This is part 2 of our “twelve volt series” where we discuss everything to do with wiring and equipping your four wheel drive. If you’re new to the auto electrical side of things, I suggest taking a look at part 1, where we’ve given a basic run-down on all components, so you know what we’re talking about.
Introduction – What’s a Dual Battery System & Why Would I Want One?
Dual battery systems are a key component of any touring four wheel drive.
It’s true that a lot of people these days are running around with overpowered; needlessly complex; and overpriced electrical systems. However, even if you’re an old-school minimalist cruising around with nothing more than a swag and a can opener, it’s still worth considering your electrical system. The reason for that, is the same basic reason that underpins all 12v set-ups, that you don’t want to drain your starter battery, leaving you stranded in the bush.
Which is essentially the key component of a dual battery system, regardless of how complex: Having a second battery to power your accessories while the car isn’t running, but for it to automatically isolate from the starter battery, when not running, so you can’t drain your cranking/starter battery.
Understanding Dual Battery Systems
The key components to a dual battery system are:
- A second battery.
- All camping related accessories (fridges, lights etc) are powered by this second battery, not the starter battery.
- This second battery is isolated from the starter battery, when the vehicle is not running.
- The second battery is reconnected to the alternator/charging-system/cranking battery, when the car is running. This recharges the second battery.
For clarity, we’re going to refer to this second battery from now on as the “auxiliary battery”. This is to differentiate from some vehicles that have a second battery to increase capacity, but without being isolated from the cranking battery. That’s not a dual battery system, in the way that it’s usually referred to.
Primary, Starting, & Cranking battery all mean the same thing – the battery that starts the engine.
Types of Dual Battery Systems
When we talk about the “dual battery system”, we’re discussing what’s being used to separate and connect the two batteries.
There’s three main types of dual battery system and they all cover the same basic aspects: isolating the auxiliary battery from the starting battery when the car is turned off, and reconnecting it when the car is running, so that the car’s alternator can charge both batteries. These systems are:
- Basic Isolator-Based System
- Voltage-Sensing Relay (VSR) System
- DC-DC Charger System
Basic Isolator-Based System (Solenoid) – Or Switched Relay
The cheapest and most crude way to do this, is to use a manual isolator that you manually switch when you turn the car off. At some point you will forget to do it, and you may drain your battery. I do not recommend this method.
Instead, another cost-effective solution is to use a solenoid. A solenoid, technically just describes an electro-magnet, but is commonly used to refer to a solenoid-controlled switch.
You can run a low amp wire that comes from an ignition switched feed. This just means that there is an electrical switch when the key/ignition is at the “on” position, and no power when the ignition is off. This small current then activates the solenoid switch, where it controls a much higher amperage switch that can handle the current between the two batteries and the alternator.
This is a simple way of having the batteries isolated when the car isn’t running, but connected when the car is running so that the alternator can charge the auxiliary battery.
Aside from cost, another benefit of a simple solenoid system is that if a solenoid does fail, it fails in the off position, so while your auxiliary battery may go flat, your cranking battery wont.
Sadly, as with all things in life, everything is a compromise. The cheap elegance of using a basic solenoid system, is that it will slowly degrade your starter battery.
A relay can easily replace a solenoid in this example, as you can switch a relay with a different power source to what is being switched. In this example, you would use an ignition switched feed to trigger the relay.
Voltage Sensing Relay
A Voltage-Sensing-Relay (VSR), is a relay switch that turns on when it senses a voltage above a certain threshold. This will commonly be at 13.2v and above for a VSR that is designed for dual battery systems. We’ll use 13.2v as the example, because it’s usually less than the charging voltage put out by a standard alternator, but is above what a fully charged battery will read. Because of this, the charging voltage being supplied by the alternator (more than 13.2v), will trigger the VSR to connect the second battery when the car is running. However, once the car is turned off and the alternator no longer charging, the voltage of the battery will read below 13.2v and will cause the VSR to disconnect the second battery.
Downsides to Using an Isolator or Voltage-Sensing-Relay
Battery Damage
A solenoid or VSR will work for a time and is certainly better than having nothing, but it does slowly damage the starter battery.
This is because the auxiliary battery will be discharged, from using your accessories, while the starter battery wont. This leaves them at two different voltages when they are reconnected.
Alternators work in a very simple manner. They provide a charging current to the battery, until the battery is full. They then stop charging, so that the battery doesn’t become overcharged. Overcharging batteries will overheat them, evaporate the battery acid and just generally damage them.
Because of the lowered voltage on the discharged auxiliary battery, the two batteries will read somewhat of an average between the two, as the alternator “thinks” it is connected to a single battery.
If for example, we have a fully charged starter battery at 12.8v and a fairly depleted auxiliary at 10.5v, then it would read close to 11.65v. A difference of more than one whole volt, compared to charged – which is substantial for a 12v lead-acid battery.
Thinking the starter battery is fairly flat at 11.65v, the alternator is going to keep charging the two batteries together. The problem is that the starter battery is already fully charged and there’s no way to direct the charge into just the auxiliary battery. This overcharges the starter battery, and will eventually ruin it.
This is why we use DC-DC chargers (discussed below).
“Smart Alternators” – Not Smart Enough for Voltage-Sensing-Relays
Exhaust gas recirculation; diesel particulate filters; smart alternators. Basically, any Euro-driven environmental policies for vehicles are generally a hindrance for us four wheel drivers.
The so-called smart alternators, are designed in response to tightening emission controls for vehicle manufacturers. Many things are labelled “smart”, such as smart phones; smart watches; smart fridges, for those too lazy to open the fridge door and see what’s in it. It generally just means a device that use to have a simple purpose, and was reliable, that has now had “features” added to it that don’t really improve anybody’s life, but makes it more finicky to work with.
Smart alternators, are simply just alternators that adjust their charging voltage output, to meet the demands of the car’s electrical systems, but without exceeding that. Because they’re charging less, some of the time, it’s less electro-magnetic resistance against the belt that’s driving the alternator, and therefore less load on the engine. Subsequently, less fuel use and less emissions. Talk about marginal gains.
The problem is that this charging voltage is often lower than the cut-in voltage that will activate a VSR. This means the battery won’t be charging the whole time the car is running. This doesn’t affect solenoids, as they are ignition switched.
I’ve got a 79 series Landcruiser, which I chose for its simplicity-driven reliability. I don’t have electric windows, I don’t have central locking, I don’t even have carpet. Why do I need an alternator that thinks for itself? Because some suit in Brussels, decided how a Japanese guy should be making a car.
DC-DC Charger Systems
DC-DC chargers, are as the name suggests, battery chargers that run from a DC supply.
DC-DCs have a VSR so that they know when to isolate or connect the two batteries, if your car has a conventional alternator. They also have a wire to connect to an ignition feed, if your car uses a smart alternator, and this overrides the VSR.
Aside from being versatile in how they switch, DC-DC chargers also fix the problem of overcharging starter batteries. They work by providing a tailored charge to the auxiliary battery, but they also prevent the alternator from reading the average voltage between the two batteries, so that the starter battery doesn’t get overcharged. Essentially, they protect both batteries, through a better charging profile for each.
Another benefit of a DC-DC, is that they usually come with an inbuilt solar regulator, so that if you do want to install solar panels, you don’t need to purchase a regulator for them. This also simplifies your wiring setup, which is good. For four wheel driving where there’s lots of corrugations, friction from rubbing and other imperfect conditions, it’s always better to simplify the amount of wires and accessories that you have.
Some DC-DC chargers also have the function to override the ignition and re-connect both batteries when the car isn’t running. If you have managed to drain your starter battery somehow, this can help you jump start it from the auxiliary battery.
Choosing the Right Batteries
- Lead-Acid (AGM, Gel, Calcium, Wet Cell)
- Lithium
- Capacity (Ah) Considerations
Deep cycle batteries, are what we want to use for the auxiliary battery. Deep cycle batteries can tolerate a greater depth of discharge without being damaged, compared to starter batteries.
Starter batteries are designed for very high output, for a short time and their capacity is measured in Cold Cranking Amps (CCA).
Deep cycle batteries are designed for a low amperage output, for an extended time. Their capacity is measured in Amp hours (Ah). One amp hour, is the amount of power required to power something drawing 1A, for one hour.
Lead-Acid Deep Cycle Batteries
When it comes to discussing deep cycle lead-acid batteries, AGM batteries are likely what’s being talked about.
Absorbed Glass Mat (AGM) batteries, are a lead-acid battery where the acid is absorbed by a fibreglass mat that is placed between the lead plates. AGMs are cost effective, reliable and will tolerate a wide range of conditions.
Gel batteries are similar to AGM, but instead of a mat, they use silica to thicken the acid solution making it behave like a gel. Gel batteries are more expensive than AGM and are less suitable for many of the conditions that a 4WD auxiliary battery may be subject to, such as heat. I’m guessing that more than half of the time when someone/articles mention gel batteries for 4WD use, they’re mistaking an AGM.
AGM and gel batteries are both maintenance free.
You may have heard of Calcium batteries in the context of off-road set-ups, but much like gel, may be a bit misunderstood. Calcium battery technology generally lends itself better for cranking, rather than deep cycle use. They also require being charged to a different voltage. As they’re less common, many current DC-DC charger models won’t have a charge profile for calcium.
If you’ve ever wondered about “wet cell”, it’s a broad term referring to batteries that have a normal acid solution in them. It will usually be referring to your typical cranking battery.
Lithium Deep Cycle Batteries
Lithium have some really alluring benefits, but some glaring pitfalls as well.
Lithium batteries are much, much lighter than lead-acid batteries.
Their other great benefit, is that they can be discharged much more deeply, so you can use a lot more power from them, for the same Ah capacity compared to AGM etc.
Lithium is also currently much more expensive than lead-acid. There are some cheaper lithium batteries coming onto the market, but it’s a case of buyer beware for the following reasons:
Lithium likes to catch fire or explode, if charged or discharged too fast, and if overcharged. Lithium batteries consequently come with in-built circuitry to regulate their maximum charge/discharge and to prevent them from being overcharged. On the aforementioned cheaper batteries, it’s hard to know just how good this inbuilt protection is. There are a lot of videos on Youtube of lithium auxiliary batteries catching fire and destroying vehicles.
Total discharge of a lithium battery (running it completely flat), can also cause them to catch fire. The chemical process causes copper dendrites to form on the cells, which can bridge them and short circuit. If you’re lucky enough for it not to catch fire, it may still completely ruin the battery.
If you are using lithium batteries – which are a great option if used correctly – its imperative that you have your dual battery system correctly set up. Use a reputable brand of DC-DC charger such as Redarc or Enerdrive, that has a charging profile for lithium to avoid over charging. Make sure you have a low voltage cut-out in the system and also be wary of all the accessories you’re running, to make sure you don’t exceed a suitable current draw from them.
Lithium vs AGM Capacity – Are All Amp Hours Equal?
As mentioned in the lithium section above, they can be discharged to a much greater depth compared to AGM or other lead-acid batteries. Because of this, a 100Ah lithium battery, is more equivalent to a 200Ah AGM, in terms of its usable power.
So if you’re considering the weight savings, you can double the figures as you’ll only need to compare a 100Ah lithium battery, to a 200Ah AGM. Likewise, you can do the same for your cost projections. It may be that the lithium battery is only half the difference in additional cost, that you originally thought it was.
