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12 volt Electrics
a simplified explanation

Most modern caravans have two electrical systems, one for Mains Electricity 230V and one for the 12 volt side of things.

They are connected in that there is a Mains powered transformer which runs the 12v side and charges the Leisure Battery.

When no Mains is available the Battery takes over and runs the 12v appliances.

To call the system 12v is slightly misleading. A fully charged 12v battery in good condition will in fact give a reading of 12.7v and be only 25% charged when the voltage drops to 12v.

When the Mains power supply is on the voltage in the 12v system of the Caravan will in fact be 13.8v because this is the voltage used to keep the battery topped up. The devices, fans, lights, pumps, etc. in the van will run quite happily on this voltage.

Slightly less simple bit!

A 12 volt battery requires a charging voltage of 14.4v for it to become fully charged. In most Caravan systems the voltage does not rise above 13.8v, this is so that the battery does not overcharge and start 'gassing'. Hence, in a Caravan with this charging mechanism the maximum the battery can reach is 80% of its full charge. This is why you see people recommending that now and again you should fully charge the battery using a separate 'intelligent' or 'smart' or sometimes called 'stage' charger. This type of charger monitors the battery during the charge process and changes the charging voltage accordingly to ensure a fully charged but not overcharged battery. Some very modern Caravans come with this as standard and you don't need to separately charge the battery


The Amps and Volts and Watts thing!

People get confused about how much they are 'taking out' of their battery.

The Voltage used for these simple calculations is 12v

For Appliances, lights, pumps, fans etc you need to know the amount of Watts they take e.g. 20W light bulb takes 20 watts.

The battery capacity is given as Ampere Hours, simplistically this means that an 85 Ampere Hour battery will give 1 Amp of power for 85 hours or 2 Amps for 42.5 hours etc. etc.

Lets look at a real case, you switch on three of the lights that have 20W bulbs, giving a total of 60W being used.

You need to use the calculation that says when you divide the voltage (V) into the power (Watts) being used you get the current being taken (Amps)

12v into 60W gives the answer 5 Amps.

This means if you leave these three lights on for one hour you will have used 5 of the 85 Ampere Hours available from the battery.
If you use the lights for three hours in an evening you will have used 15 of the 85 available.