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Your starting battery needs a mate in the bush.


These days very few people travel without a portable fridge and camp lighting, and that dictates some form of auxiliary battery power. What do you need?


The typical 4WD traveller has a fridge in the back, filled with food and beer. Failure to keep beer at the right temperature is disappointing, but failure to keep food at the right temperature can be health-threatening.

The best way to ensure your fridge will run overnight, when the engine isn’t operating, is with a deep-cycle battery. If the fridge is connected to the deep-cycle battery all the time, it will keep the power flowing as long as it has sufficient charge.

Dual battery systems are designed to ensure the deep-cycle battery is charged, without the chance of the vehicle’s starting battery being discharged.

There are several ways of achieving that, from a simple, manual rotary switch to mini-computerised electronic devices. Before the arrival of electronic dual-battery isolators the state of the art solution was a solenoid device and these are still popular for fitment to older vehicles.

The function of an isolator is to disconnect the deep-cycle battery from the vehicle’s starting battery when the deep-cycle battery is operating the fridge. It also must maintain that disconnection when the engine starts, until the starting battery is fully charged, after which it allows charging current to flow to the deep-cycle battery until it’s charged.

The latest electronic devices take the process a stage further, by monitoring both batteries and adjusting charge rate to suit battery age and condition.

dual battery kit Some dual battery isolators have a function that allows the deep-cycle battery to act in conjunction with the starting battery, to boost starting power should the starting battery drop voltage.

That system functions well where cranking loads are small, but a dead-flat starting battery may not receive enough power from the deep-cycle battery to start a large-capacity diesel engine. You can then be stuck with a dead engine and a dead fridge.

Some electronic isolators with override have a block on the dual connection that prevents draining the deep-cycle battery if the starting battery has too little power to effect a dual-battery start.

However, the message is clear: you need some form of backup battery charging with every dual-battery installation, to make sure you won’t get stranded in the bush.


Do It Right

dual battery As important as the type of isolator you select is the way the dual-battery system is fitted. Traditionally, a second battery goes into the engine bay, but most of today’s engine bays are full of other stuff. Also, today’s engine bay temperatures are higher than those of yesterday and batteries hate too much heat – particularly absorbed glass mat (AGM) types.

A deep-cycle battery or a power pack and most types of isolator can be fitted into the back of a wagon or ute, but don’t scrimp on the size of cable connecting the starting battery to the auxiliary, or you’ll suffer from voltage drop. You can buy protective cases for deep-cycle batteries that have to be stowed inside vehicles.

A simple isolator won’t be enough for many modern 4WDs that have electronically regulated alternator outputs. What’s required is a purpose-designed auxiliary battery DC-DC charger, sometimes called a BC-DC charger.

Be particularly careful with packaged power packs that come with their own chargers. Many of these chargers won’t work with modern alternators and you’ll need to buy a separate DC-DC charger as well, making the exercise very expensive.

All connections, including earth terminals, must be of top quality material and securely fastened with Nyloc nuts. A small, loose connection will cause big trouble. If you’re uncertain where fuses or circuit breakers should be fitted, you need to consult an auto electrician. For example, if you think there’s nothing wrong with the photo at right, you definitely need a sparky!

There are several different battery and portable power pack types and the best of the conventional types cost plenty: budget $500. (Lithium is even more expensive.) A cheap starting battery won’t run a fridge reliably or for long.

A truck-type starting battery is best for dual-battery winching – not fridge operation. Deep-cycle batteries tolerate overnight drain better than starting batteries, but lead-acid deep-cycles need externally-plumbed ventilation and are high-maintenance.

AGM batteries with 100 amp-hour ratings are heavy and expensive, but don’t need ventilation, can’t spill acid, are zero maintenance and tolerate charging abuse better than gel types. However, AGM batteries don’t like heat, so an under-bonnet location isn’t ideal and won’t be warranted unless the battery casing is a special heat-insulated type.

Lithium-ion batteries are now available and we’ve covered that topic at the end of this article. We’ve been testing a Revolution battery kit for years and it’s performing brilliantly. It runs our fridge for two days without recharging.

No matter what isolator and non-lithium deep-cycle battery you choose you’ll still need to charge the battery every two days at least. Running the engine isn’t usually ideal.

Solar power is useful, if the sun is shining, but can’t be relied upon in all locations at all times of the year. However, sunshine is reliable in the desert regions during winter and in monsoon-affected regions during the Dry Season.

A solar panel needs a controller that’s compatible with your battery/isolator system.


Beware the cheap solar controller

There are many different solar panels available these days and many on-line kits come with a solar charge controller as part of a package that’s usually around $120 or even less.

The charge controllers in these kits have a retail value around twenty bucks and should be treated with great suspicion. At OTA we bought one and fitted it to our LandCruiser 75 Series, in conjunction with a 130-watt solar panel.

The panel was connected to our under-bonnet second battery via the low-cost controller and we measured the input voltage at 14.4 volts. So far, so good.

However, even with the second battery fully charged the voltage remained at 14.4V, meaning the controller wasn’t dropping to recommended ‘float’ level, around 13.6V.

We checked the controller’s ‘diode’ capability by running the engine, so that the alternator charged the second battery at the same time as the solar panel was charging it.

The controller failed almost immediately and never worked again.

We replaced this cheap controller with a Redarc SRP0120 solar regulator – RRP around $85 – and it’s been functioning perfectly ever since. The regulator is programmable to suit standard lead-acid, AGM, Gel and Calcium lead-acid batteries. It also has a diode function that prevents alternator charge harming the regulator or the solar panel.


How Many Amps

It’s easy enough to calculate what size battery you need and how long you can expect it to last before recharging.

If we assume four hours of fridge operation each day you’ll need 300Watt-hours/day (75W x 4hrs). Two hours of two-fluoro light operation (2 x 15W x 2 hrs) or about three times that of LED lighting is another 60Wh/day. That’s about the minimum power consumption a campsite can expect: 360Wh/day.

On the face of it the battery capacity needed is 360 divided by 12V = 30 amp-hours (AH).

However, battery makers recommend no more than a 70 percent discharged level so, to have a 30AH daily supply, a non-lithium battery needs to be 100AH capacity.

Reducing your power consumption is one way of extending battery charge life. Our experience is that LEDs use much less power than incandescent or fluoro globes and we find that an LED light near the stove and food preparation area, in combination with LED head torches, provides ample camp lighting.


Lithium deep-cycle battery

Everyone these days is familiar with the rechargeable lithium-ion battery that powers most mobile devices, from phones to power drills, so it seems strange that it’s taken so long for battery makers to produce a reliable, automotive deep cycle lithium battery.

The principal issues that have delayed the lithium deep-cycle battery are safety improvements and development of a specific charging system. Well-publicised fires in aircraft using lithium-ion batteries had to be avoided.

Where the familiar rechargeable appliance lithium-ion battery is a lithium-cobalt type (LiCoO2) the automotive deep-cycle version uses lithium iron phosphate (LiFePO4) technology.

This LFP battery (Lithium Ferro-Phosphate) uses LiFePO4 as a cathode material, because it’s a more stable compound that resists breakdown much better than LiCoO2 if short-circuited or overheated. The LFP battery won’t catch fire in the way an LCoO can and also offers longer life, a better power delivery rate and a constant discharge voltage.

The downside is heavier weight than an LCoO, but both types weigh only around one-third of lead-acid batteries.

Also, where a lead-acid, gel or AGM battery should not be discharged below around 70-percent of its amp-hour capacity the LFP battery is said to be fine with discharge as low as 20 percent. That allows the LFP battery to deliver more than twice the power of a traditional battery, from around one-third the weight.

On top of that, the LFP battery holds 12.8-12.5V until it reaches that 20-percent point, allowing the battery to deliver virtually full power until it is discharged, whereas a traditional battery loses voltage progressively as it discharges.

But there’s more: charging cycle life is said to be up to 10 times that of a traditional battery and charging times are typically 1.5-four hours.

However, the LFP equivalent of a 120 amp-hour AGM battery weighs only 10-15kg – up to 23kg less.

Put another way, an LFP battery of the same weight as an AGM can produce constant power for up to four times as long.

But, before you rush out to buy a lithium replacement for your deep-cycle battery, there’s a catch. To avoid damage to the LFP cells that could be caused by excessive charging voltages, temperature-based voltage compensation, equalisation or continuous trickle charging, it’s vital that the LFP battery is connected to a purpose-designed charger. Your existing charger cannot be used with an LFP battery.





























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