Updated 16 May 2017
Electrical converter problems in caravans result in flat batteries when away for more than a few hours from 230 volts. Here’s how to fix this. These converters are fitted to almost all caravans and motor homes worldwide. They are thus not suitable for RVs free camping.
Electrical converter problems in caravans – nominal battery capacity
These converters run 12 volt appliances directly from 230 volt power. Their battery back-up is consequently nominal. One maker states – they ‘provide a [230 volt run] 12 volt system, with optional battery backup’. In the USA it is often specifically called an ’emergency’ supply. Few supply 12 volt power for more than one night away from 230 volts.
Electrical converter problems in caravans – too low voltage
There are two further electrical converter problems (when free camping). In particular, most converters produce about 13.65 volts. This is too low to speedily charge the batteries typically used. This is, however, usually made clear in the specifications. One reveals a deeply discharged 120 Ah battery is likely ‘to take 10 hours to attain 80% charge.’ ‘And a further ten hours to fully charge.’ Another advises that charging ‘requires 70 hours’. Vendors may explain how to use these systems. They may not, however, reveal this preclude extended free-camping.
Converters cannot be modified. Charging is speeded by replacing it by a multi-stage battery charger. If frequently driving, it’s worth adding a dc-dc alternator charger. But this only partially assists. (See also Charge Batteries Faster and Deeper).
Electrical converter problems in caravans – too thin wiring
The 13.6-13.65 volts converter output is a volt higher than needed by lights and appliances. RV makers realise this. Many accordingly use thinner cabling. This drops the better part of one volt. It’s fine when running from 13.6-13.65 volts. It is not, however, when powered by conventional batteries. These fall from 12.7 to about 11.8 volts as they discharge. Thin cable thus drops this to 12.3-12.4 volts whilst battery fed. And to 11.8 volts or so when still 50% charged.
Replacing that converter assists converter problems. But necessary also is upgrading charging, water pumping and fridge cabling. (Voltage drop is rarely an issue with LEDs.)
Electrical converter problems in caravans – a new approach
The voltage drop problem can be substantially overcome by using lithium ion batteries.
The manufacturer claims that these batteries charge from most constant voltage chargers.
Lithium ion (LiFePO4)* batteries (in RVs) supply 13.1-13.0 volts (from 90%-20% charge). They charge very much faster. All can thus be used in deep cycle applications. Too light cable, however, still introduces undesirable voltage drop. This is less of an issue with that high constant voltage. It only marginally falls under truly heavy loads.
Individual cell voltage for a typical LiFePO4. A 12 volt such battery has four cells. Typical RV usage is about 0.1 volt below the blue line. The red line is at discharge levels close to starter motor draw).
LiFePO4 batteries require a management system. This system ensures equal cell voltage. It is built into some such batteries, but nevertheless, not all. They also require a specific charging regime.
Individual cell charging voltage for typical levels of charge
Given that management system, it’s feasible to charge them (to about 80% full) from a converter. LiFePO4s can be routinely discharged to 20% or less. Usable output is thus greater than of lead acid batteries similarly charged. LiFEPO4s charge well from alternators of 13.8-14.2 volts. They may be damaged if above that. Specialised dc-dc LiFePO4 alternator chargers are now available.
Redarc LiFePO4 dc-dc alternator charger. It is available in 25 amp and 40 amp versions. It also accepts solar input.
Lithium batteries are very different from lead acid batteries. Seek expert advise before buying. See also Lithium-ion Batteries in Caravans and Motorhomes.
LiFePO4 batteries charge in ambients of -180 C. If below that, they can be warmed by switching on a load for a minute or two. (Vehicle headlights are fine for that.)
Charge absorption is highest in ambient temperatures around 250 C (770 F), to 400 C (1040 degrees F). Perfect for Australia. Less so in Alaska.
How a converter works
Most converters have 230/12 volt (or 110/12 volt) transformers plus a bridge rectifier. Some have smoothing capacitance. There is a direct 12 volt input: – of a few centimetres of wire. It may include a diode to prevent reverse current flow, but that introduces up to 0.6 volt drop!
The RV battery floats across that 13.60-13.65 volts output. It normally charges at a mere 0.8-1.5 amps. That typically increases if battery voltage drops below 10.5 volts. But it still only charges at 13.6 or so volts. Far too low for adequately charging lead acid or AGM batteries.
A few converters include multi-phase charging, but most have fixed voltages. They lack the constant current essential for deep rapid charging.
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This article relates specifically to converters used for purposes other than intended. They work fine for that intended.