24V Starter on 12V Boat

[kungfoo]

A buddy of mine is currently in Japan and his starter has gone out. His boat is 12V and he's concerned about missing the weather window.

One options we're currently exploring is the prospect of him using an otherwise compatible 24V starter motor.

It looks like there are some decent 12V -> 24V converters on the market that could be used to keep a new pair of 12V cranking batteries charged at 24V, but in case he can't source those I wanted to have an option available to him.

I put together a schematic (attached) of a way that it seems like it could be done using 5 readily-available SPDT relays and two new cranking batteries (even the 12V->24V converter plan involves those batteries).

It may not be the best option, but I am a big fan of getting all the options on the table before making decisions, or maybe just bringing the schematic and 6 (5 + 1 spare) SPDT relays along for the ride.

Would love some thoughts on how best to help him across the Pacific.

Thanks,
Harrison

Edit: I'd probably also have him throw in a battery isolator that can handle a 24V reverse bias as an extra layer of protection to ensure you never blow things up.

[Avionics_Joe]

Not how this diagram is drawn, you have a short from the 12 v charging points at the isolation relays to the charging relay.
The charging relay needs to be a double pole to work.
All this needs to be ripped out upon arrival!

The start battery really doesn't need much power. Run the alternator to the house bank and either a 110-volt charger for the 24v start battery or a 12v to 24v upconverter would be simpler.

What size motor, is it diesel or gas? Makes a big difference in cranking amps.

[kungfoo]

Originally Posted by Avionics_Joe

Not how this diagram is drawn, you have a short from the 12 v charging points at the isolation relays to the charging relay.

Not sure what you're referencing there. I forgot to add a 12V_charging line to the second battery, but whenever the switch_on line is asserted, the Charging Relay will open the circuit, and isolate the both batteries from the house 12V system. Does that address your concern?

If not, could you rephrase the issue?

Originally Posted by Avionics_Joe

The charging relay needs to be a double pole to work.

What are you recommending be added to the second pole of the Charging Relay?

All relays are currently listed as SPDT so that they can all be the same model number.

Originally Posted by Avionics_Joe

The start battery really doesn't need much power. Run the alternator to the house bank and either a 110-volt charger

Not a bad idea to add a simple automotive inverter + 24V marine charger, but I'd still want him to have the relays on board.

Originally Posted by Avionics_Joe

a 12v to 24v upconverter would be simpler.

Just to make sure we're on the same page, here, my original post specified "It looks like there are some decent 12V -> 24V converters on the market that" -- we talking about the same thing?

Originally Posted by Avionics_Joe

What size motor, is it diesel or gas? Makes a big difference in cranking amps.

Big diesel in cold climate. I'd want him to have north of a thousand amps at his disposal if it were a 12V system, guessing he could get by with half at 24V.

[DougR]

THis is/was a very common situation in the truck world, where large diesel starters were 24 volt, but the rest of the truck was 12 volt.

The problem was solved with a “series-parallel switch”. During normal operation the two batteries are connected in parallel and operate as a 12 volt source, but during engine cranking the parallel connections are disconnected and the batteries are connected in series to crank the starter. Reverts to 12 volt when the cranking is finished.

[kungfoo]

Originally Posted by DougR

THis is/was a very common situation in the truck world, where large diesel starters were 24 volt, but the rest of the truck was 12 volt.

The problem was solved with a “series-parallel switch”. During normal operation the two batteries are connected in parallel and operate as a 12 volt source, but during engine cranking the parallel connections are disconnected and the batteries are connected in series to crank the starter. Reverts to 12 volt when the cranking is finished.

Sounds like there's probably a product out there that does something similar to my home-grown schematic.

I'm already seeing some products using "series parallel" and "starter" in the search queries.

Thanks!

[Avionics_Joe]

I drew the lines connecting 12v charging.

When K4 closes you will short the batteries out. K3 needs to be a double-pole relay.
But my worry would be what if one of the relays sticks? Smoke, fire????
The only thing that scares me on a boat is fire and a calendar.

"Just to make sure we're on the same page, here, my original post specified "It looks like there are some decent 12V -> 24V converters on the market that" -- we talking about the same thing?"
Yes, we are.

I have no experience with cold weather or large diesels. I have 2 29HP diesels and I use a 4 amp 110 volt charger, works great to isolate my start batteries from the LiFePO4 main bank.

[skipperpete]

DougR is totally correct, the old 1418 model Mercedes truck used a series parallel switch for the start. It was a big chunky critter about 4” square.

[kungfoo]

Originally Posted by Avionics_Joe

I drew the lines connecting 12v charging.

Whoops -- I actually did have that correct to begin with.

Originally Posted by Avionics_Joe

When K4 closes you will short the batteries out.

Ah, so, K4 only fires when switch_crank is asserted. switch_on MUST be asserted before switch_crank can be asserted. The key has 3 positions: Off, switch_on, and switch_on + switch_crank. That third position (where switch_on and switch_crank are both asserted) is a momentary switch (so when you stop holding it, it'll go back to the middle position).

Originally Posted by Avionics_Joe

K3 needs to be a double-pole relay.

I was originally thinking of doing this with latching relays so it was easy to get it INTO the engine-drive position, but it needed manual intervention to re-connect it to the house bank.

I'm liking this idea of using a DPDT relay instead and trying to do it all in one go if I can. I could use the DPDT in-line with the SPDT so that even if switch_on unasserts while switch_crank is asserted, it doesn't harm the system...I'll take a whack at that.

Thanks!

Originally Posted by Avionics_Joe

But my worry would be what if one of the relays sticks? Smoke, fire????

hahaha, right there with ya!

Originally Posted by Avionics_Joe

The only thing that scares me on a boat is fire and a calendar.

Cuz if you get lost, you can always just pull in somewheres, and ask for directions!

[kungfoo]

Looks like fate has decided this one for me. The option for an otherwise compatible 24V replacement starter is a no-go.

I guess our next investigation is how to find a starter that could be hacked into (and by hacked into, I of course mean adapted to by the skilled craftsmanship of Japanese machinists) his engine.

[Fuss]

Just get it fixed, how bad can it be, it probably just stopped working and needs a few parts like brushes, bushes and a cleanup...if worse then take it to a specialist and get it rewound

[kungfoo]

Originally Posted by Fuss

Just get it fixed, how bad can it be, it probably just stopped working and needs a few parts like brushes, bushes and a cleanup...if worse then take it to a specialist and get it rewound

Primary coil :-)

This one isn't getting fixed easily.

[skipperpete]

So, what make of engine is it that you can’t find a replacement 12 volt starter for? Gardner, Cummins, perkins, Detroit Diesel? Give us a name and I reckon we can find you a 12 volt starter that’ll fit.

[Audeamus]

We have had 4 packages DHL to Japan from USA, UK, China in the last month. Duty paid online while in transit. Quickest 5 days longest 10 days (to outer southern island).......

We also had parts sent to Thailand from one of the big Italian vendors that specialize in engine/trans parts - sent same day.

[kungfoo]

I priced out shipping, and it would be $2,500. It's not worth it. Appreciate the enthusiasm, but if we can build an adapter piece that allows a modern off-the-shelf starter to be used then it's probably the right call.

It's also not my call.

[skipperpete]

Originally Posted by kungfoo

I priced out shipping, and it would be $2,500. It's not worth it. Appreciate the enthusiasm, but if we can build an adapter piece that allows a modern off-the-shelf starter to be used then it's probably the right call.

It's also not my call.

So, you don’t know what make of engine it is??