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12V vs 24V.. worth it in my case?

Islander_00

New Member
Joined
Nov 4, 2020
Messages
4
Location
Greece
Greetings to the hive mind from locked down Greece…

First post here but have been avidly reading through all available resources for some time.

Right… on with the questions…


I am in the process of refitting most electronics as well as power and distribution components on a newly acquired but rather old 37ft sailboat (1990 Moody 375 CC).

At Present:

Battery banks:
  • 2x LA 110Ah (220Ah@12V i.e. around 100Ah@12V available) house batteries
  • 2x LA 90AH (180Ah@12V) starting batteries for a Volvo D1-30B of 2006
Charging Sources:
  • Original 115A Alternator connected to above starting batteries
  • Old and weathered 105W Solar panel and PWM controller connected to House batteries (not enough to sustain 12V fridge, even on sunny days…)
  • Unknown (not working?) shore battery charger
Biggest Consumers
  • New 1000W Windlass @ 12V drawing power directly from house batteries through thick 9m (+9m for return) cable run
  • New compressor 40W (-ish) 12V Fridge/Freezer
There is a 220V AC ring circuit but as no inverter is present, the ring is active only when boat is connected to shore power.


Plan:
  • Install a beefy isolation transformer (will probably go with Victron at around 3KW)
  • Install an Inverter/charger for around 3KVA 220 VAC to support
    • Induction cook top
    • Microwave
  • BMV 712 and shunt for battery monitoring
  • Upgrade the Solar controller to a Victron mppt
  • Research options for a solar arch to house around 400W-600W of solar. This being a Center Cockpit boat space on the back is somewhat challenging..
  • LFP house Battery bank (looking into DIYing EVE 280 cells or similar), while keeping the LA starter bank.
    • Two independent 4s1p blocks each with a single smart bms (Bluetooth 150A-200A max for 12V, something like xiaoxiang or daly smart etc.)
    • -or- 8s1p if going 24V (bms options??)
Ok…

Main Question is: 12V inverter/charger combo VS a 24V unit?

As this is a new install, now is the time to decide. Decision will also effect battery configuration...
I understand the complications of running the existing 12V devices but, the only -difficult to manage- existing consumer will be the windlass. I could always power it from engine bank and call it a day, or maybe exchanging the 12V windlass motor for a 24V one and keeping the existing cabling to the house bank…

The cable run for the inverter/charger to the house battery bank will be about 4m (+4), so not enough to justify cable savings I think..

Only other option is to go the simplest route possible and stick to a “purely” 12V system…

Thoughts??

PS. Sorry for the long post!!!
 
Welcome! Great to have you post finally :)

Biggest reasons to consider upgrading from an existing 12v to a 24v system would be if you need to go with an inverter larger than around 3000w...
Other advantages are cable gauge can be smaller.

Main disadvantage would be you'd have to run all your 12v stuff through buck converters.
If your alternator is 12v you might need a DC-DC 12v>24v charger, or swap it out for a 24v alternator.
 
Really need to figure your power consumption.

In my case with my RV which I’m guesssing is the same size as your sailboat, running my microwave for three minutes uses about an hour of normal power consumption. If I were to use that for 20 minutes a day, I’d want to add two golf cart sized six volt batteries. The induction stove top will suck down the power more than the microwave.

I have a 12 volt system, and when the microwave is turned on, it sucks 140 amps of power. My normal consumption is 5 to 10 amps. I turn the generator on instead of taking that power from the batteries when I power the microwave.

At peak output, my 1000 watts of panels will feed 40 amps of power to my 12 volt batteries.

I have a 700 watt coffee maker that will pull betweven 50 and 70 amps for three minutes when making a cup of coffee. If my panels are putting out above 20 amps of power, I will use he coffee maker off the solar powered battery bank, but if its too early in the morning, I put the generator on.

IMO, if you go forward with your plan, definitely 24 volts. That keeps the die of the wires down and will be less of a fire hazard with the stuff you mention. Going 24 volts or 12 volts won’t change the size of the battery bank at all, but doubling the voltage lets Half the amps flow through the wires which means less voltage loss, thinner wires, and less heat for those high wattage draws.
 
In my similar situation, I chose 24v to keep the current at a more manageable level. For 12v, I have a small aux 12v battery and a 24-->12v 40A buck converter to ensure I always have basic lighting, pumps, etc that run on 12vdc.

My inverter is 3k although it rarely ever sees 2k. It is capable of running the A/C and the microwave at the same time if ever needed. I almost went to 48v, but started seeing problems with the solar side of the plan so ultimately chose 24v. In a 12v, high current system - every connection must be nearly perfect in every way to limit voltage drop. Cable length is limiting. 24v offers more fogivness for connections and cable length which makes the whole design and install easier.....along with maintenance and modifications in the future.

Easy to install with smaller cables/connectors
Safer - fuses break at half the current.
24v gear choices are plentiful for the whole system (48v choices are more limited)
 
I have lived with 12V and 24V on my boat (in Greece), and I have to say that (as an engineer) I prefer the 24V system, for all the reasons given above.

A few months ago I bought a caravan which came with a commercial (Enerdrive)12Vx 200Ah LFP system with a 2kW Inverter. I don't think about the engineering of that system, I just turn it on and use it. I have found it to be really good, and much much better than my original 12V boat system. Making coffee draws 150A (Milk Frother + Nespresso Machine) and the battery drops to 12.6V then bounces immediately back to 13.2V. The only thing that the inverter can't operate is the air conditioner. However, I'd have reservations in going to a 3kW inverter on 12V, unless you're talking a MultiPLus 3000 (which is actually only a 2400W inverter).

Changing to 24V can be done but will involve a fair bit of cost and headache. Look critically at your max loads and expected kWh/Ah usage to see if you really need 3kW of inverter power. I think 2.5kW is a bit of a break point. If you can live with 2.5kW or less than maybe 12V is just fine.
I would not have undertaken the hassle of changing the 12V boat to 24V (I changed boats instead), but that's because I like to maximise sailing time and minimise boat jobs.

I can't recommend a good solar system enough. We had 600W on the old (12V) boat which worked great in Greece, and 750W on the new (24V) boat which is also fine, but I'm hoping to get more value from that by changing to LFP this year! My Eve 280Ah Cells should be arriving very soon, and I'm starting off by building a bench test hybrid LFP/LA system.

Whereabouts in Greece is your boat? We stored at Cleopatra for a few years before heading West in 2019, and COVID has locked us out, haven't been to boat since Oct 2019 :-(
 
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