Dual battery systems

A dual battery system adds a second, auxiliary battery to a vehicle so that accessories such as a fridge, lights or communications gear run from the auxiliary battery instead of the starter battery. A charger or isolator connects the two while the engine runs, so the auxiliary battery is topped up as you drive. The parts and layout vary with the vehicle and how it is used.

The main reason is to keep the starter battery for starting. Running a fridge, lights or other loads from the auxiliary battery means those loads are not drawing on the battery you need to start the engine. It also lets you size the auxiliary battery to the loads you actually run.

Not always. If you only run small loads for short periods, a dual battery system may not be necessary. It tends to make sense when you run a 12V fridge, camp lighting or other gear while the engine is off, camp for more than a night, or work from the vehicle. Tell us what you run and for how long, and we will say whether a dual battery system suits your use.

The starter battery supplies the high current needed to crank the engine and is part of the vehicle's original electrical system. The auxiliary battery supplies your accessories and is generally chosen for steady, repeated discharge. Some battery types are better suited to one job than the other, so the two are not always the same type.

Both are used as auxiliary batteries, and neither is right for everyone. Lithium iron phosphate (LiFePO4) batteries are commonly lighter and are often rated to use more of their stated capacity than AGM, but they usually cost more upfront and need a charger set up for them. AGM is a well-established option and is generally heavier. Check the manufacturer's data for the exact battery, and ask us which suits your vehicle, space and budget.

It depends on what you run, for how long, and how you charge. Start by listing your loads, estimating how many amp-hours they use in a day, then allow for how much of the battery's capacity can safely be used. That usable figure differs between chemistries. We can work through this with you; see the sizing example on this page.

It depends on the fridge, the weather, how often it is opened and how long you stay. As an illustration only, Outbax, what size battery for a camping fridge notes that quality compressor fridges commonly draw about 2 to 5 amps while running and cycle on and off. A fridge using 3 amps for 8 hours in a day would use roughly 24 amp-hours that day. Your battery then needs to cover that over the days you camp, with solar or driving to recharge.

Many lithium batteries have documented temperature limits for charging, and some have internal protection that blocks charging outside them. Limits differ by manufacturer, so check the data for your battery. If you camp in cold conditions, mention it so we can take it into account when choosing the battery.

REDARC explains that an isolator is a simpler option, which it suggests for older vehicles, generally pre-2015, with fixed-voltage alternators, while a DC-DC charger suits newer vehicles with variable-voltage or smart alternators and longer cable runs, such as to a canopy. See REDARC, choosing the right charger for a dual battery system. The right choice depends on your vehicle, the battery type and where the battery is mounted.

Some vehicles vary the alternator's output voltage while driving, and may lower it, rather than holding a steady voltage. REDARC says this is more likely in post-2015 vehicles, although some earlier ones do it too. A simple connection to the auxiliary battery may not charge it well in that case, which is one reason a DC-DC charger is often used. We check this for your vehicle.

Often, yes. Solar needs a regulator, and some DC-DC chargers include a solar input. As one example, REDARC BCDC1250D product page states that the BCDC1250D has a built-in solar regulator and separate DC and solar inputs. Whether solar suits your setup depends on the charger, the battery and how you camp.

A BMS is built into most lithium batteries and monitors and protects the cells, for example against charging or discharging outside safe limits. What it protects against differs by battery, so check the manufacturer's documentation. It is not a substitute for correct fusing and wiring.

A monitor is not required for a system to work, but it shows the state of the auxiliary battery so you are not guessing. It helps you see how long you can keep running your gear. We can include one when we plan the system.

A correctly selected and installed system should not. REDARC states that a correctly wired system with a charger does not harm the alternator, and that its charger prioritises the starter battery first. Problems usually come from the wrong parts, undersized wiring or a poor install, which is why the system should be matched to your vehicle and fitted properly.

Yes, an inverter can be added to many systems. Inverters draw large currents, so the battery, cable and fusing all need to suit the inverter's size. We work with pure sine wave inverters, which can suit sensitive electronics. Tell us what you want to run so we can size it properly.

Usually something can be made to suit, but where the battery goes, how it is mounted and how it is wired depend on the vehicle. Space under the bonnet, in the tray or in a canopy is limited and varies. Give us your make, model, year and what is already fitted, and we will talk through the options.

Some owners do. However, REDARC recommends installation by a qualified auto electrician or technician. The risks include incorrect fusing, undersized cable and poor connections. We design, wire and fit systems in our own Warwick workshop, so you are not left to work it out yourself.

Batteries can deliver very high current, so a fault in unfused or undersized wiring can overheat cables and cause a fire. The right fuse and cable size depends on the charger, battery and loads, and manufacturers specify them. For example, REDARC states that the input and output fuses for the BCDC1250D are not supplied and recommends a matching fuse kit. Follow the manufacturer's instructions for every part.

Rules and policies differ, so check with your manufacturer or dealer, your insurer and your state authority before modifying the electrical system. Tell your insurer about modifications that you fit. Use a qualified installer and keep records of what was fitted. This is general information, not electrical or legal advice.

It varies a lot with the battery type and size, the charger, the wiring, the fuse panel and the vehicle, so we do not publish a single price. Tell us what you want to run and we will quote for your vehicle. Fitting time depends on the scope, so ask when you book.

Check connections and cable routing now and then, particularly after rough tracks, and keep an eye on battery condition using the monitor if you have one. Follow the battery maker's guidance for storage and charging. If something looks worn, loose or damaged, have it checked.

Yes, our work covers 12V and electrical set-ups for work and fleet vehicles as well as 4WDs used for touring. Tell us the equipment you need to run and the duty it has to do.