daycruiser avatar image

Lithium batteries on a 250hp Outboard motor, how to protect alternator (start and house batteries)

I need to increase the capacity of the house batteries for my boat. At the same time I need to keep or preferrably reduce the weight of the boat (I'm at the weight limit when trailering the boat).

Preferrably I would like to replace 2x Tudor 115Ah Dual Marine (60kg) with 1x Li starter battery and 2x Li house batteries (100Ah starter and 2x100Ah House).

Alternator puts out max 46A.

What is needed to ensure a NON overheated alternator?

See picture for separated starting and accessories batteries direcly at the alternator:


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4 Answers
wkirby avatar image
wkirby answered ·

There isn't any device to help with the start battery. Due to the very high starter current, this needs to be a direct connection. So therefore the alternator needs to be compatible with a lithium start battery, Yamaha would need to answer this question.

Accessory battery is a similar situation since it also has a dedicated charging lead from the engine. Yamaha would need to verify if this is suitable for charging lithium batteries. It's a nice feature of the engine to have a separate accessory charge circuit, but it's unusual and we don't really know how it works.

The usual situation is that there is one lead from the engine which connects to the start battery. The product that Victron do make for lithium accessory batteries is the Orion Smart series which are also known as B2B (battery to battery) chargers. These have their input connected to the start battery and then the output is connected to the accessory battery. The B2B charger is then set up to limit the charge current going to a lithium accessory battery and also prevents the start battery from being discharged when the alternator is not running.

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daycruiser avatar image
daycruiser answered ·

Thanks wkirby!

  1. If the dc-dc charger is connected to the starter battery, how does it know if the motor is on or off? I don’t want to build a system that after two nights “camping” with the family using the fridge/lights/stereo the dc-dc-charger has used up the starter battery trying to charge the house battery. The idea is that the house battery in worst case can run flat but you can still start the motor (to get home or charge the house batteries).
  2. A DC-DC-charger. Does it require to be in parallell with the starter battery/alternator. If the starter battery is removed can it charge a Lithium battery straight from the alternator?

Once this is solved I need to start figuring out how to attach the regualar chargers and solar panels in to the system.

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Normally, when the engine is running and the alternator is charging, the battery Voltage will be high ~14V for example. When the engine is stopped and the alternator is no longer providing charging power then the start battery will go lower.
Within the DC-DC charger, you can set the lower Voltage limit of the start battery at which you wish the DC-DC charger to stop taking power. You might set this to aroung 13V for example.
Here is more detail about engine on detection:

You should never run an alternator without a battery connected directly to it.
The DC-DC charger would either not work or be damaged if connected to an alternator only.
Here is the datasheet which contains an example wiring diagram:

Thanks alot for now! I'll do some reading and make some drawings to understand a bit better :)
daycruiser avatar image
daycruiser answered ·

Would a system like this work? Missing pieces?


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That looks like a good plan to me.
daycruiser avatar image
daycruiser answered ·

I did some research on the outboard motor.

  • The alternator deliveres max 45A.
  • Of the 45A max only 27A are available for charging batteries (motor uses the rest itself). i.e. 45-27=18A.
  • This is rpm dependant 600rpm=15A 2500-6000rpm=45A.
  • I guess that motor always need 18A so at 600 rpm nothing is sent to batteries.
  • But I don't know how a alternator (or stator/rotor) works.

The risk is that a lithiumbattery connected directly to the alternator overheat the alternator so a DC/DC in paralell with starter battery are proposed as a solution.

IF i choose a Victron Orion smart DC/DC 12V 30A. Can that also overload the alternator? would it be "safer" to go with a 12/12 18A instead?

I will also add a BMV712 to the system, still considering a smart shunt instead but like to have a gauge (feels more future proof). I hope to have this system at least 10 years.


f250-alternator.png (95.4 KiB)
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Daycruiser, What did you go with 30 amp or 18 amp DC to DC Converter? I am looking at the same situation. I have the 30 amp but didn't install it yet debating to exchange and go lower.
Did you go with the 18 or 30
Bit late now, but check the lithium starter battery is designed for alternator charging as well as start current.

I would either upgrade the alternator, or use the 18A Orion.

ww2bar avatar image ww2bar kevgermany ♦♦ ·
Start is a lead, House lithium, going with the orion 18.