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Adding an alternator to your lithium battery setup

Posted by Peter Kennedy on 12/19/2017 to Lithium Ion Batteries

A discussion on the best way to incorporate an alternator into a Lithium-Ion battery system.

There are a few things to consider when incorporating an alternator into a lithium-ion battery system.  The first is:  "Is the alternator up to the job?"  A Lithium Ion battery can put a lot of load on an alternator and have it working flat out for an extended time.  There might need to be some way to limit the demand to prevent the alternator from self destructing.  Other considerations include making sure the alternator will charge the lithium battery at the correct voltage and figuring out what happens when the lithium battery is full.

The relationship between battery capacity and alternator output

Lithium batteries have a maximum charge current and a recommended charge current.  For maximum battery life the alternator charge should be sized to not exceed the recommended charge current. Here is the table for Victron 12.8 volt Lithium Ion batteries:

You can see in the table above that the 300 Amp Hour battery can accept a charge at up to 750 Amps but the recommended continuous value is 150 Amps or less.

Charge Voltage

As you can see in the table above the recommended charge voltage is 14.0 - 14.4 Volts    Most alternators will not be able to get the battery to that voltage until the battery is nearly full. When it does get to the full voltage the BMS (Battery Management System) needs to shut off the charge to the battery.

What happens when the battery gets full?

When the battery gets full the BMS needs to shut off the charge.  One way to achieve that is to interrupt the connection between the alternator and the lithium battery.  In this case there needs to be another conventional battery, usually a start battery, for the alternator to charge. You cant disconnect an alternator's output while it is running because with nowhere for the power to go its internal voltage would soar and destroy the diodes.

Another way to achieve the same result is to have the BMS turn off the alternator regulator. This is normally only possible with an externally regulated alternator.  When its turned off the alternator isn't available to charge any other batteries, such as a starting battery.



Ways to limit the load on the alternator

If you have a monster alternator then it may not need limiting.  That would be the kind of heavy duty truck alternator that is designed to run at high output all day.  For any other alternator it would be wise to consider some kind of limiting to prevent it from self destructing when charging Lithium Batteries at high output for an extended time.

Alternators that have an external regulator with alternator temperature sensing offer the first and easiest way to limit the current.  When the alternator gets too hot the regulator dials it down a bit to keep it within the allowed temperature.  The latest version of the Balmar MC-614-H regulator offers a continuously adjustable output to keep the alternator within temperature.  

Other alternators may need some additional external limiting device.  The Victron 12/1200 BMS has a built in feature that limits the alternator to a pre-determined output up to 100 Amps.  It works by putting a fuse of the desired limit in the BMS.  If you want to limit the alternator to 80 Amps you put in an 80 Amp fuse.  As the fuse nears its rated capacity it begins to heat up and its resistance changes.  The BMS recognizes that and reduces the current accordingly.   Using this device requires you to have an alternate battery for the alternator to charge.  This 12/1200 BMS can be used as a current limiting device even if it isnt being used as the BMS that controls the batteries.

Another high tech device used to limit the alternator is the Victron Buck Boost DC DC Converter.  This is especially appropriate to Sprinter and EuroVans and other vehicles with high tech alternators.  The Buck Boost DC DC converter has a vibration sensor to turn it on automatically when the engine is running.  It takes the input voltage and changes it to whatever voltage is required by the Lithium Battery, and in the process limits it to the programmed value, up to a maximum of 50 Amps.  With this you can use a 24 volt alternator to charge a 12 volt battery or vice versa

Circuit Diagrams

In the layout above the alternator is charging a conventional start battery at all times.  The start battery is connected to the Lithium Ion battery via the Cyrix-Li-Ct which is a battery combiner controlled by the BMS.  When the Lithium Battery is full the combiner gets shut off but the alternator continues to charge the start battery.  There is no current limiting shown, the alternator had better be up to the job or have active temperature control.


Victron 12/1200 BMS
In this next diagram (above) the Victron 12/1200 BMS is used and this has a built-in way to limit the alternator to up to 100 Amps depending on what size fuse is placed in the left hand fuse holder.  Note that the alternator always has a conventional battery to charge even when its output to the lithium battery is interrupted.

The Victron 12/1200 BMS has its limitations though so you might prefer to use the VE Bus BMS.  Any system with a Multi-Plus or Quattro inverter charger is going to want to use the VE Bus BMS.  The 12/1200 is really for smaller systems and has a limit of 100 Amps on the output side.  In that case you can still use the VE Bus BMS as the BMS that is controlling the batteries but use the 12/1200 solely in its function as a current limiter.  The wiring diagram would look like the one below.  Again, as before, the alternator always has a backup battery to charge.




In the next drawing, below, the alternator is only charging the Lithium battery.  The alternator is externally regulated by a Balmar MC-614-H regulator.  The setup includes a temperature sensor on the alternator which limits its output by keeping the alternator within the allowed temperature.  It is also possible to limit the alternator to a certain percentage of its rating in the regulator software.  The ignition circuit of the regulator is controlled by the "allow to charge" signal from the BMS.  When the battery is full the "allow to charge" signal turns off and so does the regulator and therefore the alternator.  A small relay may be required to allow the BMS charge signal to interrupt the ignition wire of the regulator.


The last way shown below is the most hi tech (and the most expensive)  It was designed for use with the type of smart alternators and dynamos used on vehicles such as the Sprinter Van  and Euro Van where the output voltage can vary dramatically. To avoid interfering with the vehicles electronics, which might cause warranty issues, the buck boost converter can be programmed to use its built in vibration sensor to only operate when the engine is running. The output produced by the converter can be regulated to suit the requirements of the Lithium batteries it can prevent excessive load on the alternator by limiting how much charge is available for the Lithium battery.  This sophisticated device is a clever workaround to all kinds of difficult problems that can be encountered when making modifications to these vehicles. The wiring diagram looks like this:

Buck Boost DC DC Converter

A note about these diagrams

The diagrams on this page are block diagrams to illustrate a concept.  They were all taken from published Victron Energy drawings available on the company website www.victronenergy.com  They were edited to remove extraneous information not relevant to the immediate topic in hand. In general they don't show circuit protection, battery switches or other similar items.

I did have a quesrion about all these systems, what happens when the engine is stopped?  Does the lithium battery drain into the conventional battery?  Here is the official answer:  "The Cyrix-Li-ct will disconnect when the engine isn’t running and the starter voltage starts to drop much like the regular Cyrix. The BMS 12/200 blocks energy from flowing back to the starter battery altogether."


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