Updated 25 February 2016
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. Some 80% plus of caravans and motor homes sold in Australia and New Zealand use these converters.
Electrical converter problems in caravans – nominal battery capacity
Electrical converter problems in caravans and motor homes, when away from mains power, are due to converters being primarily intended to run 12 volt appliances where 230 volt power is available. They have only nominal battery back-up.
As one major converter maker states – their function is to ‘provide a [230 volt run] 12 volt dc power system, with optional battery backup’. That ‘battery backup’ has limited capacity. In the USA it is often referred to as an ’emergency’ supply. Few can 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 in caravans and motor homes. Most converters produce about 13.65 volts. This is too low to speedily charge the lead acid deep cycle and AGM batteries typically supplied. This is usually made clear in the makers’ literature. One advises that 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 that (the same size) battery ‘requires 70 hours’. Vendors may explain how to use these systems, but not that they preclude extended free-camping.
Converters cannot be modified. Slow charging can be improved by replacing the converter by a high quality multi-stage battery charger. They charge at up to 14.7 volts. If driving a few hours each day it is worth adding a dc-dc alternator charger. (See Charge Batteries Faster and Deeper). As shown below however, changing the charger may only partially assist.
Electrical converter problems in caravans – thin wiring
The 13.6-13.65 volts converter output is higher than the 12.7-12.3 volts needed by 12 volt lights and appliances. Many caravan and motor home makers thus install thinner cabling than needed for a battery driven system. This is fine when running from 230 volts, but lead acid and AGM batteries fall from 12.7 or so volts to about 11.8 volts as they discharge. The cable voltage drop reduces this to a typical 12.3-12.4 maximum when battery fed. And too a less than useful 11.8 volts when still 50% charged.
Replacing that converter as described above thus assists converter problems in caravans and motor homes. Necessary also is to upgrade the charging, water pumping and fridge cabling. Voltage drop is less of an issue with LEDs lights.
Electrical converter problems in caravans – a new approach
The voltage drop problem can now be partially or almost fully overcome by using lithium ion batteries.
The manufacturer claims that these batteries can be charged from most constant voltage battery chargers.
On the relatively light loads drawn drawn by caravans and motor homes, lithium ion (LiFePO4)* batteries produce a virtually constant 13.1-13.0 volts (from 90%-20% charge). They charge very much faster and can deliver virtually starter motor current with next to no drop in voltage. All can be used in deep cycle applications. The too light cable still introduces an undesirable voltage drop. This is far less of an issue however with LiFePO4’s virtually constant voltage output. Further, unlike lead acid batteries, their voltage only marginally falls under truly heavy loads (e.g.microwave oven, or air compressor).
This graph shows individual cell voltage for a typical LiFePO4 battery. (A 12 volt such battery has four cells – so you need to multiply the voltage shown by four). The most likely RV usage is about 0.1 volt below the blue line – so voltage output stays almost constant. The red line is at discharge levels of some three times the battery’s amp hour capacity (e.g, 300 amps for a 100 amp hour battery – close to starter motor current draw).
LiFePO4 batteries require a battery management system that ensure equal cell voltage, and a specific charging regime.The management system is built into some such batteries – but not all..
This graph shows the individual cell voltage required to achieve typical levels of charge (a 12 volt LiFePO4 battery has four such cells). As can be seen the 13.65 volts (3.41 volts/cell) is too low full charging – but balanced by the ability to discharge routinely to 10%-20% remaining charge.
Given a battery that does have a full battery management system, it is possible to charge them (to about 80% full) from the converter’s 13.65 volts. As LiFePO4s can be routinely discharged to 20% or less whilst retaining close to full voltage. That output is way ahead of lead acid batteries charged from that converter.
These batteries will fully and rapidly charge from a vehicle alternator that has an output of 13.8 – 14.2 volts. They should not be used with one above that. Specialised dc-dc alternator chargers are now on the UK and Australian market.
This Redarc LiFePO4 dc-dc alternator charger is available in 25 amp and 40 amp versions. It also accepts solar panel input.
Lithium batteries are very different from lead acid batteries. Unless you have an electrical background I advise against using this approach right now. If you have (or know someone who has) read my article Lithium-ion Batteries in Caravans and Motorhomes. It is revised and updated frequently.
LiFePO4 batteries will charge in ambients of -18 degrees C. If below that they can be readily warmed up by switching on a load (such as vehicle headlights) for a minute or two. Charge absorption is highest in ambient temperatures around 25 degrees C (77 degrees F) to 40 degrees C (104 degrees F). Perfect for most of Australia – but less so in Alaska.
How a converter works
Most converters have 230/12 volt (or 110/12 volt) transformers plus a full-wave bridge rectifier and possible smoothing capacitance. Some have a direct 12 volt input. As shown below that input is a few centimetres of wire plus (with some) a diode to prevent reverse current flow. That diode 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 about 10.5 volts. But it still only charges from 13.6 or so volts. This is far too low for adequately charging lead acid or AGM batteries. .
A few converters include multi-phase charging, but usually via fixed voltages for bulk, absorption and floating. They do not employ the constant current required for effective initial bulk cycle (typically 80% of the entire charge time).
Click here for a larger image
This article relates specifically to the use of converters used for a purpose for which they are not intended. No part is intended to imply or suggest that they are deficient if used for the purpose for which they are made.