Fit a Defender dual battery system


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Dual batteries will allow you to plug in accessories and start your engine : credit: © Alisdair Cusick
Dual battery systems are a popular modification in Defenders. Alisdair Cusick looks at why you might use one, what you need to consider, and how a system is fitted

Need to know

Time: 2 days
Cost: See below
Difficulty: 4 out of 5
Models: All 90/110/130 and Defenders.
Tools needed: Screwdrivers, crimping tools, open-ended and ring spanners, plus sockets, drill and bits, hot air gun.
Parts & costs: 12BS7622KIT3 – £1440.
Work safely:
• Use the right tool, for the right job.
• Always be fire safe in the workshop.
• Use the correctly rated lifting and support equipment.
• Get an expert to do the job, if in any doubt.
• Always leave battery post insulators on until the last moment to reduce the risk of shorting them out on tools or bodywork.
• Ensure metallic tools do not unintentionally contact electrical terminals. 
• Always wear safety glasses and safety shoes when handling batteries.
Thanks to: Tim Consolante of Mobile Centre, Ian Baughan of IRB Developments for their help with this feature:;


It is a fact that if you run auxiliary electrical devices in your Defender, sooner or later you’ll encounter battery issues. Most likely that will be as the vehicle’s battery ages – perhaps the temperature drops, and one morning the vehicle won’t start. That is because the battery is nothing more than a storage device that relies on a chemical reaction to produce electrical energy. As batteries age (or are mistreated) that process loses efficiency and the battery’s ability to hold charge depletes, eventually reaching a critical point where it can’t hold enough energy to start the vehicle. If you add additional load on a battery, say plug in a fridge, run exterior lighting or a winch, then obviously that electricity has to come from the battery. Without the vehicle running, you won’t be replacing any of the energy used. Again, you may find you can’t start the vehicle after running a device for a surprisingly short amount of time.

A dual battery system gets around this, because instead of one electrical storage device to run everything, you have two: an engine starter battery dedicated to the vehicle circuits, and another for auxiliary electrics. Should one battery deplete, you can use the other battery to start the car. Once the engine is running, the alternator will recharge both batteries.

There are a few crucial elements to consider for the system to work properly. The batteries need to be wired correctly so the alternator sees voltage across both batteries, rather than just the nearest one. Then, the two batteries need a way to be separated when the engine is off, otherwise you could flatten both batteries, say by running auxiliary equipment overnight.

That task is done by a voltage-sensing Automatic Charging Relay (ACR). This is a critical part of the system, allowing separation of the batteries when the engine is off, but when the car is running, automatically connects them and strives to balance the state of charge in both. It will detect a certain voltage (on this system 13.5 volts for 30 seconds or 13 volts for 90 seconds) then combine the batteries. When the vehicle is stopped, it isolates the batteries, meaning you could flatten the auxiliary battery, but the starter battery remains ready to start the car. The ACR is a vital component, and must be suitably rated to cope with the current demand of the vehicle. If you fit dual batteries to run a 4-amp fridge, you’ll need a smaller, cheaper ACR than someone needing dual batteries to run a 400-amp winch.

It is also recommended to use an auxiliary fuse box because, if you’re intending to add multiple devices, it makes sense to wire them from a central point, rather than have a rat’s nest on the battery terminals. An auxiliary fuse box means all the extra kit can be fused, and because all wiring is in one place, that means fault finding (or adding future accessories) is made much simpler.

For owners wanting independent secondary power sources, a dual battery system is a very worthwhile modification. Allow a day and a half or more to fit it, as there’s a large amount of thinking, measuring and making involved, as well as just fitting. Here, Tim Consolante from Mobile Centre and Ian Baughan of IRB Developments walk us through fitting a dual battery kit to a 2.4 Puma Defender 110.


Battery facts

A battery is nothing more than a storage device that holds enough electrical energy to start a vehicle. Once it is running, the alternator (or dynamo on older cars) generates enough electrical energy to run the vehicle, and in the process recharges the battery. Until then, the battery sits ready to be used.

They have a finite lifespan, but one that can’t be readily defined, for there are many variables that affect longevity. Repeated short journeys in cold conditions shorten the lifespan because the highest demand is created on the battery without proper time to replenish the energy removed. Likewise, allowing them to go flat and then jump-starting them, can also create problems, not least because it was allowed to go flat in the first place. Remember, they rely on a chemical reaction, and all elements required to achieve that will lose effectiveness unless you look after them.


How to wire the system correctly

Mobile Centre’s Tim Consolante explains that the most common error (DIY and professional) is in not connecting the batteries correctly when using an ACR-based system. We all recall connecting batteries in parallel from school but, on a vehicle, the connection needs to be different. For the vehicle to read the charge across both batteries, they should be wired in parallel. So, positive on one battery to positive on the other (via the ACR in between) and negative to negative, but crucially, the car’s positive feed should then connect to one battery, and the car’s earth should connect to the second battery. This way, the alternator can read the state of charge across both batteries. The ACR’s job is then to balance that charge, or to separate it when the vehicle isn’t running, in order to preserve the starter battery.

Both batteries should be the same make, age and construction. The ACR is trying to balance both batteries. It will struggle to do that if both don’t behave similarly.

The kit: The comprehensive kit from Mobile Centre includes batteries, fuse block, charging relay, busbars, stainless steel battery tray, cables, connectors, sleeving and all mountings.

Battery location: The existing vehicle battery is, of course, under the passenger seat; but there’s sufficient space for the two dual batteries and the ACR and fuse box in here.

Typical: Defender single battery installations are rarely standard under here, so all dual battery fittings differ. It’s common to find the original battery to be unsecured – an MoT fail.

Make it easy: To help access, remove the seat runners with backrest, too. Just four bolts, two minutes and we can get right in. Useful when dealing with heavy batteries.

Alarming: Before disconnecting battery, there’s a procedure: ignition on, off, then disconnecting the battery within 30 secs. This stops battery back-up siren triggering.

Out with the old: The battery can now be removed. Ours was unsecured, which went unnoticed at the last MoT. That’s because testers aren’t allowed to remove any trim.

Not just a pretty cover: Sound-deadening under seat box is removed. This also serves as thermal protection, aiding battery performance in the cold. Battery tray is unbolted and removed.

Holder for two: In goes the first new part, the tray to hold both batteries. It will need holes drilling in the seat box floor for the four fixings, before fitting.

Assessing the space: Ian uses two dummy battery cases as we decide where the ACR should mount. Using dummies (cardboard in the correct shape) makes life easier at this stage.

Pro crimp: The first ACR to battery positive lead is made up. Tim demonstrates how the professionals crimp a terminal on, using this battery-powered handheld crimper.

DIY crimp: Ian crimps the other end using his crimping tool. Both valid, both effective. If you don’t want to buy a manual crimp tool, pay a local garage.

Test fit: On goes the first lead and, at the same time, the ACR location is double-checked. The ACR is being held upside down here while the lead is attached.

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Fit it: Happy all is good, the batteries and lead are removed, and the ACR is fitted. Fit four rivnuts, so the ACR can simply be screwed into position.

Pro finish: Connectors are crimped, then heat-shrinked; conduit adds mechanical protection, but is also heat-shrinked. This makes joints airtight and prevents corrosion.

Take the bus: Fit a busbar, which is a neat way to have more than one connector, whilst keeping them fused. Better than having all feeds on one battery post.

Connecting the ACR: New cables are made for the vehicle battery mega fuse to ACR, busbar to ACR, aux battery mega fuse to ACR. The existing winch supply is also connected.

Fuse box trial: A suitable position is found for the fuse box bracket, allowing for clearance around the batteries and fixings. The bracket is trimmed a little, then is fitted using rivnuts.

Negative bus: Tim and Ian decide to use a busbar for the earth, too. Again, it allows more than one feed, and is the neatest way to achieve it.

New earth: Original earth lead had previously been trimmed, with a poor-fitting terminal. Tim makes a replacement, to run from transfer box to chassis, then to the busbar.

Common fail: A standard Puma earth lead (right) and ours (left). The armoured right-angle joint is a common failure point on these leads, according to Ian. Something to watch.

Fuse box: The fuse box is fitted onto the bracket (pic 17), then Tim makes and connects the power and earth leads for it. Both leads go to their corresponding busbar.

Factory fuse box: The positive feed from factory fuse box, leading into the battery box, goes onto the vehicle battery positive post. Both fuse boxes now sorted for positive and earth.

Testing, testing: Batteries and positive leads go back, to check clearance/cable routing. Negative lead length measured against dummy batteries: never use a steel rule near real ones.

Tight fit: The battery negative joining lead is fitted, then another lead to the busbar is measured. Extra length is needed to allow the battery to fit into the battery tray.

Dash switch: The ACR dash switch wiring is fitted with conduit, then has a terminal crimped on. We can now begin to wire our devices into the auxiliary fuse box.

Populate it: Ian wires in the ACR dash switch (plus an existing screen wash system), wiring them from the rear for a neat appearance – we don’t want an octopus graveyard.

Switch decision: The owner wants the ACR switch under the dash vent near the driver. Ian drills the corners of the hole, then trims it to size with a Dremel.

Switch fit: To mount the switch neatly, remove the four centre console fixings, then use a trim tool to prise out the centre section and disconnect the radio connectors.

Feed the wiring: Wiring runs forward from the fuse box, under the transmission mat and up through the dash. Put the pins into the connector and connect the switch.

Connect the switch: Replace the centre dash, then, back at the seat box, connect the Deutsch plug for the switch harness, then fit the cover to the fuse box. That’s almost it.

Batteries in: Wearing safety glasses, fit the two batteries. You may need to fit the wiring after they are in the car. Take great care to avoid inadvertent shorts.

Last job: Finally, the battery clamp is fitted, and the terminal insulators. It is worth a check all wiring is intelligently routed, and not going to rub or catch anywhere.

Test: When the car is started, after a short period the ACR will be seen and heard to operate, pulling down the yellow button and with two solid red lights on the switch.


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