DC or AC generator??

[ohiobenz]

I would appreciate some input on a decision for my LMTV expedition/off-grid RV project.
The habitat will have about 1300W of 24V solar panels on the roof, Tesla 24V storage batteries, a 10KW 24VDC/120VAC pure sine inverter and a diesel powered backup generator. I have a small 120VAC 8A Air conditioner for the habitat, and will be running a 120VAC cook top like Adventure Driven.
The question is about the generator....

I currently have a 3.5KW 24VDC unit. Yesterday I picked up a 5KW 120/240AC gas generator from an ODOT/Iron Planet auction. I have seen others who have swapped the VAC generators to a diesel VDC unit making it a diesel powered VAC unit.

With my basic system being DC, should I run the 3.5KW 24VDC generator or the 5KW 120/240 VAC unit and a second AC/DC inverter for charging the batteries????

Thanks!

 

[john61ct]

A/B test efficiency, grams of fuel per AH charged

Deplete your bank to 11.00V just before starting to recharge.

Use an AH counting Battery Monitor to measure power produced.

Temps similar.

Precise measure of fuel consumption.

 

[ohiobenz]

So far all the off grid home posts are saying DC. Its a military gen. I'm still in the specification steps, so no physical tests to run. Also the system is entirely 24vdc, including lighting.
I think the key is the fact this is a backup. Running 24VDC will recharge the battery bank without power loss, running 120VAC means loss through an inverter to 24VDC... or am I missing a component in the equation?

 

[Hillbilly Heaven]

Best fuel efficiency will be at 50% load on either generator. So 1.75KW@24VDC or 2.5KW at 120/240 VAC. You say inverter but what you need is a charger/power supply to charge the Tesla battery bank and also run the 24VDC systems. I am not familiar with the Tesla battery management system and how you would incorporate what amounts to using shore power when using the 5kw generator. How is the 3.5Kw DC generator incorporated into the BMS?

You will lose some power efficiency converting 120/240 VAC to 24VDC. In other words you will have to generate more wattage at 120/240VAC than you consume at 24VDC.

 

[ohiobenz]

Best fuel efficiency will be at 50% load on either generator. So 1.75KW@24VDC or 2.5KW at 120/240 VAC. You say inverter but what you need is a charger/power supply to charge the Tesla battery bank and also run the 24VDC systems. I am not familiar with the Tesla battery management system and how you would incorporate what amounts to using shore power when using the 5kw generator. How is the 3.5Kw DC generator incorporated into the BMS?

You will lose some power efficiency converting 120/240 VAC to 24VDC. In other words you will have to generate more wattage at 120/240VAC than you consume at 24VDC.

The pure sine inverter I have is to convert 24vdc to 120AC for use of the air conditioner and cooktop.

 

[john61ct]

Yes, but here we're talking charging, not loads.

If you are a mr fixit DIY then get what you think is more reliable and easier to get parts for from remote locations.

The big advantage of AC gensets is cheap, sold in consumer channels, well known.

You likely need a big AC charger anyway for when shore power is available.

The efficiency difference will likely be a minor issue compared to maintenance / replacement.

 

[luthj]

Is the DC gens output voltage suitable for charging the Telsa bank quickly? Will the bank overload the gen?

If fuel economy is your primary goal then the DC gen would likely win. For shorter runtime the AC gen would likely win, but depends on the chargers output.

 

[Hillbilly Heaven]

Going back re-reading is important to not miss what he is thinking.
He wants to swap the diesel engine from the 3.5 KW DC generator to the 5KW AC generator. The diesel is probably too small to achieve 5KW from the AC generator.

 

[luthj]

Going back re-reading is important to not miss what he is thinking.
He wants to swap the diesel engine from the 3.5 KW DC generator to the 5KW AC generator. The diesel is probably too small to achieve 5KW from the AC generator.

Good catch. I doubt the diesel would be able to make 5kw without modifications. And running at 5kw would cut its useful life down.

What kind Generator is the AC unit? If its an inverter type, it won't care about RPM. Otherwise it is going to need a specific RPM, is the diesel engines governor set to the right RPM?

 

[john61ct]

Looks to me like the question is

use DC genset or the AC one ?

not modifying either

 

[Hillbilly Heaven]

@john61ct

I currently have a 3.5KW 24VDC unit. Yesterday I picked up a 5KW 120/240AC gas generator from an ODOT/Iron Planet auction. I have seen others who have swapped the VAC generators to a diesel VDC unit making it a diesel powered VAC unit.

His 3.5Kw 24VDC generator is diesel. The 5KW 120/240VAC generator he just bought is Gasoline. He is talking about seeing others swapping generator heads between gasoline and diesel engines.

 

[john61ct]

in that case, as stated, would require some engineering, best for them to match

 

[dwh]

Efficiency isn't the big issue - the big issue is what will it take to charge the battery? What voltage/amperage does the battery want?

Will the DC unit properly charge the Tesla battery? If so, then save the hassle and just run that.

If it won't, then consider hacking together a diesel AC unit and using Tesla's approved AC battery charging method (which might want 240vac).

 

[john61ct]

No, this is not the whole bank, just a pack / chunk / segment of it.