Protecting your alternator when switching to lithium

In this post, I will try to explain how to protect your alternator and what we decided to do regarding our alternator. Especially regarding the following

  • Protecting the alternator from overheating
  • Protecting the alternator from a load dump
  • Increase charging from the alternator 

All references to lithium batteries in this post are related to LFP / LiFePO4 / LiFeYPO4 / lithium iron phosphate batteries. 

With lithium batteries, we have a great source of power. But this source also needs to be refilled. Lithium batteries also have the potential to damage the alternator since most alternators are made for lead-acid batteries. 

Our primary source for charging the batteries will be solar panels. But we also wanted one more fail-safe way of charging our batteries using the alternator. 

To keep things simple and cheap, not everything that we did might be right for you. But regardless, it is good to consider these topics even if you end up deciding not to change anything regarding your alternator when switching to lithium batteries.

A lot of people are not changing anything regarding their alternator when switching go lithium and it works fine for them. But it is still important to know the risks and decide what is best for you.

Protecting the alternator from overheating

The small automotive alternators fitted on yacht engines are not made to handle lithium batteries in several different ways. 

With lead-acid batteries, the alternator would not run on max all the time. Since lithium batteries have such a low resistance, the lithium battery will take all the charge current the alternator can deliver. This causes the alternator to deliver the max charge current until the battery is fully charged. This might not be an issue if you have a small battery, and you don’t charge your battery with the alternator at low SOC values (State of Charge).  

With a small case alternator running at max, it’s a high risk that it would overheat, especially at low RPM, since the internal cooling will not be as effective at these low RPM:s. 

The Victron blog has a great article and video about this and how the small case alternators overheat. In this article, they end up using a different type of protection of the alternator then us. Keep in mind that in the Victron tests the alternator sits on a workbench surrounded by nice fresh air. The environment with the alternator sitting close to a hot engine will not be as good for the alternator as in the tests. 

Victron post on charging lithium batteries with alternators

There are basically two ways of reducing the risk of overheating the alternator. 

  • Current limit the alternator
  • Temperature control the alternator

Current limit the alternator

The most common way of current limiting the alternator is to use a lead-acid battery for the alternator. The alternator is then charging the lead-acid battery, and the lead-acid battery is charging the lithium battery through a battery-battery charger. 

This is a reasonably cheap solution that both current limits the alternator (B-B charger) and protects the alternator form a load dump (lead-acid battery). There are several options for DC-DC chargers, both from Victron and Sterling. To effectively current limit the alternator, you would have to use a DC-DC charger that is rated much less than the alternator. At the same time, there have been some other discussions about the risk of overheating the DC-DC charger if it has to run at max. 

But this solution has some significant drawbacks. Due to the disadvantages of using a B-B charger, this was never an option for us, and I have not researched them in depth. 

One of our main issues is space. For this solution, we would have to keep both a lead-acid start battery and add a DC-DC charger. We simply have no space for this. 

The second objective for us was to add a secondary reliable charging source e.g., charging from the alternator while at the same time doing an Atlantic crossing or charging while at anchor. If we added a current limiting solution, we would reduce and not improve our charging from the alternator. 

One of the key features with the lithium batteries is the low internal resistance that enables the quick charging. By current limit the alternator by charging a lead-acid battery from the alternator, you are back to the poor charging efficiency you have with lead-acid. You are then missing out on some of the key features of the lithium batteries. 

Temperature control the alternator

The other option of protecting your alternator from overheating is to use an external alternator regulator that can temperature control the alternator. 

There are several options for external alternator regulators. Some of the best ones are the Balmar MC-614 and the WakeSpeed WS500. The MC-614 has some nice features such as the Belt Load Manager that can limit the max load of the alternator, regulate the alternator based on temperature etc. The WS500 can be connected to the BMS with CAN and is the only external alternator regulator that can deliver a true CC/CV charge cycle. 

Keep in mind that you first need to make sure that your existing alternator can be controlled by an external regulator instead of the internal regulator. 

By adding an external regulator to the alternator, you don’t only protect your alternator from overheating. You also have the option to configure all the settings on the alternator for optimal charging of lithium batteries, such as optimal absorption, bulk, and float charge values. While at the same time, disable options such as temperature voltage compensation that you don’t want with lihtium. 

Protecting the alternator for a load dump

If the alternator is disconnected from the battery, there is a high chance of damaging either the alternator or the different loads (or both). 

There are several ways in how an alternator can get disconnected while running. 

  • Someone accidentally disconnects the battery with a switch while the engine is running
  • The BMS, under voltage or over voltage protection disconnects the battery for some reason
  • A fuse is blown disconnecting either the battery or the alternator

If the alternator suddenly gets disconnected from the battery, the alternator has no route to deliver it’s current. If the alternator has no other way to deliver it’s current, the load dump will most likely burn the alternator diodes and destroy the alternator. 

If the loads are still connected, there is also a high risk of these loads getting the load dump with extremely high voltage levels destroying all the loads. Imagine what would happen to your navigation equipment if they would get a load dump from the alternator of more than 80 V.

Load dump protection device

The simplest and most likely cheapest solution to handle the load dump from the alternator is to add a load dump protection device such as the Sterling Power Alternator Protection Device (APD) that is built to handle an alternator load dump.

Safe shutdown of the alternator from BMS 

The best protection is if the BMS safely shuts down the alternator instead of the BMS disconnecting the battery in case of an issue. But this requires both a BMS that can talk to an external alternator regulator as well as an external alternator regulator that can get signals from the BMS to shut down. 

Safe shut down from external regulator

Some alternators will also safely shut down if the external alternator regulator loses power. While discussing this issue with Balmar, I was informed that the Balmar alternator would safely shut down if the MC-614 regulator loses power at the same time as the alternator is disconnected from the battery.

Lead-acid battery with a B-B charger (battery to battery charger)

With the B-B option where the alternator is connected to a lead-acid battery, both the lead acid-battery and the B-B charger will act as a load dump protector. 

What did we end up with?

To increase charging from the alternator, we replaced our stock 60A alternator with a Balmar 120A alternator. But since we only had a V-belt, we also had to upgrade to a serpentine belt since single V-belts can handle max 70A alternators. 

The alternator is controlled with a Balmar MC-614 external regulator so we can set all the charging parameters for lithium as well as temperature control the alternator. The power to the external alternator regulator is wired, so the regulator will lose power at the same time as the alternator is disconnected from the battery, which will cause the alternator to shut down safely. 

The installation of the alternator and regulator, conversion to the serpentine belt, and calibration of the tachometer will be in a later post. 

6 thoughts on “Protecting your alternator when switching to lithium

  1. Hej Jonas
    Intressant artikel, jag är också i skedet att byta till LiFePO4 och är sedan tidigare utrustad med en MC 614.
    Det skulle vara intressant att se hur du kopplat regulatorn och vilka inställningar du gjort.



    1. Ska försöka skriva om installationen av vår generator, regulator samt konverteringen till serpentinbälte inom en vecka.

      Spontant kan jag säga att vi är väldigt nöjda.

      Däremot var installationen väldigt mycket mer krävande än vad jag trodde den skulle vara. Dels för att VolvoPenta inte har en standardlösning för anslutningen av varningslampa samt varvtalsräknare.

      Men även för att vi fick modifiera svänghjulet som hade en diameter på 2-3 mm större än vad den borde ha haft. Vi fick även byta den trasiga displayen på varvtalsräknaren för att kunna kalibrera varvtalsräknaren.


  2. I think overloading problem which has seen after replacing lead acid batteries with Lifepo4 can be avoided by motoring in high rpms to keep the Alternator cooler. Alternator temperature should be checked with a temperature controled relay which disconnects the Lifepo4 service battery bank from the Alternator.

    Boats have two battery banks in common for service and cranking. The Alternator charges both of them simultianously. These two battery banks are separeted with a diode isolator that blocks the current drawn from the crank battery for service use. The lead-acid crank battery remains in its place and it is going to be charged by the alternator even if the Lifepo4 service bank is full and disconnected by its own BMS or the Alternator’s temp control shut off circuit.

    I am not quiet sure if you need a dump protection for the Alternator when you replace only lead-acid service batteries with Lifepo4.


    1. As long as you don’t get an open circuit there is no need for dump protection. Keeping the lead-acid for cranking will prevent an open-circuit in case of BMS-disconnect.


  3. Hi there! Thank you for the detailed information. I only have lithium on my boat, no lead acid so i’m doing the sterling alternator protector and i am adding the 614 regulator. Can tell me how you acomplished “The power to the external alternator regulator is wired, so the regulator will lose power at the same time as the alternator is disconnected from the battery”.

    Many thanks,


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