How to charge lithium and lead-acid batteries with an alternator?

Q&A: How to Charge Lithium-Ion and Lead-Acid Batteries Using an Alternator?

When installing a lithium-ion battery in a boat or RV, you must decide if and how the alternator will charge it at the same time as the starter battery. An incorrect installation can damage the alternator. With the help of functional diagrams, this article addresses some key issues and highlights some challenges that users face.


Q 1: Do I need to keep the lead-acid starter battery?


Yes. Most Li-ion batteries are not designed to deliver the cranking current required to start a diesel engine, and if used that way, they would age faster. In the case of “drop-in” lithium-ion batteries, there is a risk that the internal EMS will shut down the battery. Always having a full starter battery operating independently of the house bank is a useful safety precaution.


Q 2: Can I connect lithium-ion and lead-acid batteries in parallel?


Yes. Most of the time, the lithium-ion battery will trickle-charge the lead-acid battery (it has a higher resting voltage: 13.2V vs. 12.8V) and that will keep the lead-acid battery full and healthy for a long time. Having two battery banks allows the user to keep the lead-acid battery as a backup in case the EMS shuts down the lithium-ion battery, to avoid being left in the dark. (Instructions are given at the end of this article.)


Q 3: Can I charge the lithium-ion battery with the same voltage I use for the lead-acid battery?


This is possible, but it will cause the lithium-ion battery to age faster (capacity loss). Multiple studies show that charging a Li-ion battery at 13.8V is enough to fill it to more than 95% SOC. The battery will age faster if it is charged at a higher voltage or if it is kept at a “float” voltage above 13.2V. Users should be aware of this risk.


Q 4: Can my existing standard alternator charge the lithium-ion battery?


Doing so can damage or destroy the alternator. Although the charge voltage will probably not destroy the battery, this is not the best thing to do for battery health. The real risk is on the alternator side. The lithium-ion battery has a very low internal resistance that forces the alternator to deliver as much current as possible. Standard alternators have an efficiency of about 60%, so when charging at 100A the alternator generates 1,000W of heat. The alternator dissipates that heat with its internal fan, but at low RPM or over extended periods, the fan will not be able to extract enough heat and the alternator will start to smoke. Some people have reported that they have charged their lithium-ion battery directly with a non-regulated alternator for years and never had a problem. Our guess is that their charge circuit has a resistance high enough to limit the current.


Q 5: What are my options for charging the lithium-ion battery with the alternator?


  1. Add an external regulator to the alternator and ensure that:
    • it can be adjusted to suit your lithium charging parameters (do not trust or use the default “lithium” setting)
    • it has a temperature sensor to reduce the charge current as the temperature rises.
  2. Keep the alternator and the lead-acid battery as they are, and add a DC/DC charger to recharge the lithium-ion battery from the lead-acid battery when the engine is running. For efficiency and cost considerations, this would work best when the charge current is low (< 40–60 A).


Challenges to bear in mind:


1 – An alternator cannot be disconnected from its load while it is charging

This would create a high voltage spike that can destroy some of the alternator’s components. This situation will arise if the EMS disconnects the lithium-ion battery while the engine is running.

  • The easiest way to avoid that situation is to have the lead-acid battery permanently connected to the alternator. Even if the lithium-ion battery is disconnected by the EMS, the alternator still has the lead-acid battery to charge.
  • Another approach, if the alternator has an external regulator, is for the EMS to shut down the regulator properly before disconnecting the charge bus.


2 – If both batteries are connected in parallel you could have a flat starter battery and not be able to crank the engine

When the lithium-ion battery’s voltage drops below 12.8V (about 40% SOC), any load will draw current from both batteries. If the SOC drops too low, there may not be enough energy left to start the engine. (This recently happened to a friend after three days without sunlight).

Two approaches may be considered:

  1. Connect the lithium-ion battery to the starter battery only when the engine is running (use the engine ignition to close a contactor). This may not be effective if you do not run the engine regularly, as the starter battery will self-discharge.
  2. Keep both batteries connected in parallel most of the time and let the TAO EMS disconnect them when the lithium-ion battery voltage dips below 12.8 V. Reconnect them when the voltage rises above 13.2 V. The advantage here is that the starter battery is trickle-charged most of the time.


Sample Functional Diagrams:

Disclaimer: These functional diagrams are intended to illustrate how the different components are interconnected and work together. They are not wiring schematics. Negative wires and other irrelevant components have been omitted for greater readability. These diagrams can be used as a basis for planning and documenting the wiring schematic for your installation. The wiring schematic should be prepared by a professional who knows the regulations and safety rules applicable to your installation.

These examples are based on TAO EMS’s capabilities. If using a different BMS, please ensure it has the relevant outputs and logic to control them. The EMS configuration is not presented in detail here, as it is shown in the manual. If you need help with the configuration, please ask on the forum.


1 – Lithium-Ion Battery Charged Using a DC/DC Charger:



  • The alternator is always connected to the starter battery.
  • The starter and lithium-ion batteries are not connected together?
  • Charge and load disconnect relays are closed during normal operation (outputs 5 and 6).
  • EMS output 1 is closed during normal operation to enable starting the charger with the ignition key.



  • When the engine is running, the DC/DC charger draws energy from the alternator to charge the lithium-ion battery (depending on the charger’s capabilities, this can be done via the ignition key or automatically when the voltage of the starter battery is above a threshold).
  • If the EMS detects a full lithium-ion battery or a high voltage warning situation, it shuts off the DC/DC charger (output 1).
  • If the EMS detects a high voltage fault (perhaps caused by a defective DC/DC charger) it will:
    1. give an early visual and audible warning
    2. then, after a moment, it will shut off the charger (output 1)
    3. lastly, it will disconnect all charge sources (output 5).


2 – Lithium-Ion Battery Charged Using the Alternator




  • The alternator is always connected to the starter battery.
  • The lithium-ion and starter batteries are connected in parallel via a relay that is controlled by EMS output 2; this relay is important as it prevents the starter battery from discharging in the event that the lithium-ion battery is deeply discharged (this can be done using a manual switch to save money, but it increases the risk of human error).
  • The battery interconnect relay is also controlled by a manual override switch with 3 positions (EMS – OFF – ON). It should be set to the “EMS” position for normal operation.
  • The charge parameters of the alternator are set by the external regulator to suit the lithium-ion battery.
  • EMS output 1 is closed during normal operation to enable starting the regulator/alternator with the ignition key.
  • Charge and load disconnect relays are closed during normal operation (outputs 5 and 6).



Note: All voltages mentioned are examples (you can set them to suit your requirements).

  • When the EMS measures a cell voltage above 3.25V (13V for a 12V battery), it connects both batteries in parallel to allow the lithium-ion battery to trickle-charge the lead-acid battery (output 2).
  • When the engine is started, the regulator is turned on by the ignition key and both batteries are charged.
  • If the EMS detects a full lithium-ion battery or a high voltage warning situation, it shuts off the regulator/alternator (output 1).
  • If the EMS detects a high voltage fault (possibly caused by a defective regulator) it will:
    1. give an early visual and audible warning
    2. shut off the regulator/alternator (output 1)
    3. then, it will disconnect the lithium-ion battery from the alternator and the lead-acid battery (output 2)
    4. lastly, it will disconnect all charge sources from the lithium-ion battery (output 5).
  • If the EMS measures a cell voltage below 3.2V (12.8V for a 12V battery), it disconnects the starter battery from the lithium-ion battery to avoid discharging the starter battery (output 2).
  • If the engine is started while the “battery interconnect” relay is opened, it is necessary to close it using the manual override switch.


Option of Using the Lead-Acid Battery as a Backup:

If a lithium-ion cell’s voltage drops below the minimum allowed, the EMS will disconnect the loads and leave you in the dark. TAO EMS offers a way to avoid that:



  • Use a relay to connect the backup battery to the load bus (dotted red line on the diagram) and control that relay with EMS output 4.
  • Configure output 4 so that the “backup battery connection” relay is opened in normal operation.
  • Configure the “load disconnect” trigger to also activate output 4 (in this way, the “backup battery connection” relay will be closed just before the “load disconnect” relay is opened and the lithium-ion battery will be disconnected from the load bus).



If the EMS detects a low voltage fault requiring a load disconnect:

  • it gives an early visual and audible warning
  • if you have not corrected the situation within an interval you have preset (default time is 5 minutes), it will:
    • connect the starter battery to the load bus (output 4)
    • disconnect the lithium-ion battery from the load bus (output 6)

and you will not be left in the dark

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