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henrique-eipeldauer asked

DC-DC converter with outboard engine alternator

My sailboat’s electrical system is very simple.
I have a dual purpose 12V 200Ah flooded cell battery bank. I run the daily loads and also start my Honda 20hp outboard from this single bank.

The bank is charged from the outboard and from a 300W solar array connected to an MPPT 100/20.

The outboard has a starter motor and a 12A regulated alternator that reaches ultimately 14.8V,

The recommended absorption voltage for my batteries is 14.4V.
So in my understanding I should install some device such as an Orion Tr DC-DC converter to get the 14.8V brought down to 14.4V.

However, the outboard’s harness is solely a positive and a negative wire. These are connected directly to the battery bank. There’s no outside separation for the starter motor and for the alternator circuits.

I may be wrong, but it seems obvious to me that I cannot connect these two wires to a DC-DC converter, since the current would be reversed whenever the starter motor is used.

Does anyone have any suggestions on how to regulate the (already internally regulated) alternator output? Or any other solution for my system?

Thank you!

Henrique

Orion DC-DC Converters not smart
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markus avatar image
markus answered ·

Hi.

"My sailboat’s electrical system is very simple"

I would suggest, to keep it that way.

There are aftermarket alternator regulators for outboard motors available.

There is a good chance, that there is a solution for your specific model, I would ask a capable marine outboard mechanic first.

Alternatively if you have a stable 14.8V output, you could make use of the Voltage drop on power diodes.

Shottky diodes have ~0.4V drop.

Look at something like this:

Datasheet - STPS200170TV1Y - Automotive 170 V, 2 x 100 A, high voltage power Schottky rectifier

If you connect the 2 diodes in opposite direction and parallel, you get conductivity and a Voltage drop in both directions.

Good for your batteries and should not bother your starter.

BR

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henrique-eipeldauer avatar image henrique-eipeldauer commented ·

Hello Markus,

Thank you very much for your simple and clever suggestion.
A couple questions here:

1- Is the 0.4V voltage drop constant for these diodes, or does this value depend on the current?


2- In case it depends on the current, the starter motor would cause a significant drop when it's cranking the engine at 60-70A, right? I'd be afraid such increased drop could turn the motor too powerless to crank the engine.

Since my solar MPPT delivers temperature-compensated, 3-stage, customizable charging, I initially thought of a similar solution for the outboard alternator, so a DC-DC converter came to mind. However, I have this physical issue in which I cannot separate the alternator and starter leads, and I don't think a converter would accept a reverse (cranking) current.

Thank you again!

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markus avatar image markus ♦♦ henrique-eipeldauer commented ·

Hi.
According to the (example) datasheet of the diodes I posted above, the Voltage drop at 100Amps should be 0.63 V.

No. A DC-DC converter is not the right solution in your case.

BR

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henrique-eipeldauer avatar image henrique-eipeldauer markus ♦♦ commented ·

Markus,

Looking closely to your datasheet, we can see on page 3 a graph relating voltage drop, temperature and current.

Assuming the 25C curve, when charging at 1A (finish of absorption phase), we'd have a 0.4V drop, which fits perfectly my 14.8-0.4=14.4V demand.

However, for cranking at 65A, we'd have a 0.75V drop. Don't you think going down from ~12.6V to 11.85V could possibly render the starter ineffective?

Thanks once again for your patience.

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