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Old 02-12-2019, 12:54 PM   #1
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24v versus 12v system

Hello all,
I am building a new 38' downeaster that will have a Scania for power. This engine uses 24v for starting. The question is whether to try to go 100% 24v or perhaps 50/50.
Some have suggested to go 100% 24v and while some agree it seems the availability of 24v components are not usually readily available.
It also seems that it has hard to get around the need for some 12v components.
The solution could be two panels; 12v and 24v, but then how best to accomplish that.
I have to confirm but I was told that the Scania may accommodate two alternators so that one could be 12v and the other 24v.
some have suggested to use one alternator at 24v to charge start batteries and then an inverter to charge the 12v batts.
thoughts?
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Old 02-12-2019, 01:15 PM   #2
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Like you said, you definitely need a 24V start battery to source the current necessary for starting.

I would have Scania put a 12V alternator on. You can use a 12-24 transformer to charge your start battery. Put an ACR between the 12V side of the transformer and a 12V house bank. This lets you take advantage of having a 12V rail for the majority of the house stuff (like you said, way easier to find parts) and your alternator (and charger/inverter) can charge the house bank directly. Your 12-24 transformer doesn't have to be that big because you don't need a ton of current to charge a start battery.

Just keep an extra transformer on hand in case that one breaks - they're not too expensive. If you were really in a bind and your 24V battery was dead, you could disconnect your house bank, parallel two of the batteries to get 24V, start your engines, then un-parallel the house bank and reattach the alternator.

Sorry - follow up question - does the Scania need 24V to run, or just to start?
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Old 02-12-2019, 01:17 PM   #3
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Quite a few marine electrical companies make 24V to 12V step downs. It should be easy enough to add a small panel for any devices that are 12V only and power it from the step down.
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Old 02-12-2019, 01:22 PM   #4
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Assume the starting motor IS 24v?

If you have a lot of other 24v items on the boat.... thrusters, windlass, etc. I could argue to set up a 24v system. Most power, less wire for those high power items is a plus. Alternators may be a tad harder to get but suspect not much.

Have the common runs for 12v with a converter, which would be easier and fairly common.
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Old 02-12-2019, 01:28 PM   #5
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It depends on how much electrical load your boat will have. If running lots of dc stuff, I would go with a 12v alt and then a DC-to-DC converter to step up to 24 to charge the start batts. The 24v charge would only be a few amps.

Only complication would be if engine is electronic control, that could take some power and throw a wrench in the matter. Then I would look at two alts, one 24v and one 12v.
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Old 02-12-2019, 01:35 PM   #6
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Unless there are a LOT of big loads, like thrusters and windlass, needing to go a long way, I'd think just keep a small 24v start/run bank from the 24v Alt and either add a 12v Alt. or a 24-12 charger and properly sized 12v house bank for the rest of the boat.

What about other power sources on the boat needed to be considered...? Is there a genny? Solar?
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Old 02-12-2019, 01:44 PM   #7
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I'd go 100% 24V, except where not possible. I built a custom sailboat, it is all 24V with a few exceptions. The advantages are smaller wire throughout. This is particularly good for winches, windlasses, inverters, thrusters, and other high amperage devices. Most equipment is available 24V, for those things that weren't I use Mastervolt DC-DC converters. There is a small 12V panel fed by one for miscellaneous 12V loads.

In my case, the motor was available only as 12V. It has two 24V alternators, the motor and its 12V start battery are powered/charged by a DC-DC. The solar and line charger are 24V. The only other substantial load I was unable to get in 24V is the ITR diesel heater, which has its own 24-12 converter. The installation has three 10A converters: 12V panel, engine, heater. More that enough was saved in wire to pay for them.

I've not regretted the decision, and wish the trawler was 24V as well.
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Old 02-12-2019, 01:50 PM   #8
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Its practically impossible to totally avoid 12V for some electronics and accessories that you will want or need, but for most of the boat its not a big issue, just that your local chandlery may not have a replacement you need on the shelf the day you want it. Sounds like you're not dealing with legacy equipment on board so make the main system and all your significant equipment 24 and use a DC-DC converter to supply a 12V panel, or panel section, probably at the helm. Choose as much as possible 24V Nav & Com and a 15 Amp converter should be adequate, 20A ample and 30A "robust". Actually I think the 15 could be ample, 20 robust and 30 overkill, but a 12V only SSB would require re-thinking. The converter setup could be with or without a battery. A second converter for backup would provide redundancy at a lower cost than an alternator, battery and panel feed. I would do that and have a second 24V alternator backup.
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Old 02-12-2019, 03:01 PM   #9
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Pretty easy to add a second alternator to a single engine boat. Although my John Deere is a 12 volt engine, I put a second 12 volt 220 amp alternator with a 3 stage smart regulator to charge the house bank, on the engine. System is very simple with the start battery running only the engine and guages, being changed by the original alternator. Everything else runs off the house bank and is recharged by the 2nd alternator.

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Old 02-12-2019, 03:15 PM   #10
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thanks all, most of you hit on some of what I've already heard. boat will have a genset, thruster, small fridge, and windlass. comments like DDW's reinforce the idea of going mostly 24v. I agree it seems unlikely to avoid some 12v applications.
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Old 02-13-2019, 07:01 AM   #11
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The most common method on large trucks and buses since 1990 or so is an equalizer.

The 24V system is std for the engine , usually (2) 12V series 31 start batts .Everything engine 24v is stock.

The 120V charger is 24V output to the start batts .

A house set of any size you choose frequently (4 ) 6v deep cycle batts is charged at 12V by the equalizer.

An equalizer is very reliable , and usually lasts "forever". 100A 12V no problem.

The Vanner is common and has great schematics to show various wiring requirements.

70-M Series Equalizer - Vanner

www.vanner.com/manuals/EQUALIZER-70M-SERIES.pdf
The Vanner VANN-Guard Power Management System is an efficient and highly reliable method of obtaining a 12 volt DC power source from a 24 volt DC ...

Vanner70-100 12v-24v Battery Equalizer 100 Amp | eBay

https://www.ebay.com › ... › Car & Truck Parts › Charging & Starting Systems › Other
$799.95 to $945.41 - ‎In stock
Find great deals for Vanner 70-100 12v-24v Battery Equalizer 100 Amp. Shop with confidence on eBay!
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