Updated March 2021
Before 2000, most battery and alternator charging used a more or less fixed voltage. This was typically 13.8-14.4 Vdc. This worked well for starter batteries but not for batteries that fed lights and fridges etc. This was because as charging voltage approached battery voltage, the voltage difference decreases. The charge rate accordingly falls.
Fixed voltage charging brings batteries to about 60% and 70%-75% within a few hours. It may then, however, takes a day or more to fully charge. If following makers advice not to discharge below 50%, this limits usable battery capacity to 20%.
Battery and alternator charging – increasing the charge
Charging voltage and current available limits battery charging. Increasing charger or alternator size, however, can only assist if the battery bank accepts that charge. Only really big ones (and most LiFePO4s) can.
Most low-priced mains battery chargers commence charging at a high voltage. They switch to a lower voltage toward the end. But, if they fail to do so, they wreck the battery.
Battery and alternator charging – multi-stage charging
Recommended by battery makers, multi-stage charging is effective. It charges deeply, quickly, and safely. Such units initially charge at constant current. They constantly increase charging voltage as battery voltage rises. There is typically a ‘bulk’ stage, sometimes preceded by short ‘warming-up’. During both, charging is at constant current. This continues until the battery reaches about 85% of full capacity. It charges at the maximum current the battery safely absorbs. This is typically 15-20 amps for a 100 Ah deep-cycle battery. About 25-30 amps for AGM batteries. It is yet higher for lithium batteries.
This is followed by an absorption stage. Current is reduced to about half that of the above. It is then held constant for one to two hours. This assists that charge to be evenly absorbed. Following this, charging drops to that required to balance the battery’s internal self-discharge. This also caters for minor constant loads – e.g. electric clocks.
Battery charging is temperature related. The maximum is usually 40 degrees C. Some chargers have an optional battery temperature probe. This automates charge voltage reduction on hot days . It is worthwhile in tropical areas.
DC-DC charging systems accept whatever the alternator puts out. They ‘juggle amps and volts’ to that required for optimum charging. A voltage sensing relay ensures starter battery charge priority. This relay also protects the starter battery if it drops below about 12.7 volts.
Redarc BMS 1215 – dc-dc alternator charger and mains battery charger and solar regulator in one unit. Pic: Redarc.
Some units combine alternator, solar and grid voltage battery charging and energy monitoring. Doing so reduces failures (most faults occur in exterior interconnections).
DC-DC charging enabled alternator charging faster and more effectively. It also helped computer engine and general management systems. These need a rigidly controlled voltage. DC-DC charging thus electrically isolates the auxiliary charging system from the alternator. The vehicle’s computer system ‘perceives’ the DC-DC charger much as a pair of spot lights.
DC-DC charging solves many previous problems, e.g., of some alternators producing only 13.6 volts. This is too low for effective charging. A few alternators, however, produce 14.7 volts or more. This introduces a risk of overcharging. DC-DC alternator charging described below is usually able to cope. This needs checking with individual manufacturers. Some have differing specifications. DC-DC charging also overcomes the poor charging of temperature-controlled voltage alternators. These may output 14.2 volts or more when the engine is cold. They may then drop to 13.6 volts at the engine’s running temperature. Such alternators were fitted to many vehicles from 2000-2010. A few until 2017.
The DC-DC charging method combines multi-stage charging (described above), with a ‘switch-mode function’. This function optimises the alternator’s output to that required for charging specific batteries. Most DC-DC charging copes with a typical 9-18 volts. This solves most issues.
This type of charging has been widely used from 2000. More specialised version later became required for the variable voltage alternators described below. These became increasingly used from 2014.
Battery and alternator charging – main types of alternator
Fixed voltage: these produce a theoretically fixed voltage. This typically exceed 14 volts, and never drops below 12.7 volts whilst driving. Such alternators were fitted to virtually all vehicles from about 1980-2000, and some until 2010-2011.
Temperature compensating: voltage varies with engine temperature. It is typically 14 or so volts when cold. It decreases as the engine warms up, to about 13.6 volts. These alternators were fitted to many vehicles from around 2000-2014 (and some later).
Variable voltage: these vary from 12.3 volts to 15-15.4 volts whilst braking. A few may drop to zero volts in vehicles that have the regenerative charging described below. These became increasingly used from 2012-2013 in hybrid vehicles.
As a generalisation, DC-DC alternator chargers can be used with any alternator that never (whilst driving) falls below 12.7 volts. Variable voltage alternators require a variant of that approach. They are described below.
Variable voltage alternators
This type of alternator recovers energy lost as heat whilst braking and decelerating. It normally charges the vehicle’s main battery to 80% charge. When the vehicle brakes or de-accelerates, the alternator increases charging to 15 volts or more. This rapidly brings that main battery to full charge. That (now extra) 20% supplies electrical needs until the starter battery falls to 80% remaining. During that period alternator output drops to about 12.3 volts or, with some, to zero. This eliminates the need for a voltage sensing relay. Charging ceases for up to two minutes every time the alternator drops below 12.7 or so volts. That relay may still be used – but connected in a different manner from before. (The DC-DC unit makers show how.)
Fixed voltage charging
A fixed voltage alternator has a high enough voltage to charge a secondary battery for leisure or auxiliary use. They are, however, becoming less common as more stringent fuel consumption, environmental and emissions standards, are legally required.
Modified DC-DC charging
Modified DC-DC charging mostly overcomes the ‘12.3 volt’ issue mentioned above. It uses the starter battery voltage as a reference for the alternator’s ongoing output. The specialised DC-DC unit optimises battery charging accordingly. These specialised chargers are complex units made by a relatively small number of makers. They are also referred to as BC-DC units.
Some variable voltage alternators (also known as ‘smart’ alternators) allow the vehicle to control their output voltage and current based on vehicle operating conditions. The intent is to reduce the alternators’ energy draw. It may, however, preclude it from charging a secondary battery system to a usable level.
How do I tell if I have a variable voltage alternator?
Smart alternators are now fitted to most recently-made vehicles. Their output may be from 12.3 volts to 15 or more volts. If you are electrically-minded, measure that voltage. Another way is to look at the vehicle start battery. A variable voltage alternator has an associated battery sensor. This is a small module mounted on or close to a battery terminal. The simplest way, however, is to ask an auto-electrician.
Does a smart alternator affect my dual battery system?
A variable voltage alternator charging a dual battery system can have unforeseen or detrimental effects. There can be situations where your starter battery is fully charged. This may cause the vehicle’s Engine Control Unit (ECU) to reduce the alternator voltage. This reduces the load on the engine thereby reducing fuel consumption. It also reduces emissions. While both are beneficial, if your voltage sensitive relay (VSR) is thereby ‘off’, your auxiliary battery receives no charge. Even if you override your VSR to stay on, the voltage will be too low for any useful charging. This shortens run-time for auxiliary loads and reduces battery life. Have an auto-electrician ensure your dual battery system is fully compatible with your variable voltage alternator.
Can I use a battery isolator with a smart alternator?
Relying on the alternator and a simple battery isolator may not provide the auxiliary battery with adequate voltage to charge effectively. This is the main reason why BCDC Dual Input DC-DC chargers are required in these applications. They are the only sure way to ensure you are fully maintaining your auxiliary batteries.
What is a DC-DC charger?
A DC-DC charger sources a DC power supply from a vehicle battery. It tailors that energy for multi-stage battery charging. A smart alternator’s output voltage, however, may not be suitable for charging an auxiliary battery. By installing a DC-DC charger, you can ensure your auxiliary battery is getting the correct charge and maintained via the vehicle input. Many include a solar regulator function.
Variable voltage alternators look like that shown below. They have a much wider pulley than most. Some have multiple belts – enabling them to handle up to 2 kW.
A certain way to determine your type of alternator is by measuring its voltage in typical driving. This involves connecting a voltmeter across the main battery and is likely to need extending the meter leads. Secure the leads to ensure they do not get caught up in the fan belt. Drive the vehicle across a range of conditions that including prolonged braking. Have an assistant observe and record alternator voltage. If at any time (while driving) the alternator voltage drops below 12.7 volts, that indicates it is a variable voltage unit.
Typical variable voltage alternator. Pic: source unknown.