Battery Power

In this article we explain why you might need an additional power source independent of the starting battery installed into your vehicle and we look in detail at types of batteries, how and where to mount them, the role of isolators and the different types available, battery monitors and we have some useful tips for wiring up your accessories along with tons of clear pictures to guide you.

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These days travellers have many power hungry gadgets and accessories that they won't leave home without - be it a day trip or extended time away from home. All of these accessories have been adapted in some way to be powered from the vehicle's battery - this may be in the form of a power inverter to convert 12 volts DC to 240 volts AC, or a simple cigarette lighter 12 volt adaptor.

The vehicle's standard battery is not designed, nor does it have the capacity, to power these additional accessories and as most modern vehicles have their own built in accessories they already have a high power demand from the starting battery. Basic accessories like mobile phone chargers, iPod and portable DVD players can be used on the standard battery while the engine is running and in moderation when the engine is switched off.

It is obviously very inconvenient to have a dead starting battery and quite seriously is a life threatening possibility for travellers to isolated areas. It is recommended that if you need to power any of the above-mentioned products or accessories including fridges, inverters, and camp lights, that you should have an additional power source independent of the starting battery installed into your vehicle.

Auxiliary Batteries & Power Packs

Additional power sources fall into two main categories. Permanently mounted auxiliary battery systems and portable power packs. The latter becoming increasingly more popular now that newer vehicles on the market have limited under bonnet space.

In some cases, these auxiliary batteries may be mounted in the tool box of a caravan or camper trailer, or even on the rear tray of a ute. These auxiliary batteries in both fixed and portable systems are available in varying capacities and depending on your needs, will determine which system will best suit you and which battery type you should have fitted. There are also fridge packs available but as they take a longer time to recharge and have a lower capacity than conventional batteries they are not recommended for quick car charging and will not suit most travellers.

Conventional lead acid batteries fitted as auxiliary batteries to vehicles (or power packs), average around 90 to 100 amp hours. Broadly speaking this is the available amps in the battery for use to power accessories.

Device Usage Guide

Some typical amp hour usage values for some devices are listed below:

  • 2.5 to 3.0 amp hours for a 12 volt fridge (compressor type)

  • 1.0 amp hour for a 12v fluorescent light (twin tube)

  • 2.0 amp hours for a 12v portable DVD player

  • 5.0 amp hours for an inverter powering a laptop

  • 1.5 amp hours for an inverter powering a camera charger

  • 4.0 amp hours for an inverter powering a small TV

An Example

Let's look at a typical real-life situation and make some calculations:

Between 6pm and 8am the vehicle is at camp and not being driven - the fridge is on; the camera and laptop chargers run off a 600 watt inverter for 3 hours; and the 12 volt lead light is used for 5 hours. The total capacity taken from the battery is estimated over this 14 hour period to be 60 ampere hours. With a 90 ampere hour wet cell cranking battery and with 14 hours of usage overnight it's down 60 ampere hours. This will have to be replenished by the vehicle's charging system.


When a battery recharges, the charge is not consistent - as a guide it will bulk charge until about 60 - 80% full and then slow down and the balance of the charge may take as long as the first 80%. In the example above, the 60 amp hours used will take roughly 2 hours to fully recover on a 100 amp alternator, lets say 45 amps in the first hour and 15 amps in the next. During this time the accessories will be using charged power off the alternator via the auxiliary battery. If you look at the graph below, you will see the charge zone of a 85 amp alternator is 2000 to 4000 rpm (of the alternator, not the engine), which means a very fast idle speed or a slow drive for the required charging time.

The hours during the day that the vehicle is not driven also needs to be taken into account and allowances made for charge time in these calculations. Bear in mind that this is a guide only, and power figures will vary depending on make/model of accessory, type, and condition of the auxiliary battery and the charge rate of the alternator. You can also keep the batteries maintained when 240 volt AC is available by using a good quality 3 stage float charger.

Starting & Deep Cycle Batteries

When fitting a battery to your vehicle, caravan, camper trailer or power pack, make sure you fit one to suit your needs - this is also governed by your budget! Batteries come broadly as either Starting, or Deep Cycle. Starting (cranking) batteries are designed to supply large current for a short period.. Deep cycle batteries are more suited to long slow power draw and slow recharge so are better suited as the auxiliary battery and solar system set ups (More on Solar)
A good wet cell cranking battery will cost you around $140 but will have a limited life when being used as an auxiliary battery and if looked after will last 12 - 24 months. A marine battery is a semi deep cycle battery and costs around $200. It will last longer than a standard wet cell battery and again if looked after will last 24 - 36 months.

AGM means "Absorbed Glass Mat", which is a very durable sealed and maintenance free battery. It is made up of a saturated mat and will not leak even if broken. They have a very low internal resistance and will accept a fast charge with no damage or boiling. A typical 90 amp hour AGM will cost you $290 but with an expected service life of 10 years they are well worth it. AGM's are the replacement for your standard heavy duty battery (eg. the N70ZZ on a Land Cruiser or Patrol) and has the same dimensions but instead of around 17kgs the AGM weighs 30kgs - which leads me to the next topic. AGM batteries are available as either Starting or Deep Cycle.

Mounting Batteries

Mounting the battery is very important and must be done using good quality parts and brackets. Mounting the battery in a tool box or caravan boot is fairly easy but make sure you allow for ventilation, use a sturdy hold down clamp and the battery must be placed in a marine battery box or kept separate from tools, canvas and other loose items with a divider to prevent acid damage, short circuiting and possible battery rupture.
12 volt power pack units have a battery compartment and the power pack basically just needs to be secured in a safe upright position. When fitting the battery as a dual battery under a bonnet, there are a few things to keep in mind if you can't find a custom-made battery tray or carrier to suit your vehicle. Check your bonnet clearance before making a carrier or buying the battery - some systems require a low mount battery with recessed terminals similar to the BMW style batteries to obtain the bonnet clearance and where possible ensure the positive is the further away from the outside edge of the vehicle in case of off-road body damage. When making the carrier, use at least 25x25 angle and support the battery in at least 4 solid mounting points bearing in mind that there are corrugations and potholes on the roads you will likely travel.

With the battery (or power pack) mounted in a secure safe position, you can now connect them up to the charging system. Power Packs are supplied with cables and instructions and have built-in isolators to prevent draw from the starting battery when the engine is not running and with these it is just a matter of connecting the supplied cable to the starting battery and you are done. These cables are normally fitted with Anderson plugs for easy removal of the power pack when not in use, or when power is needed away from the vehicle for use at a camp site or in a boat.

With permanent mounted batteries, or dual battery systems, you will need to do the wiring to the charging system. To prevent voltage drop over long runs of cable it is necessary to use fairly heavy cable between the starting battery and auxiliary battery. The minimum cable thickness that should be used is 100 amp 8 B&S cable. In extreme situations it may be needed to use 140 amp 6 B&S cable. These cables must be run safely away from stone damage and sharp edges. When wiring up to a caravan or camper trailer use Anderson plugs at the hitch and use a fresh clean earth point - never use the hitch as an earth source. Use good quality lugs and brass battery terminals for all the connections between the two batteries.


To prevent the starting battery draining from the new connection you need to fit an isolator as well as spike/surge protection and an optional battery monitor. Some isolators have built in spike and surge protection so an external one is not needed, but it is important to know that you have one - most new vehicles have a computer and will possibly be damaged if there is a voltage spike as the two batteries engage and disengage especially at high engine revs when the amperage could be as great as 50 amps at 14 volts or higher. There are 3 basic ways of isolating the auxiliary battery.

Manual Switch

1) You can use a basic battery master switch, which requires opening the bonnet and turning the knob.
2) There is a simple solenoid, which can be fitted with a toggle switch via the ignition to manually engage the charging system.

Automatic Switch

Both these systems work but people tend to forget to switch them on/off and this causes problems especially if the battery is in the caravan and you crank the engine while the switch is on as this can melt the wiring and plugs.

3) So the preferred system is Battery Management or fully automatic system. These consist of voltage sensing relays or "Smart" solenoids. They engage when the engine is running and the starting battery reaches 12.6 volts. They are simple three wire systems and work off the main battery cables between the batteries. There is no need to alter any wiring on the vehicle and they are a fit and forget system!


You may fit fuses at either end of the positive battery cable but they will need to be rated at 100 amps and are very expensive. Most owners do not fit fuses on these cables for the following reasons:

  • Vehicle manufacturers do not fit fuses between the battery and the solenoid of the starter motor

  • There is very little chance of a short circuit between the battery and isolator

  • Fuses are a cause of voltage drop and will decrease the charge rate of the auxiliary battery

Battery Monitors

Battery monitors are optional. They are a great way to keep an eye on your auxiliary battery. There are basic LED models, and multifunction LCD models - both of which will show you battery status and charge voltage. 12 volt power packs will all have a similar monitor.

Wiring Up the Accessories - DIY

Once the vehicle is setup with an auxiliary battery and is connected and charging you can wire up your accessories. If you wish to DIY then here's some practical tips and reminders.

It is always best to plan ahead and fit wiring, fuses and plugs for future accessories even if you have not yet purchased them. When choosing cable, always go heavy, look at the products maximum amp draw and use at least twice the rated cable. In the case of fridges and inverters that are sensitive to voltage drop, it is best to use 100 amp 8 B&S cable to prevent voltage drop over the distance to the rear of the vehicle. In many cases it is easier to run this 100 amp cable (fused at the battery) to the rear of the vehicle and use a fuse distribution box to wire up the accessories. If possible, use auto reset circuit breakers, marine switches and connector strips for ease of wiring. Label fuses and wiring for future reference and carry spare fuses. Fit an extra lighter socket to the rear as well as an Anderson plug. It is also advisable to wire any existing factory sockets to your new auxiliary wiring.

Good Practices

Remember to always insulate cable, use rubber grommets when routing cable through metal, rustproof any holes drilled into the body of the vehicle, solder all joints, fit fuses or circuit breakers and make a wiring diagram for future reference. And if all this has just been too technical, seek the expert assistance of a professional 4WD Auto-Electrician.

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Created: June 2008
Revised: March 2013
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