Battery for 3000W 12v inverter

[goeasy123]

I'm new to the practicalities on Lithium and PV, but I do understand power electric generally. I'm struggling to convert my knowledge into practical component selection. Any help is much appreciated. Is this possible and how do I run a 3000W 230VAC inverter off a 200Ah 12V LiFePO battery?

- I'm unlikely to ever run the inverter at 3000W. The highest load might be an induction hob at 2400W and it is likely to run for less than 10 minutes per charge cycle.
- I'd like to limit the battery to a single unit and around 200Ah so that the system can be portable. The SOK 206Ah marine case would be ideal or maybe I need to go DIY to get a big enough BMS in the circuit?
- It will live on a boat that has a 12V lead acid (LA) base system. I'd like the LiFePO system to have a high current 12VDC out. probably on and Anderson plug as occasional back up for the boats LA system. This might mean putting dissimilar chemistries in parallel on occasions and when they re sitting a different voltages. How do I do this safely?
- How do I charge my new LiFePO systems from the boats system. Can I use a VSR to sense the LA is being charged and then charge the LiFePO though a DC - DC converter or is there a better way? The LA common load/charge buss has 460W of PV, a basic engine alternator and a 3500W diesel generator on it.

I've ask these questions of some commercial suppliers? The answer is usually 1000's of $'s if big heavy Victron kit. I'd like budget, small and light for this project.

 

[chrisski]

Please do a power audit with a power requirement in kWh and max wattage. IMO a system with 2000 watts is limited to 24 volts and more, but 3000 watts and 12 volts exceeds that.

since a 3000 watt inverter will pull around 330 amps when the power is low and it surges, what size Anderson plugs do you plan on using?

Also, a 3000 watt 12 volt inverter to be used for anything real is not portable. My 3000 watt 24 bolt inverter has a 200LBS battery pack with solar that can push 2100 watts.

 

[goeasy123]

Please do a power audit with a power requirement in kWh and max wattage. IMO a system with 2000 watts is limited to 24 volts and more, but 3000 watts and 12 volts exceeds that.

since a 3000 watt inverter will pull around 330 amps when the power is low and it surges, what size Anderson plugs do you plan on using?

Also, a 3000 watt 12 volt inverter to be used for anything real is not portable. My 3000 watt 24 bolt inverter has a 200LBS battery pack with solar that can push 2100 watts.

That's a good question. In a typical scenario we might be on a long sea passage running navigation electronics and 2 fridges. The 420Ah 12V lead acid would deplete by 170Ah per day (~2kWh). The PV recharges to 100% by mid afternoon on an ocean passage. The current would rarely exceed 10A. On those, high charge days the spare charging capacity would keep the LiFePO topped up. On a string of cloudy days I could use the LiFePO to make up the gap while the sun didn't shine. With the LA I have 3 cloudy days before I need to worry about recharging. 200Ah of lithium would give me another 5 days in emergency mode. An ocean passage may take from 20 to 40 days.

So why the big Anderson plug and a 3000w inverter? In an absolute emergency when the LA had completely failed I might need to use the 1000W 12V windlass to bring up the anchor. This would be a last resort emergency situation and if it damaged the battery it wouldn't be a problem. The 3000w inverter, because occasionally we might run out of cooking gas in remote locations and need to resort to a cheap induction hub.

I hope this makes sense?

 

[RCinFLA]

Battery cable current greater than 150 amps is tough requiring a lot of attention to details.

A 3kW 12v inverter is pushing a max cable current of 250 to 300 amps. That is not impossible but very difficult. You have to watch every fraction of milliohm of resistance for terminals, clamping connections, current shunt, and cables. Figure 0.05 milliohm for every compression connection and 0.07 milliohm for every bus bar.

Forget about using a high amperage breaker. They have too much series resistance. You are pretty much stuck with using high amperage fuses.

I use 250 amp large breaker on my 48v system. They have 4 milliohms of series resistance with all their guts for thermal and magnetic trip mechanisms. 4 milliohms at 250 amps is 1 volt drop across breaker. High amperage fuses are 0.5 to 1 milliohm resistance.

12v system cannot afford that 1 volt drop for a breaker. That is also 250 watts of breaker heating, so they get quite hot under sustained load.

For batteries, minimum two parallel 280 AH batteries. That is only about 6.5 kWH extractable at moderate to high current if you don't lose too much getting from battery to inverter. Keep below 0.5 C(A) maximum sustained current draw from each LFP battery string.

 

[chrisski]

Last thing I will add is a 3000 watt inverter uses 24 watts an hour for idle draw per hour or 600 watt hours, or half a 100 ah battery.

To me, you have a good plan minus the 12 volt. You can get 24 volt to 12 volt converters to run the dc side. I have a 24 volt to 12 volt 70 amp converter, and up to three of those converters can be placed in parallel.

My wiring from the 24 volt battery to the 3000 watt inverter is 4/0. I could not find quick disconnects for that sized wire when I looked.

 

[time2roll]

3000w could go 300 amps and I recommend a 600 AH lithium as a minimum. Yes to a certain extent it all can work on less. I don't recommend maxing out all the equipment and getting by with some minimally acceptable combination.

 

[goeasy123]

Last thing I will add is a 3000 watt inverter uses 24 watts an hour for idle draw per hour or 600 watt hours, or half a 100 ah battery.

To me, you have a good plan minus the 12 volt. You can get 24 volt to 12 volt converters to run the dc side. I have a 24 volt to 12 volt 70 amp converter, and up to three of those converters can be placed in parallel.

My wiring from the 24 volt battery to the 3000 watt inverter is 4/0. I could not find quick disconnects for that sized wire when I looked.

I was trying to keep things simple with 12V batteries. A 24V to 12V step step down sounds like a much better idea. Can you point me at a suitable device. Also, If I want to use the boats 12V system to charge the 24V LiFePO how do I do that?

 

[chrisski]

For the step down converter, I use this

https://www.amazon.com/Victron-Orion-12-70A-DC-DC-Converter/dp/B01BF5BQLA

These 12/24 volt chargers charge up to 360 watts and could work if your engine is on enough

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-Smart-DC-DC-chargers-isolated-250-400W-EN.pdf

 

[goeasy123]

Thanks all. I might have to do something with the alternator.

If they were handing out Oscars for 'Best Solar Forum' this one would get it!!