Relocating the Battery

This article was first published in 2008.

There are three primary reasons why you might want to move the battery from under the bonnet to the other end of the car. It could be because you'd like to have a larger battery (for better starting, or for running a big sound system); it could be because you want to improve weight distribution (most front-engined cars are at least a little nose-heavy); or it could be because of the reason that we are making the move for here. And what's that? - to create more room under the bonnet. In this case we needed to fit in an air/air intercooler - and it went right where the battery used to sit.

Relocating the battery can be done very cheaply, a little bit more expensively, or lavishly. Here we did a combination of those things - some budget bits and some more expensive. Cheap was the cable that was used - very heavy-duty welding cable bought by the metre from a disposals yard. More expensive was the high current circuit breaker mounted on the battery box.

But let's start at the beginning.

Under the Bonnet

Here's what the battery mounting arrangement of the standard car looks like. The battery is retained by two vertical rods and a crossover bracket. The rods are anchored to a battery tray, which in turn is bolted to the bodywork. Small diameter electrical clamps are used around the battery posts to connect the battery to the car's wiring. Next to the positive clamp are located fusible links. In short, a typical battery arrangement.

The first steps were to remove the battery and then unbolt its tray. In some cars the tray will be spot-welded - rather than bolted - into place. To remove this type of tray the welds will need to be drilled out - or if it's not in the way, it can be left in place and a new battery tray bought for the relocation project.

The original earth lead (it normally connects the negative terminal of the battery straight to the engine) needs to be terminated on the bodywork. Find a convenient bolt (here one of the threaded holes from a bolt that had held the battery tray in place has been used) and remove the paint from around the hole. You want to see bare, shiny metal all around this spot.

The earth battery post clamp looked like this. It was easy enough to use a hacksaw to cut off the loop (ie cut along the red line), leaving a well-terminated lug that could then be bolted to the bodywork.

The earth strap bolted in place. Only the one positive cable runs to the new battery location, so the earth connections here and at the battery need to be very good if there's not to be a voltage drop between the battery and the electrical loads.

The positive battery clamp was also cut off in the same way, leaving a pair of lugs. (One connected to a cable heading straight to the starter motor, the other to a cable going to the main loom.) Using a nut and bolt, these lugs were connected to a new terminal that had been soldered to the new heavy duty cable that will run to the relocated battery. If you don't have a big - and I mean big - soldering iron, pick terminals that can be attached to the new cable by crimping or with set screws. Again, these connections need to be electrically good.

You must make absolutely certain that this connection can't come into electrical contact with the body or engine. We slipped heatshrink over the new join and then....

...made a clamp from a piece of aluminium sheet to hold the cable in place. Some rubber fuel hose was slipped over the connection to further protect it from the clamp. So, original battery cable goes in at one end... and new battery cable comes out at the other end. At the top right of the pic you can also see the revised earth cable connection.

Running the Cable

There are two paths that the cable can take - through the cabin or under the floor. We chose the latter, for two reasons. Firstly, it's usually a lot easier drilling a hole in the boot floor than trying to drill through the firewall (often firewalls are double layer and/or insulated). Secondly, the cable (complete with insulation) is very thick, and clearances along a sill panel inside the cabin are often tight. This is one cable that you don't want crushed, eg under a seatbelt retaining bolt! We placed one side of the car on ramps and chocked the other wheels, allowing easy access under one side.

The easiest route from the engine bay to the floor of the car was to go around the engine bay until the cable reached the centreline of the car, then head southwards. Here the cable has been clamped into place, using the clamps available from hardware stores for holding copper plumbing. From the point where the cable might be subjected to exhaust heat and/or abrasion, the battery cable has been run inside clear plastic hose. This acts as an additional protectant and adds only a few more dollars to the cost.

Make sure that you buy at least 2 metres more cable than you think that you will need - it is very difficult to join on a little bit extra if it turns out to be too short.

The cable - complete with its clear plastic tube cover - was cable-tied into place alongside the plumbing that already runs from the back to the front of the car. Heavy-duty cable ties were used, being tightened up only when everything was positioned correctly.

At the rear of the car the cable goes its own way - it separates from the plumbing - and from hereon it was clamped into place. Note how it has been kept higher than the chassis rail, helping to protect it from impacts and flying rocks.

The cable passes through a rubber grommet in the floor of the boot. This hole will probably need to be a new one drilled for the cable but first have a good look around - often there are holes through the boot floor which are filled by rubber plugs. Don't be tempted to leave out the grommet - it's a must-have to protect the cable against the sharp metal edge.

The Battery Box

The first steps in installing the battery box was to retrieve the original battery tray, cut off the protruding brackets and give the modified tray a quick splash of paint. If the original battery tray isn't suitable for use in the box, buy another - don't be tempted to just place the battery in the (relatively flimsy) plastic box.

Talking of battery boxes, here it is bolted to the floor of the boot. Note that large washers that have been used on the high tensile bolts - the washers help spread the load and so resist tearing through the tray. Three high tensile bolts have been used here - the shape of the tray lent itself to the use of this number. Normally you'd use four bolts.

Here the underside of two of the bolts holding down the battery tray (and so box) can be seen. Again large washers have been used to spread the load - one of the washers needed a flat cut on it to clear the chassis rail. Placed either side of the chassis rail in this way makes the assembly strong and rigid - important as the battery is heavy and you don't want it coming loose in any circumstances... including an accident. The third bolt holding the box in place has been attached to an aluminium plate that is supported by the rear bumper brackets, located on top of the chassis rail.

Unless a completely sealed battery is used, ventilation should be provided to allow the escape of gases emitted when the battery is being charged. Most boots in cars are ventilated (the air outflow vents are in the lower parts of the boot) but if there could be a problem, the battery box should be sealed (eg with foam rubber weather-sealing strips) and a hose placed from the box to outside air for ventilation. Alternatively, sealed batteries designed for use inside the cabin of cars often have a vent tube provided - this can easily be run to the outside.

The battery terminals are of the screw-on type: the cable slips through an opening and then a bolt tightens on the cable from the side. This shows the earth cable. It's red cable like that already used but it has been wrapped in black tape to prevent colour confusion. It's earthed to the body in the same way as the cable in the engine bay.

A Blue Sea Systems 75 amp circuit breaker is mounted on the side of the box. This product is available from marine stores and sells for about AUD$70. (Since this installation I have found the same device at Jaycar Electronics... for AUD$33!) Without a circuit breaker or fuse, any short-circuit that develops between the battery and the original fusible link in the engine bay has the potential to cause a fire. The breaker can tolerate twice its rated current for 10 seconds, allowing its rating to be lower than the starter motor current draw.

To prevent anything coming into contact with the live terminals, a cover over the circuit breaker was made from acrylic sheet. The press-button (power off) and the reset button (power on) can still be easily reached as required.

Conclusion

I already had the cable, terminals, heatshrink, cable ties and acrylic sheet. The battery box cost AUD$20, the circuit breaker AUD$70(but see the note above), the nuts and bolts AUD$8 and the clear plastic hose AUD$10. So moving the battery in this way cost me a bit over a hundred bucks - a cheap way to make underbonnet space. And as a bonus I now also have a main battery shut-off switch!

Cable Thickness? The multistrand copper welding cable that I used has a total conductor diameter of about 7mm. According to the Bosch Automotive Handbook, this is good for a bit over 100 amps continuous (nearly 1.4kW at a running car voltage) - way above what the on-going electrical loads of the car will ever be. The peak (but intermittent!) load is the starter motor, and the voltage drop to the starter needs to be kept as low as practicable. It's for this reason that only super heavy duty cable should be used. In practice, the cable I used was plenty thick enough - the car started just as easily with the battery mounted in the boot as it did when it was under the bonnet. However, if you have a large high compression engine, even thicker cable might be needed. Diameter of Conductor (ie excludes insulation) Continuous Current Capability (at 30 degrees C) 3.4mm 54 amps 4.5mm 73 amps 6.3mm 98 amps 7.8mm 129 amps 9.0mm 158 amps 10.5mm 198 amps 12.5mm 245 amps 14.8mm 292 amps 16.5mm 344 amps