My first caravan did not need a battery. It had no electrical equipment and the lights were powered by gas. However, as some of us found out on the 2009 AGM rally, today’s caravans hardly function at all without 12 volt power.

The more recent the caravan or motorhome the more critical it seems to be. For example, the LCD control panel found on many Thetford fridges will not work without 12 volt power. Also those caravans fitted with Alde wet heating need 12 volt power to operate the boiler and central heating pump.

Full marks to caravan manufacturers for their increasing use of power saving LED lights but, if you want to last more than a couple of days without a mains hook up, then it may be sensible to think about having a second battery.

One way of doing this is simply to take a fully charged battery along and swap over when the main one runs down. This approach does have some advantages. For example, if the awning light is accidentally left on all night, then only one battery is affected.

This article however concentrates on how to connect two batteries together so that the effective amount of power available is doubled without any need to do anything.

The first thing to say is that it is better if both batteries are identical. It’s not an absolute must but it will help to ensure optimum performance. Whilst on this point, I have found that carbon fibre plated batteries, such as those from Elecsol, seem to have slightly different characteristics and so are best not mixed with other types.

Batteries are inherently very heavy so care needs to be taken in deciding where to site a second battery. This is not so critical on a motorhome but on a caravan it’s much more important. Heavy items should be positioned as near to the axle as possible and ideally at floor level. Avoid using the extremities of the van, especially the rear, as this will make the outfit less stable when towing. If the second battery is to live inside the van then it is Important to put it in a suitable plastic box that will contain any acid spills or leakage. If there is any chance of the box moving around during transit it should be screwed to the floor.

It is also important to vent the battery to outside as explosive gases can be given off during charging. Most leisure batteries come with a piece of plastic tubing for this purpose. Here on the right you can see the tubing plugged in to the vent hole at the top of the battery.

In my case I put the second battery in a side dinette bunk near the axle and drilled an 8mm diameter hole through the floor for the vent tube. Afterwards the hole was sealed from above and below with silicone mastic. This helps to make it gas tight as well as keeping out any weather.

Next comes the task of connecting the batteries together. This requires heavy duty flexible cabling with a fuse at each end. Why at each end? Well, if the cables are accidentally shorted, perhaps by a nail or screw, both batteries will feed the fault so each must be fused. Of course this arrangement means that the rating of the cable in amps should be at least the sum of the two fuses.

The batteries should be connected together like this - positive to positive with a fuse at each end, and negative to negative

A couple of examples might help to explain this better. For caravans without movers a fuse rating of 20amps at each end should be sufficient. The cable used should therefore be capable of taking at least 40 amps as this is the current that would need to flow to be sure of blowing both fuses in the event of a short on the cables themsleves. Where a mover is fitted the fuses should be ideally be a minimum 30 amps each and the cable rated at a minimum of 60 amps, or the sum of the two fuses, whichever is the greater. In the picture on the left the black square between the two batterrty terminals is a fuse holder containing a 60 amp fuse. There is a similar fuse at the other end of the cable where it connects to the main battery.

Cables may be referred to by their AWG (American Wire Gauge) number, or by their conductor diameter, or by their conductor area - see table below.

AWG number	Conductor diameter	Conductor area	Approx Current rating	Max fuse rating (2 fuses)
12	2.05mm	3.3mm2	22 amps	11 amps
9	2.9mm	6.6mm2	35 amps	17 amps
8	3.3mm	8.5 mm2	40 amps	20 amps
5	4.6mm	16.6mm2	64 amps	32 amps
4	5.1mm	20.4mm2	73 amps	36 amps

If necessary, cables can be doubled up to increase their rating. For our purpose the cables should contain many individual strands and be very flexible.

Halfords and Maplins each sell suitable 8 AWG cable. Maplins call it ‘Power Cable for In-Car Entertainment Systems’ and Halfords ‘42 amp Power Cable’.  Maplins also sell 4 AWG ‘Car Battery Power Cable’. It contains over 1000 strands and is very flexible. This gauge can take 73amps and is ideal where the caravan is fitted with a mover. Both companies also sell suitable in-line fuse holders. Maplins call them ‘Water-Resistant Maxi Fuse Holders’ (£1.99) and Halfords ‘Maxi Blade Fuse Holders HFS206’ (£5.39). They look remarkably similar to me so I’d buy from Maplins if possible. The Maplins fuseholder is shown here on the right. Before use the cable loop is cut and wired in series with the circuit to be protected.

So there we have it – a simple job that should take little more than an hour to complete but that effectively doubles the power available for running your outfit.