Alternator Charging Technology
In modern engines with smart alternators, the ECU changes the way the alternator charges. It may change the charging set-point between 9V and 16V.
This is done to meet engine emmissions control regulations and optimize fuel consumption.
These regulations require the starter battery to be charged at no more than 80% when running under power. When the engine is "braking" and there is no acceleration , the ECU directs a high charging voltage and the battery can then max out at 100% capacity. When back under acceleration, the ECU will direct a lower charge voltage to put less load on the engine from the alternator saving fuel and reducing carbon emmissions. The ECU will also direct a voltage change if the vehicle has auto start-stop.
To charge a second battery system requires a battery charging profile at a series of set voltage levels.
A DC to DC alternator controller bridges these two conditions so the battery is charged correctly. Some units, particularly the Victron unit, can be switched between "power supply" and "battery charging" mode. In power supply mode, the unit applies power regardless of the battery condition and is commonly used to put power into a flat battery.
In battery charging mode, the DC to DC applies a load to the alternator. Whilst the level of load can be pre-set in some models, it is generally on or off. Victron DC to DC have an engine detection system based on a voltage range that automatically turns the unit on and a separate range for isolating the starter battery, that is called "lock out".
Fixed power DC to DC units are generally kept at a low level os less than 600W (50A nominal). This is because if on when the engine is idling, the load may cause the alternator to overheat as there is less air flow. Fixed power DC to DC units range in efficicency of 85-90%. The generate significant heat.
Placing 4 units in parallel as suggested in the brochure of an Australian manufacturer, would produce around 15% of 3000W in heat which is 450W. That amount of heat is difficult to manage.
Time To Charge
The time taken for a DC to DC power (amps) to charge the target battery charging capacity (amp hours) is easy to calculate. Typical values are 5 - 8 hours for charging from a low State of Charge (SOC) to 100% SOC. That has worked well in the past when touring involved long drives and battery capacities were small to moderate.
However, increased battery capacities now see 400-800Ahs in a caravan and at least 200Ah in a canopy. Touring patterns have also changed with shorter drives between locations or short drives frm a stationary base camp. Charging in 1-2 hours is very convenient. This requires 100-250A of charging at 12V. For this level of charging, the batteries have to be Lithium.
Some Lithium batteries are designed to charge at a 0.5C or lower rate. There may be a variety of reasons for this but generally it is cost based. Going to the effort to increase the charge rate and then find that the Lithium battery system cant accept that level of charge would be a major disappointment.
In the discussion that follows, it is assumed that the Lithium battery bank can accept the level of power (amps).
It is now possible to charge a 400Ah Lithium bank in less than 2 hours in one of two ways:
1) Direct Control of the Alternator combining an alternator regulator and battery charger.
2) Using a "Buck Boost" planar transformer that can vary the load on the alternator and charge a battery bank.
Direct Control of the Alternator
This is an external alternator regulator and battery charger combined. There are several brands that achieve this with a high degree of efficiency. It could be argued they are 99% efficient as there is only a minute amount of heat loss. However, there are pre-cautions needed in designing systems that use them. For that reason they are used by system designers who provide and end to end solution. The designer must be very knowledgable on the Lithium battery BMS technology being used. If the BMS calls for a "stop charge" and the charging circuit opens suddenly, the accumulated energy in the alternator has to be dumped suddenly. Such a load dump may damage the system.
In the marine environment, these controllers have been used for some time. The product Safiery supports has extensive range of parameters to program for big alternators on small engines as in sailing boats. The largest alternator power Safiery supports is 12.5kW for marine. That needs to be a 48V alternator for practical cable management. At this level, 250A will be flowing.
Dual alternators for port and starboard engines can be controlled in sync with a CANbus connection between them.
In the vehicle environment, the alternator is small compared to the engine but the charging process is more complex. Understanding the algorithm to move from a bulk or absorption charge level to float and then back again to re-bulk requires a carefull choice of parameters. The power level is high and the system design requires quality products that all talk togther.
Toyota Lancruisers, both the 79 and 200 series have an easy to control alternator. With a replacement alternator at 250A output, it is easy to extract 220A+ from them for charging.
The alternator controller is perfect for second alternator systems. There are a standard option on nearly all the USA "muscle" trucks, Mercedes Benz Sprinters, VW Craters, Ford transit Vans, Isuzu trucks, MAN trucks and even the new Hilux can have a powerfull second alternator. The largest system installed by Safiery is a 9kW alternator and charging on a MAN Expedition vehicle. The typical size installed is 5kW.
Using a "Buck Boost" planar transformer
Vehicles with some degree of electification require multiple voltage systems with high power transfer between them. A "mild hybrid" vehicle uses a 48V battery bank to drive the motor on a ring gear of a petrol engine for added torque. The motor may used be used to start vehicles and becomes as a generator when the engine is braking. Both the Dodge RAM 1500 and 2500 have such a system. Performance vehicles like the Aston Martin Valkrie have electric super chargers that are likely to be powered at 48V from lithium. The Bentley Bentayga and Audi S6 also are mild hybrids at 48V.
Mild hybrid technology suits the Australian 4WD market well as range is increased with more efficient petrol engines yet torque can be added by the 48V battery.
The "buck Boost" Safiery uses hails from the electric vehicle market and is manufactured to "automotive grade" which is a very high standard. Being an automotive product it requires ECU control. It wont operate with out that. Safiery have an ECU embedded in the case that controls the Buck boost with CANbus (as all vehicles do) at 500Bps. It can switch direction in the power flow in less than 25ms.
A "buck boost" demands power when the low side "pin" voltage is less than the available alternator voltage. This makes it very effective for alternator control. The largest "Buck Boost" installed by Safiery is 3,000W or 250A on the 12V DC alternator side.
Compliance to Emmision's Control
Back to where this article started, emmissions control, particularly with European engines has a strict requirement on the ability of the vehicle to reduce alternator load at times. Adding charging of a second battery with a regular DC to DC will immediately change the fuel consumption level. In many countries this is presently allowed. However, it may be restricted in the future. Safiery has (Patent Pending) technology that will only operates the "Buck Boost" when the vehicle is in regenerative braking and then puts no load on the alternator when it is not. This makes this technology absolutle leading edge and future proof.
Lithium batteries may have a BMS that communicates with CANbus. There are several advantages:
- No Shunts are required as the current measurement using hall effect sensors is in the battery itself.
- An accurate voltage, current and temperature measurement can be transmitted by CANbus - there is no loss
- A pre-warning of BMS action to "stop charge" can be issued by CANbus
Safiery links CANbus Lithium batteries both at 12V and 48V with the 1) Buck Boost, 2)Alternator controller and the 3) Victron GX operating system. It is a simple and elegant system that is all digital. The design is modelled on a vehicle's CANbus system.
Examples of alternator control
“Simplicity is at the very heart of this system”
Use your 4WD as a power station for your caravan or camper trailer “plug in and run 240V”.