The Wanderer
The 230 ac/12 dc volt converters increasingly fitted in caravans and motorhomes to run their 12 volt electrical systems are being claimed to cause problems for owners who, unaware that they are intended for a fairly specific usage, attempt to use them for a different usage (particularly free camping).
These converters primarily address the needs of RV rental users, and of private owners for casual weekend use. They provide a 12 volt dc supply from 230 volt mains power. To quote one converter maker, their function is, to ‘Provide a [230 volt run] dc power system, with optional battery back up’.
It is not always realised by users that the ‘battery back up’ is intended for limited use - to the extent that, in US-built RVs, that back-up is often described as an ‘emergency supply’.
There is a (typically 100-120 Ah) battery but as long as 230 volts is available, all lights and appliances are powered (at about 13.65 volts dc) directly from the converter. With most, power is only drawn from the battery if 230 volts ac is not available.
Converter systems work well and reliably for their intended usage, so forum comments to the effect that ‘they are deficient’ miss the point. Most such units are well made, and they work well and reliably for their intended use.
Free Camping
For the large number of RV owners who free camp, the system required is virtually the opposite of a converter system. They need to run routinely from some form of energy source such as the RV’s alternator/or solar, and if 230 volts is available, it is that which provides back up power.
Converter-based systems are not intended for that type of use - nor can they realistically be modified to do so. Most will cope with a single overnight stay if the RV has LED lights and only the appliances originally installed.
The units include a charger (and a few have good ones) but with virtually all, charging is at one or more constant voltages that rarely exceed 13.8 or so volts – let alone the constant current mode required for speedy full charging. This, say some vendors, is to safeguard the battery from being overcharged.
The limitations are made clear in most converter makers’ literature.
One advises that a deeply discharged 120 Ah battery connected to its product is likely “to take 10 hours to attain 80% charge, and a further ten hours to fully charge.” In other words, if the battery is typically 50% discharged it cannot be fully charged overnight.
Another notes: “in our testing, a 125 Ah battery was fully discharged to 10.5 volts and then connected to [one of our] Converter/Chargers set to our standard output voltage of 13.6 volts. The battery reached full charge in 70 hours.” (Almost three days!).
RV vendors may explain how to use the systems, but not necessarily that (as may be obvious from the above) such usage does not include extended free camping.
Voltage Drop Issues
The problems can sometimes be solved by replacing that converter (it cannot be modified) by a high quality battery management system. But with some RVs, this alone may only partly assist.
This is because as typical converters units produce 13.6-13.65 volts (about one volt higher than that of a reasonably charged deep cycle or AGM battery) some converter makers recommend cable that may be only a quarter or so of the size needed for a primarily battery driven system. If that recommendation has been followed by the RV maker, part or all of that cable may have up to one volt voltage drop.
It is currently unclear how many or which brands of converter-equipped RVs have such light cabling, but there is anecdotal evidence that some do.
If that cable is installed, and the intent is to use that vehicle for extended free camping, there is really no choice but to replace it. No matter what else is done, retaining that cable will substantially preclude the system working satisfactorily.
Fixing the Problems
As noted above, it is not possible to modify the converter. Here is what one of the USA’s largest RV charger maker’s (Ample Power) says about attempting to do so:
“What can you do about the RV converter? If you want to live unplugged from the AC power pole, the RV converter has to be removed and replaced. Call your local recycler and have them recover the copper wire in the transformer. There’s no fix that can be done.”
If the existing wiring is adequate, or replaced if not, the ideal solution is to install a BMS (Battery Management System) that includes the 100% recommended dc-dc alternator charging, plus solar regulation, and often a 15-25 amp multi-phase charger, plus energy monitoring.
It can also be done using separate units, but it is advisable to buy all from the same maker (or at least vendor) to ensure that they are compatible.
For the Technically Minded
A typical converter works much as shown here. Most are 230/12 volt transformers with a full-wave bridge rectifier and possible smoothing capacitance. Some have a direct 12 volt input - but as shown - that is simply a few centimetres of wire, that bypasses the converter’s internals, plus, with some, a diode (to prevent possible reverse flow). That diode however introduces up to 0.6 volt drop.
In most, the battery is floated across the 13.60-13.65 volts output via a sensor that typically limits float current to 0.8-1.5 amps. An override usually enables charging at higher current if the battery drops below a certain level of charge (typically 10.5 volts), but at a fixed 13.6 or so volts – not nearly sufficient for deep and reasonably rapid charging.
A few converters include multi-phase charging, but usually via fixed voltages for bulk, absorption and floating – not the constant current required for effective initial bulk cycle (typically 80% of the entire charge time).
The matters discussed in this article only affect users who choose to use their RVs for a usage for which, electrically, it was not intended.
No part of this article is intended to imply or suggest any deficiency on the part of converters if used for the purpose for which they were sold.
Collyn Rivers
My various books in this area are all current. They are stocked by and available directly from CMCA NHQ.
Typical converter. In some, the AC/DC is a transformer and full-wave rectifier. In a few it is a switch mode unit. As can be seen, charging is effected at 13.65 or so volts. A few charge at multiple levels, but rarely at voltages high enough for speedy charging. They work very well for their intended purpose, but are best avoided for extended times away from 230 volts power. Drawing: Caravan & Motorhome Books.
Category: Technology
Written: Fri 01 Mar 2013
Printed: March, 2013
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