When considering what batteries and charging / isolation systems to use in your 4WD, there are many things to consider. These factors include;
What will the system be used for?
What environment will it operate in?
What is the design life?
What is the budget?
In my case, I was designing a system for a very specific purpose. There is no such thing as a “perfect” design for every use case, as the use cases vary so much. There is always compromise in every design. My aim was to minimise the compromises and optimise the design for my specific purpose.
So, to answer the questions above for this exercise;
What will the system be used for?
My system was designed with the following requirements in mind;
o to provide a mobile 240VAC supply for my various drone battery chargers, that would allow me to fly a drone for up to 8 hours whilst only stopping to swap drone batteries, without needing to start the car engine
o to run a 240VAC/12VDC 40 litre fridge more or less constantly
o to allow non-critical vehicle accessories such as lights, stereo and accessory sockets to remain running at least overnight in a camping scenario, without starting the car’s engine
o to allow all of the above to run indefinitely, without having to worry about leaving sufficient charge in the main battery to start the engine at anytime
o to be able to run a 14,500lb winch for up to 5-10 minutes at a time, without the engine stalling from a flat battery.
o the system will need to be mounted in a 4WD LandCruiser
o I don’t want the noise of a generator constantly running when I’m in a quiet camping spot, nor do I want to carry the extra weight and size of a generator, so that ruled out using a generator for the 240VAC supply
o If the car is parked at a place where 240VAC is available, it must be able to utilise that as a charging input
o Solar should be used to charge the batteries at all times
What environment will it operate in?
I operate mainly in the state of Victoria, Australia. This area sees extremes of geography and temperature. During summer in the desert, you would expect to see temperatures of up to about 45 degrees Celsius during the day. During winter in the mountains, you would expect to see snow, and temperatures down to about minus 20 degrees Celsius overnight.
As indicated, the geography ranges from sand dunes in the desert, to extremely steep, rough and often muddy mountain “goat-tracks”, and everything in between.
The environment also includes the physical limitations of the available space for mounting equipment. In my case, space limitations restricted the maximum inverter size to about 3500W, and restricted the maximum number of batteries inside the cabin to roughly 3 x 100Ah, plus another 2 x 100Ah under-bonnet.
What is the design life?
Since some of the components of this system are quite expensive (such as batteries) and their life can be dramatically shortened by misuse, I designed the system to pamper these components, in order to extend their life as much as possible. As such, the system should last for a minimum of 10 years (except for the lead-acid batteries), and hopefully, much longer than that!
What is the budget?
I didn’t really set a strict budget as such, I was more concerned with achieving my objectives, and maximising the life of the components used. However, if your budget is tight, I’d recommend focussing first on getting a solution that guarantees your car is always going to start, before worrying about powering any accessories. As you will see, powering accessories can always be added later.
Battery Selection
There are now two major types of battery for consideration; lead-acid, and lithium. Within each of these battery types there are many variations.
For the purposes of this discussion, we will only be looking at Flooded-Lead-Acid (FLA), Sealed-Lead-Acid (SLA) and LiFePO4 (lithium) batteries.
Lithium Vs Lead-Acid Battery - General Design Considerations
Lithium batteries are very light, and can be discharged regularly to 80-100% Depth of Discharge (DoD) without harming the batteries. They have a life cycle of 2,000-5,000 cycles or even more, if looked after.
Lead-acid batteries are very heavy (typically over double the weight of an equivalent lithium battery), and can typically only be discharged to a max 50% DoD (for deep cycle) or 80% DoD (for starter batteries) without harming the batteries. They have a life cycle of several hundred to a thousand cycles, if looked after.
Lead-acid batteries can typically discharge >700CCA, so are ideal for starting motors, even in cold temperatures. CCA refers to Cold Cranking Amps. CCA is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. The rating refers to the number of amps a 12-volt battery can deliver at 0°F (= -18°C) for 30 seconds, while maintaining a voltage of at least 7.2 volts.
Lithium batteries can typically discharge approx. 100A continuous, for a 100Ah battery, at room temperature (25°C). A 100Ah lithium battery is typically about the same size as a standard 100Ah lead-acid starter battery for a 4WD. The maximum discharge rate for this type of lithium battery is typically 1.5-2C, ie 150-200A for a 100Ah battery, but only for a short time (typically between 5-30 seconds). However, it is not recommended to discharge a lithium battery at more than 1C, (ie 100A for a 100Ah battery), on a regular basis, as it will prematurely age the battery. Note that there is NO equivalent CCA rating for lithium batteries.
Lithium batteries should NOT be charged when their temperature is below zero degrees Celsius, as doing so will permanently damage the battery (they can be discharged in temperatures of minus 20 degrees Celsius or lower, however any discharge below zero will age the battery more rapidly). They usually will not even accept any charge, if below zero degrees Celsius. So they are NOT suitable as a starter battery in cold climate areas, as the alternator always needs to stay connected to the battery to avoid damage, hence the battery will be charging as soon as the engine starts running.
Lead-acid batteries can operate down to -20 degrees Celsius and beyond without harm, so are ideal as starter batteries, even in cold climate areas.
High temperatures also adversely affect all types of batteries. Charging/discharging or even storing a lithium battery above 30 degrees Celsius will start to age it; above 45 degrees, it will age rapidly. Since under-bonnet temperatures can reach 70 degrees Celsius or more, lithium batteries are NOT suitable for under-bonnet operation. Lead-acid batteries can handle high temperatures better, but high temperatures still adversely affect them.
Lead-acid battery voltages vary by several volts, according to their State of Charge. A 12V lead-acid battery should never be discharged less than 12V, if it is to stay healthy; so a voltage-based disconnect, set at 12V, can be used to isolate a lead-acid battery.
Lithium batteries have a very flat voltage discharge curve (see below), and the level of this curve varies according to temperature; so a voltage-based disconnect cannot be used as a reliable disconnect method. A more accurate way to disconnect a load from a lithium battery is to use a State-of-Charge method that looks at the entire history of the battery’s usage. This can be done by using an inline shunt to measure the total current drawn from the battery bank, another sensor to measure voltage, and a chip to monitor the history of both parameters and make calculations. The Victron Energy Battery Monitor model BMV-712 Smart does all this, and has Bluetooth communication to an app on your Smartphone, to enable you to change the operating parameters.
Lead-acid batteries for vehicles generally only fall into two types: Flooded-Lead-Acid (FLA), and Sealed-Lead-Acid (SLA). Batteries designed exclusively for starting most cars and 4WDs are typically FLA types. Lead-acid batteries designed for deep cycle purposes are generally SLA. Deep cycle SLA batteries are ideal for running small, constant loads, such as a fridge. There are also some backup batteries that are dual-purpose, ie can be deep-cycle, but can also be used as a backup to start a car. These batteries are also typically SLA type.
Conclusions around battery selection
The main battery for starting the car should be a Flooded-Lead-Acid type, mounted under-bonnet.
The backup battery for starting the car should also be a lead-acid battery, mounted under-bonnet. I chose a dual-purpose SLA type, which I also use to run the fridge.
Use lithium batteries to power auxiliary devices, and for non-critical accessories. These should be mounted inside the cabin, where temperature variations are minimised.
Rewire all non-critical car accessories over to the lithium batteries, so as not to drain the main starter battery when stationary for long periods.
Use automatic isolation devices on all batteries to prevent over-discharging the batteries, in order to maximise their service life, and so they still have a reserve capacity when needed to start the car.
Logic Rules for Design of Isolation Circuit for Lithium Batteries
State of Charge (SoC) smart meter has a programmable changeover relay, set this to trip at 20% SoC
If SoC is ok AND (ACC OR ACC O’RIDE * is on) then Non-Critical Accessories Rail **(NCAR) is on. (* ACC = +12V supply from main battery, switched by ignition key. ACC O’RIDE = manually switched +12V supply. This is used to keep car accessories going, without needing to leave the key in the car!) (** Non-Critical Accessories Rail is used to supply +12V to all non-critical vehicle accessories, as required).
If SoC is low, NCAR is off.
If ACC AND ACC O’RIDE are off, NCAR is off.
Inverter has a remote control, but the on/off button is a toggle operation (ie press once to turn on, press again to turn off, etc)
There is a +12V switched rail inside the inverter’s remote control, that can be used to indicate the state of the inverter (ie +12V rail is on means inverter is on)
Inverter can be independently switched on or off manually at any time, via remote control on/off button
When SoC becomes low and inverter is on, then toggle inverter on/off button to turn inverter off.
Design Guidelines for Charging Circuit for Lithium Batteries
MUST use a charger that has a lithium battery charge profile, to ensure optimum battery longevity.
Need a dual input charger, ie inputs for alternator and solar panels
Solar circuitry should utilise Maximum Power Point Tracking (MPPT) to optimise efficiency of solar panels
DC-DC charging is preferable, so that the lithium batteries will always be charged at the optimum rate, regardless of what the alternator is supplying, or the voltage of the supply battery, or the voltage drop in the charge cables. DC-DC charging essentially allows any battery to charge any other battery, regardless of voltages.
The charger should have an isolator built-in, to stop the charging process from the supply battery, when the supply falls to a certain level (eg, when the motor stops running). The solar input should still work independently.
Maximum charge current should not exceed maximum charge rate of lithium batteries. For a 100Ah lithium battery, this is typically 50A.
Redarc BCDC 1250D meets all of these guidelines, and has one of the highest outputs available in the market (50A). For even higher outputs (eg to charge multiple 100Ah batteries), Redarc BCDC chargers can be connected in parallel.
Logic Rules for Design of Isolation Circuit for Lead-Acid Batteries
Isolation (circuit opens) should happen at 12V or above
Re-engagement (circuit closes) should happen at least 0.5V above circuit opens setting, to prevent hysteresis effects. This ensures that the isolation circuit won’t close unless charging is taking place.
Isolation and re-engagement should happen bidirectionally, so that charging of either battery will eventually lead to charging of the other battery, once the first battery is sufficiently charged.
A manual override switch should be available, to allow the batteries to be forced into a parallel connection. This is useful for when winching, or if the main battery has been accidentally drained and needs a boost, or develops a fault.
If a bidirectional isolator is being used, current rating of the isolator should take into account the maximum current flow in either direction! This should include maximum current drawn when winching!
Redarc SBI 212D meets all of these guidelines, and has one of the highest current ratings available in the market (200A).
Headlights
When engine is running (ie, IGN is on), all headlights should operate from main battery/alternator.
When engine is off (ie IGN is off), all headlights should operate from NCAR, so they can be used as an auxiliary lighting source at campsites.
Driving Lights
Driving lights should only operate when high beam is on.
For simplicity, driving lights can be powered from main battery/alternator. Don’t use driving lights as auxiliary lighting, when stationary.
Current Drain Considerations
LED headlights: high beams draw 4 x 60W = 240W total (includes draw from low beams, which stay on) = 20A max. Put onto changeover battery selector relay.
Fridge: rated constant draw is 2.5A. Is on constantly, on camping trips. Surge may be approx. 13A. Put on Aux 1. Also fed by alternator / solar panel.
Audio System: max draw is 10A. Used intermittently. Put onto Aux 2-4.
Accessories Sockets: fused @ 15A each. Put onto Aux 2-4.
14,500 lb winch: max draw is approx. 300A. Leave winch on Main, use Aux 1 in parallel when winching. SBI 212D isolator has a manual switch input to enable parallel operation. (Optionally wire up SBI 212D isolator so that this comes on automatically). Lithium batteries can also be manually switched in, in parallel with the lead-acid batteries, to assist with winching (this is wired so that it can only be done when the chargers are off, so as to not short-circuit the chargers). Hand throttle can also be used to increase engine revs to enable maximum alternator output, when winching.
Inverter: max draw is approx. (3500W/12) x 1.05 (efficiency drop) = 306A. Max continuous current drain of 3 x 100Ah lithium batteries is 300A, should be ok if this is marginally over, as the inverter is also fed by alternator / solar panel, and these lithium batteries can handle up to 2C.
Non-essential accessories moved to Aux 2-4 (lithium batteries)
Headlights: max 20A
Audio system: max 10A
Cigarette lighter: max 15A
Interior lights: max 10A
Switchable accessory sockets: max 15A each
Non-switchable accessory sockets: max 15A each
Charging considerations for AC Charger
Main battery: “Century Ultra Hi-Performance, model N70ZZL4WD” (now replaced by N70ZZLXHD)
No special charging considerations mentioned on the website.
Aux 1 battery: “Century Dual Force + , model 27X MF”
The charger has to be AGM compatible and rated at no more than 15 - 20% of the batteries amp hour capacity.
The absorption / bulk charge rate should be set between 14.6v – 14.8v
The float charge rate should be set between 13.6v – 13.8v
A Victron Energy Blue Smart Charger IP65, 15A model meets all of these requirements. It is also waterproof, and has Bluetooth monitoring.
Summary
Below is the circuit diagram that shows my final design. It essentially combines a “traditional” dual-battery under-bonnet lead-acid solution using a bidirectional isolator, with a “piggy-back” lithium battery solution in the cabin, fed by multiple DC-DC chargers in parallel.
The dual-battery lead-acid system’s main role is to guarantee that the car will always start, when needed. The lithium batteries’ main role is to power all the non-critical accessories. There is also some overlap between roles, and each system can assist the other, if needed.
The system has gone through numerous iterations and upgrades to arrive at this point, and is now at the stage where it is meeting all my specified design requirements. It has been extremely reliable, with no issues at all over the last year of operation.
There are many other factors involved in the design that I have not touched on, such as fuse and wire size selection, protection circuits, alternator upgrades, issues around “smart” alternators used in newer cars that may affect your installation, and so on. Hopefully, however, this has given you enough information to make your own decisions about how to get started, from here. Good luck!
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