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

When installing a lithium battery on a boat or a camping car you need to decide if and how the alternator will charge it at the same time as the starter battery. If not done the right way the alternator could be damaged. In this article I address some key questions you may ask yourself, I point out some challenges to be aware of, and I propose example of functional diagrams.

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

Yes. Most lithium batteries are not designed to deliver the cranking current required to start a diesel engine and if it does it would make it age faster. If it is “drop-in” lithium battery there is a risk that the internal BMS will shutdown the battery. It is also a safety to always have a full starter battery independent from the house bank.

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

Yes. Most of the time the lithium 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 is the opportunity to use the lead-acid battery as a backup in case the BMS shuts down the lithium battery… like that you will not be left in the dark! (see how to do it at the end of the article)

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

Yes… but you should not if you don’t want the lithium battery to age faster (capacity loss). Many studies show that charging a lithium battery at 13.8v is enough to fill it up at more than 95% SOC (state of charge). 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. So why would you do it if that will shorten the life of your battery?

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

Not without the risk of damaging or destroying the alternator. While not being the best for the battery, the charge voltage should not destroy it. The risk is on the alternator side. The lithium 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%, meaning that when charging at 100A the alternator generates 1000W of heat. The alternator dissipates that heat with its internal fan, but at low rpm or if that lasts too long the fan cannot extract enough heat and the alternator starts smoking. It is interesting to read some reports from people that have charged their lithium battery directly with a non-regulated alternator for years and never had a problem. My guess is that their charge circuit has a high enough resistance that it limits the current.

Q 5: What are my options to charge the lithium battery with the alternator?

  1. add an external regulator to the alternator and be sure 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 to DC charger to recharge the lithium battery from the lead-acid battery when the engine runs – for efficiency and cost considerations it would be best suited when the charge current is low (< 40 -60 A)

Challenges you need to think about:

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

If you do so, it will create a high voltage spike that can destroy some of the alternator’s components. This situation will happen if the BMS disconnects the lithium battery while the engine is running.

  • the easiest way to avoid that situation is to always have the lead-acid battery connected to the alternator – like that even if the lithium battery is disconnected by the BMS, the alternator still has the lead-acid battery to charge
  • another approach, if the alternator has an external regulator, is for the BMS 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 voltage of the lithium battery goes below 12.8v (about 40% SOC) any load will draw current from both batteries. If the SOC goes too low there may not be enough energy left to start the engine (recently happened to a friend after 3 days without sun).

A couple of approaches can be considered:

  1. connect the lithium 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 BMS disconnect them when the lithium battery voltage dips below 12.8 volt and reconnect them when the voltage goes above 13.2 volt – the advantage is that the starter battery is trickle charged most of the time

Example functional diagrams:

Disclaimer: These functional diagrams are designed to help understand how the different components are interconnected and work together. They are not wiring schematics. To make it easier to read, the negative wires and other components that are not relevant have been omitted. These diagrams can be used as a base to plan and document your installation wiring schematic. A wiring schematic should be developed by a professional who knows the regulations and safety rules applicable to your installation.

The examples are based on the capabilities of the TAO BMS. If using a different BMS, make sure it has the relevant outputs and logic to command them. I do not detail the BMS configuration as it is explained in the manual. If you need help with the configuration, please ask on the forum.

1 – Lithium battery charged by a DC/DC charger:

Configuration:

  • the alternator is always connected to the starter battery
  • the starter and lithium batteries are not connected together
  • charge and load disconnect relays are closed during normal operation (outputs 5 and 6)
  • the BMS output 1 is closed during normal operation to enable turning on the charger with the ignition key

Operation:

  • when the engine is running, the DC/DC charger draws energy from the alternator to charge the lithium battery (depending on the charger capabilities, this can be done via the ignition key or automatically when the voltage of the starter battery is above a certain level)
  • if the BMS detects a full lithium battery or a high voltage warning situation, it turns OFF the DC/DC charger (output 1)
  • if the BMS detects a high voltage fault (defective DC/DC charger?) it will:
    1. give advanced visual and audible warning
    2. then after a time delay, turn OFF the charger (output 1)
    3. then disconnect all charge sources (output 5)

2 – Lithium battery charged by the alternator

Configuration:

  • the alternator is always connected to the starter battery
  • the lithium and starter batteries are connected in parallel via a relay that is commanded by the BMS output 2 – this relay is important as it prevents discharging the starter battery in case the lithium battery is deeply discharged (to save money but prone to human error, this can be a manual switch)
  • the battery interconnect relay is also commanded by a manual override switch with 3 positions (BMS, OFF, ON) – it should be in the “BMS” position for normal operation
  • the charge parameters of the alternator are set by the external regulator to suit the lithium battery
  • the BMS output 1 is closed during normal operation to enable turning on the regulator / alternator with the ignition key
  • charge and load disconnect relays are closed during normal operation (outputs 5 and 6)

Operation:

note: all voltages mentioned are examples (you can set them to suit your requirements)

  • when the BMS measures a cell voltage above 3.25v (13v for a 12v battery) it connects both batteries in parallel to allow trickle charge of the lead-acid battery by the lithium battery (output 2)
  • when the engine is started the regulator is turned ON by the ignition key and both batteries are charged
  • if the BMS detects a full lithium battery or a high voltage warning situation, it turns OFF the regulator / alternator (output 1)
  • if the BMS detects a high voltage fault (defective regulator?) it will:
    1. give advanced visual and audible warning
    2. turn off the regulator / alternator (output 1)
    3. then disconnect the lithium battery from the alternator and lead-acid battery (output 2)
    4. then disconnect all charge sources from the lithium battery (output 5)
  • if the BMS measures a cell voltage below 3.2v (12.8v for a 12v battery) it disconnects the starter battery from the lithium battery to avoid discharge of the starer 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 to use the lead-acid battery as a backup:

In case the voltage of a lithium cell drops below the minimum allowed, the BMS disconnects the loads and you are in the dark. TAO BMS has a way to avoid that:

Configuration:

  • use a relay to connect the backup battery to the load bus (red dotted line on the diagram) and command that relay with the BMS 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 (meaning that the “backup battery connection” relay will be closed just before the “load disconnect” relay is opened and the lithium battery is disconnected from the load bus).

Operation:

if the BMS detects a low voltage fault requiring a load disconnect:

  • it gives advanced visual and audible warning
  • if you have not corrected the situation after a time set by you (default is 5 minutes), it will:
    • connect the starter battery to the load bus (output 4)
    • disconnect the lithium battery from the load bus (output 6)

And you will not be in the dark!

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