Wooden boat builder with questions

[Midnightsailor]

Greetings,I'm a new member and gladt to have found this group and to join it. I build and restore boats ( wood) and am in process of designing a system for my 20 ft RV trailer. I have much experience with 12 volt systems on boats ,mainly navigation electronics, radios and such. Did a small solar project on a boat many years ago before lithium batteries were a thing . Confused about the 12 vs 24 vs 48 volt options. I get why the higher voltages save money on wiring but confused when it comes to going to higher kw inverters. Seems the consensus is 12 volts is ok for inverters under about 2kw but from 3 kw and up 24 and for high output inverters say 4 or 5 kw or more 48 volts system should be the choice.
My question centers around a specific customer who has a 42 foot boat with what seems like an impressive solar system. A bank of 12 to 14 Gp31AGM batteries, two 250 panels( I believe) a top of the line TriStar MPPT controller going into a 4kw inverter charger ( Mastervolt ,I believe)
I've used his system to run everything from running lights( 12 V LED type) to his air-conditioning/ heat pump and everything in-between.. refrigerator,freezer , etc..never had any issues , batteries still in great shape after I believe it's been about 8 to 10 years,
If 12 volts are not proper for such high kw inverters,why is he not experiencd any issues? The only time bank even approaches 50% discharge is if he runs the air-conditioning for 8 hours or more( overnight)
I'm asking because I'd like to run a 3kw inverter/ charger in my setup from 2 or 3 200 amp lithiums and wonder what my issues might be if I stick to a 12 volt system because of the other ancillary items already a part of the RV.

 

[rmaddy]

It's a matter of amperage. A 4000W inverter on a 12V battery bank can result in up to about 400A of current being pulled from the batteries. That's a lot of current. It requires a lot of copper and big fuses. It can be done but it must be done correctly to be safe.

The idea of going with higher voltage is to lower the amperage. The same 4000W inverter on a 24V battery bank means only 200A of current. With 48V it would only be 100A.

Lower amps means smaller (cheaper) wires, smaller (cheaper) fuses. Everything has tradeoffs of course.

Many manufactures make 12V 3kW inverters. It's quite doable. Just be sure to use appropriate wires and fuses to be safe with the 300A of current that can pull from the batteries.

Battery capacity is only relevant when deciding how long your system can run your loads.

 

[toms]

You will be fine with 3kw@12V. As others have said ensure your cable terminations are flawless.

For systems that use RC Aircon i usually install two inverters (and often use two completely independent systems.)

Make sure with big 12V inverters that the inverter is as close as possible to the battery (try to keep the main wires less than 50cm)

Check your typical useage, it is normal for surges when motors/pumps start, and short bursts (eg microwave) to get close to full inverter capacity. Unless you are running constantly at full capacity the wiring will not get warm.