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Tech-Tip: Mounting an Auxiliary Battery in a Pickup

Considering the vast array of pickup trucks available on the market today – including the various styles they come in, such as King Cab, Extended Cab, and Dual Cab – it’s easy to see how they’ve become so popular. There really is a truck for every driver’s individual needs! With their tremendous payload capacity and optimized available storage space, today’s trucks offer seemingly limitless options for auxiliary battery mounting locations.


No longer does a truck owner have to wonder, “Can I mount a second battery in the engine compartment?” Additional mounting options abound: Inside a truck cab, you can install one behind or under a seat. In an SUV, the rear cargo area is an option, as is the rear bed/box of a pickup truck. However, some locations present potential electrical complications such as voltage drop or similar charging woes.


One reason for such issues is bad grounding wire connections. In an electrical circuit, think of the positive wire as carrying the power/current from a power source i.e. start battery to a rear mounted REDARC BCDC charger or similar, and the negative wire carries the power/current back to the power source/starter battery to complete the circuit.


A bad ground wire connection can occur if the ground wire is connected to bodywork – such as a pickup truck bed/box – that is rubber mounted to the vehicle chassis. The box mounting isn’t sufficient to support the common ground/negative connections needed to complete the electrical circuit, especially when subject to high power demands such as a DC/DC charger trying to recharge an auxiliary battery. This means the auxiliary battery might not get properly charged.


This can be overcome by installing additional negative straps called “negative earth/ground lead connections” between the pickup truck’s chassis and the rear bed/box holding the auxiliary battery. This can help ensure a reliable negative/ground connections between the vehicle’s engine block, the chassis, rear bed/box and the negative of your DC/DC charger and auxiliary battery. This will reduce the likelihood of inefficient connections leading to diminished battery performance, dead batteries, or premature battery failure.

Please note that a qualified technician should carry out the following strap installation procedures. But for technically minded DIY enthusiasts, and those who like to be well informed, read on.

These straps consist of measured lengths of battery cable with eyelet terminals attached at each end. Using existing mounting bolts, nuts, or brackets on the vehicle, you can create a solid ground connection among the previously mentioned vehicle parts – such as the truck’s engine, chassis and truck bed/box. Once these additional cables (straps) are in place, it’s time to test the ground reliability and circuit strength.


To do so, you’ll need a reliable digital multimeter to test for voltage readings. Start the truck’s engine and verify that the alternator is charging by first testing the vehicle start battery voltage, this should be somewhere between 13.2 > 14.6Volts. The voltage difference between the vehicle’s starter battery positive and device you are powering i.e. DC/DC charger input, should ideally be less than half a volt (<0.5Volt). If the voltage variance is this minimal, the ground straps are doing their jobs, and the powered device is receiving sufficient power. If however there’s a greater variation in voltage readings, you likely still need to improve your grounding connections and/or increase the size/AWG rating of your cable/s.


This simple addition of cable straps and test procedure can ensure that you enjoy a great camping weekend – with your food and drink properly chilled – instead of reaching your destination and discovering your fridge is warm, your food is spoiled, and your auxiliary battery is dead! Avoid such a disaster! Manage your grounds!


Article written by Mark Bruce, REDARC Electronics Technical Training and Development Manager – North America. Mark Bruce is a qualified Auto Electrician with over 35 years experience in the trade and wholesale automotive electrical industry. 


For more technical advice, speak with Mark directly by contacting us directly.