An Easy way to install bigger batteries.

The Hymer, and most motorhomes, has a battery box that will take 2 batteries - one for the engine and one for the van - the leisure battery (LB). If you want more power, then you have to locate the LB elsewhere.

It's not a good idea to just slap an additional battery elsewhere and wire it in in parallel, although many people do just that, usually because there is very little choice and it's the easy way out. But you will never get the full potential out of two batteries installed in this way.

But you soon understand the problem when you decide to do it properly, and install 2 or more identical batteries elsewhere in the van - how do you wire them in? All the connections go to the original battery box, where the original LB is!

The easy answer is to use a "dummy battery" and big thick cables.

Take out the original LB, and replace it with a piece of wood with two terminal blocks mounted on it - one for pos one for neg. Then attach all the wires that used to go to the pos of the LB to the pos side, and ditto for the neg side.

So now you have all the wires connected as they were, but no battery!

Install the new battery(s) wherever suitable in your van. In an S700 there is little choice - it's either in the boot or in the gas locker. Another possibility if you are handy is to make an under chassis battery box - as found in HGVs. But wherever you put them, they must be identical, and wired in parallel. 

Then you take a single thick wire - as thick as you can afford and work with - I suggest 10 to 12mm diameter - and run it from the battery, down through the floor, along the underside of the van, and up into the original battery box, and to the positive terminal block on the dummy battery.

Now all you need to do is connect the negatives. You have two choices - you can run a separate wire along with the positive one, or you can use chassis, which saves wire - thick wire is expensive. You will see from your original battery box that the neg of both batteries goes to chassis via thick black cables. So attach the original LB neg to the neg terminal block. At the other end at the new batteries, run a thick cable to the nearest point on the chassis - make sure it's a clean connection to the main chassis member - usually down through the floor. 

IMPORTANT - do not connect the chassis neg to the same battery neg as the positive if more than one battery. It must go to the opposite neg of the last battery in the pack. So if two LB's in parallel, then the neg goes to the neg of the second battery.  This keeps the system balanced, and multiple batteries are seen "as one". The rule is - pos to the pos of the first battery, neg to the neg of the last battery. (does not apply for only one battery)

And that's it basically! No modifications to original van wiring needed at all. You can tap solar into the pos wire and neg to chassis at any convenient point, and the same if you upgrade your charger as you should if you install bigger batteries. The space where the LB used to be is now a useful empty space, you can put a charger or an inverter in there, whatever you can fit. Usually you would install your charger, inverter and solar controller as close to the batteries as possible.


Dummy battery - a simple piece of wood with two terminal blocks, in place of the original LB. You can see the original thick black cable going to the neg of the starter battery, and at the bottom you can see the thick red cable that goes down out of the box, and off to the new batteries. - not needed for most installations.

The little red box is a split charge relay- not needed for most installations.


Here is an install I did using 4 x 100a/h batteries. NOTE - the colours are wrong - the lower right connection to the neg is red - it should be black - to chassis. (wire is expensive - I had no black left) But everything else is correct. The pos is top left, and the main pos wire back to the original battery box goes from here. The blue box is a Victron inverter charger, and the white is a new consumer unit, but usually you would use the original 240v consumer unit to supply 240v from your inverter tom the van plugs.

Important NOTE!! This installation of 4 batteries in the boot of an S700 proved after one year to be too heavy. Pay attention to weight distribution. See next blog post on how I solved it. This photo is for demonstration of wiring batteries in parallel - not where to put them in your van.

More photos of some other dummy battery installs I have done.

The relay on the left is Victron Cyrix intelligent relay. It replaces the Hymer split charge relay (R1) which is located behind the fuse panel. It is important to take out R1 if you fit the Cyrix. The reason for this is that the Hymer wiring and relay is only rated to pass about 20 amps of charge through to a normal single leisure battery. If you fit bigger batteries, you need more charge. Fitting a bigger relay, with thicker wiring, enables about 50 amps to get through to the Odyssey from the alternator.

The yellow thing in the middle is a 100 amp mega fuse, and a convenient bolt on way of safety fusing - there is another on the negative of the battery - so that in the event of a crash which shorted out either the wiring, a fuse will blow.

Another method of fusing using blade fuses.

Another view - a dummy battery board makes installing and fusing extras a dream.