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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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markus avatar image markus ♦♦ henrique-eipeldauer commented ·

I am no outboard mechanic, but I guess it will crank nevertheless.

But a good captain will never purely rely on what some bloke in a community says, he will always be the one responsible if something is going wrong.

Does your outboard have a manual pull rope to start or is the electric starter the only way to start your engine?

If you feel safe to try the solution I suggested, OK do it. It would be possible to short the diodes and eliminate the Voltage drop in case your battery is empty and therefore have less Voltage etc.

But it’s a DIY tinker solution you would have to deal with.

As an alternative, you could use a little starter battery capable of charging with 14.8V and use a DC-DC charger to charge your second battery from that new starter battery.
Going this way would give you 2 separate and independent batteries and therefore more safety.

What you choose to do is up to you.
You are the captain! ;o)
BR

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

Markus,

I'm not by any means seeking here liability for any of the opinions you're kindly giving me.
When you lack knowledge in a certain subject, you look for advice from people who know better, which is what I'm doing here with you.
Any decisions I make regarding my boat are solely based on MY OWN conclusions. These I usually reach by gathering knowledge from my own 40-year boating experience, my civil engineering degree and from kind fellows like you, who are willing to share their own knowledge.

Yes, my outboard has a pull rope, which is by the way light to be pulled, so any low voltage to the starter definitely wouldn't be disastrous.


In theory, I also like your suggestion of adding a dedicated starting battery and the DC converter. That would make things work at their best.
However, I like even more your very initial suggestion of keeping things simple :-)

So I'll most probably opt for the double diode solution. It's relatively inexpensive and I can always go back, in case I have trouble cranking the engine.

Once again, thanks a lot for your time and good advice!

Henrique

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markus avatar image markus ♦♦ henrique-eipeldauer commented ·

Thank you.

please keep us informed how it goes.

All the best and lots of fun with your boat!

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

Markus,

Thank you for the wishes!
Actually, the double diode model you suggested is out of stock, apparently worldwide.
By looking up on the manufacturer's website, I found this other model:
https://www.st.com/resource/en/datasheet/stps160h100tv.pdf

Its voltage drop values are just right for me. However, its datasheet shows values for IRRM(Repetitive peak reverse current) and IRSM(Non repetitive peak reverse current) at 2A and 10A respectively.
I don't know what these figures really are, but the fact is that I have a somewhat permanent 12A charging current in one direction or an occasional 60A cranking current in the other direction.
Shouldn't these two be below the 2A (or 10A) limit for these diodes?

Thank you once again,

Henrique

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markus avatar image markus ♦♦ henrique-eipeldauer commented ·

Hi, These values are important for rectifier use of the diodes.
They don’t really matter to you. Expect the diodes to get warm, when charging the battery with full current.
BR

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

Thank you once again for your quick and valuable reply!

At 0.6V(drop) x 12A=7.2W, it seems we could expect a temperature rise of 0.9degC/W x 7.2W=6.5degC, which is consonant with you saying they will "get warm".

I'll have to wait a couple months until they arrive here in Brazil. But I'll let you know how they work by then.

Best,

Henrique

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