rallies.
Figure 1. Schematic of a typical US ‘converter’. The relay
is held open by the presence of 110-volts and thus isolates the battery
from the RV’s 12-volt system: meanwhile, the battery is trickle
charged by a second part of the converter. If/when 110-volts is not available,
the relay closes and connects the 12-volt system across the small back-up
battery.
Collyn Rivers, W8054.
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