Power supply units (PSU's) can fail for a variety of reasons. One common cause is the use of generators where the PSU is switched on before the generator has stabilised. They can also fail due to age, which was the case with this 10 year old caravan. The problem with vans of this vintage is that the original PSU is often buried behind a control panel and can be difficult to get at. Here there was insufficient slack on the wiring for the unit to be slid out far enough to reach the PSU. So, even if a direct replacement had been available, it would have been very difficult to fit.

 

The failed power unit lies behind this panel but can't be accessed as there is insufficient slack on the connecting wires to pull it out any further

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The answer was to fit a new breed of power supply such as those made by Amperor. They are designed to connect directly to the leisure battery thereby making them easy to fit. The one I chose was their 18 amp 3 stage charger. It is a powerful and compact unit designed not just to charge a leisure battery but also to provide power for a caravan’s 12 volt system. It is designed to connect directly to the caravan battery.It costs around £80 and can be bought from Amazon or via ebay.

This dual role can mean that compromises have to be made. To fully charge a 12 volt battery a voltage of 14.4 volts is needed. There are two problems with this. The first is that, once the battery is fully charged, the voltage needs to be dropped back to at least 13.8 to avoid gassing the electrolyte. The second is that a sustained voltage of 14.4 volts is not good for the caravan’s appliances as these have to be able to work from the battery only on a voltage of around 12 volts.

The clever thing about Amperor’s charger is that it senses what is going on and adjusts its output voltage accordingly.It can not only give the leisure battery a full charge but also power all the caravan's 12 volt appliances without damaging them through over voltage. How does it do this?

If it senses a high load (above 3 amps) it ramps its voltage up to 14.5 volts. Allowing for a small drop in the wiring this means that the battery will receive about 14.4 volts. High loads can occur because of a low state of battery charge or because of something connected to the caravan’s 12 volt system, e.g. a television.

charge_curve

This graph shows how the output voltage of the Amperor charger varies with load. At high loads the voltage ramps up to 14.5 volts but as the load reduces below 2 amps the voltage falls back to 13.8 volts.

 

So the battery gets 14.4 volts and a full charge but what about the television. Might 14.4 volts be a bit high for that? It might but, by the time the power has gone through the caravan’s wiring harness reaches the television the chances are that the voltage will have dropped by a volt or so.

When the load reduces below 2 amps the Amperor unit drops its voltage down to 13.8 volts. This prevents the battery from over-charging whilst ensuring that the caravan’s kit does not suffer from over voltage.

Before fitting the PSU I made up a 12 volt connecting lead with a 20 amp in line fuse in readiness for connection to the battery. 2.5 mm cable is recommended for this 18 amp unit.

 
Here the new PSU has been fitted with 2.5mm cable to its output terminals. Note the 20 amp (yellow) fuse on the positive cable. This will provide protection in case of a fault.
charger

The instructions say that the charger should be mounted in a cool ventilated position that allows the maximum possible airflow around the unit. It should be close to the battery but in a separate compartment. I found this spot in the front offside bed bunk.

unit in place

The Amperor unit fitted neatly into the space between the battery box and front bulkhead. This position should help it to avoid being smothered with bedding or other items.

The 12 volt wires were routed through an existing cable hole in the battery box and connected directly to the battery via the 20 amp fuse previously fitted. The hole was resealed with silicone mastic to prevent battery fumes from entering the van.

 
 
The PSU was fed from a specially installed 240 volt socket nearby. If you are not competent to carry out this work call in an electrician who is.
socket

Next it was time to conduct some tests. I tried switching on and off various loads whilst measuring the voltage at the TV socket. The results can be seen in the table below.  The voltage varied between 12.8 volts with 5 x 20 watt ceiling lights and 13.6 volts with just one 10 watt reading light. With no load at all the reading was 13.8 volts.

 

Load Approx current Socket voltage
15” TV 2.6 amps 13.6
20” widescreen TV 3.8 amps 13.35
5 x 20 watt interior lights 10.0 amps 12.82
3 x 20 watt interior lights 6.5 amps 13.41
2 x 10 watt reading lights 1.8 amps 13.35
1 x 10 watt reading light 0.9 amps 13.60
Omnivent low speed 0.8 amps 13.63
Omnivent medium speed 1.7 amps 13.37
Omnivent high speed 3.7 amps 13.36
Radio - medium volume 0.8 amps 13.65
None 0.0 amps 13.8
Leisure battery (partly discharged) 5.0 amps 14.4

 

Lastly the PSU was switched off and the leisure battery partly discharged by switching some lights on for a while. When the PSU was switched back on with no other loads the voltage went up to 14.4 before falling back to 13.8 volts.once the battery had been topped up.

All these tests show that the Amperor unit is ideally suited to its intended use. Unlike the original charger, which had a constant output of 13.8 volts regardless, it is also capable of giving the leisure battery a full charge. After several months of testing I can report that the Amperor unit performed flawlessly with never a dim bulb in sight.

Footnote

The Amperor unit used here acts as a powerful charger and should only be used with good batteries of at least 60 amp hour capacity. Failure to observe this could result in battery overheating and damage. If in doubt about the state of your battery, monitor it carefully for the first few hours after connecting the new supply. If the battery gets warm it could be time to it send it for recycling.

For more information on Amperor chargers click this link