Please check my design.

[John Frum]

Also, please stress test the snot out of the system.

 

[Brewman]

Well here is the deal, the Inverter is only going to run a 5K BTU window shaker, it should never draw more then 15A as that would pop the breaker on a house 15A plug. If I did the math right, a full 15A at 115V would be 80A at 24V.

The converter is 40A out but I'll be lucky to ever pull 10 from it.

If I fuse and wire for 100A max and test the snot out of it I should be good.

Or am I just making this unsafe.

 

[svetz]

Plus conversion factor.
40 amps /.9 conversion factor * .5 step down factor / .8 fuse headroom = 27.777777778 fuse amps

That reminds me... probably not an issue with LFP, but power (watts) = amps x volts. So devices that are trying to supply constant power (e.g., an inverter) take more current as the battery voltage drops to make up for the loss of voltage.

It depends on the device though. Constant resistance devices (e.g., light bulb) have a current drop with the voltage (e.g., V=IR)

 

[Brewman]

I could always increase the 4AWG to 2AWG from battery to buss bar and inverter but the BMS will still have 2 8AWG wires.

 

[John Frum]

Well here is the deal, the Inverter is only going to run a 5K BTU window shaker, it should never draw more then 15A as that would pop the breaker on a house 15A plug. If I did the math right, a full 15A at 115V would be 80A at 24V.

The converter is 40A out but I'll be lucky to ever pull 10 from it.

If I fuse and wire for 100A max and test the snot out of it I should be good.

Or am I just making this unsafe.

My 10kbtu window shaker draws ~750 watts while the compressor is running.
But it draws ~3000 watts for a moment to get the compressor spinning.

 

[Brewman]

Maybe I'll drive down south about 25 miles and hit up a new friend for some learning. Lol

 

[svetz]

Well here is the deal, the Inverter is only going to run a 5K BTU window shaker, it should never draw more then 15A as that would pop the breaker on a house 15A plug. If I did the math right, a full 15A at 115V would be 80A at 24V.

Oh oh... what's the in-rush amps on the air conditioner? Not a problem about the wiring/breaker as its quick...but the in-rush can trip an inverter. I had to put a soft-starter on mine (it was pulling 86.2 amps at startup, but it was also bigger). If it's a minisplit or other variable speed start you'll be fine.

 

[John Frum]

I could always increase the 4AWG to 2AWG from battery to buss bar and inverter but the BMS will still have 2 8AWG wires.

2 awg thhn is a bit of a pain to work with.
The bend radius is pretty big and its about the limit for my hands and these cutters https://www.amazon.ca/gp/product/B00004SBDG

 

[Brewman]

2 awg thhn is a bit of a pain to work with.
The bend radius is pretty big and its about the limit for my hands and these cutters https://www.amazon.ca/gp/product/B00004SBDG

Maybe I'll just double up the 4AWG

 

[svetz]

Or am I just making this unsafe.

Nature is a wonderful thing, if there are any significant errors she'll let you know when you turn it on in her typically firey way. The good news is it's small enough to be safe enough you can handle anything that comes up and you've enough fuses and breakers nothing too expensive should suffer permanent damage.

 

[Brewman]

I'm not worried about the inverter handling the inrush, I'm more worried about the BMS handling it.

 

[John Frum]

I just checked my AC with my Klein cl390.
26.7 amps inrush@122VAC.
26.7 ac amps * 120 VAC / .85 conversion factor = 3769.411764706 dc watts
3769.411764706 dc watts / 24 volts low cutoff = 157.058823529 dc amps
I would guess that your bms could probably handle my window shaker.
but I sincerely doubt your inverter could.
The surge capacity for an high frequency inverter is usually ~20 ms or 1 cycle of ac.
Getting that compressor running might take a second or more.

 

[Brewman]

I just checked my AC with my Klein cl390.
26.7 amps inrush@122VAC.
26.7 ac amps * 120 VAC / .85 conversion factor = 3769.411764706 dc watts
3769.411764706 dc watts / 24 volts low cutoff = 157.058823529 dc amps
I would guess that your bms could probably handle my window shaker.
but I sincerely doubt your inverter could.
The surge capacity for an high frequency inverter is usually ~20 ms or 1 cycle of ac.
Getting that compressor running might take a second or more.

This is for a 10K right?

 

[Brewman]

In Wills solar shed AC video he is using the same 2K inverter to run his 5K BTU AC

 

[John Frum]

This is for a 10K right?

Yes.

 

[Brewman]

Im going to try to find out what BMS he is using.

 

[John Frum]

from the link "AWM is commonly used in a variety of applications such as general purpose wiring circuits, control circuits and internal wiring of appliances."
I suspect that is how the bms gets away with 2x 8awg.

BMS is rated for 100 amps
8 awg is rated for 55 amps at 90C and 80 amps at 105C
100 amps / 2 wires / .8 current asymmetry factor = 62.5 fuse amps
62.5 amps / .8 fuse headroom = 78.125 amps

 

[Brewman]

BMS is rated for 100 amps
8 awg is rated for 55 amps at 90C and 80 amps at 105C
100 amps / 2 wires / .8 current asymmetry factor = 62.5 fuse amps
62.5 amps / .8 fuse headroom = 78.125 amps

I'm not really understanding this

 

[John Frum]

I'm not really understanding this

@svetz and I were talking about Appliance wiring material ratings.
Pure copper 8 awg wire with 105 Celsius rated insulation is rated for 80 amps per wire.
I speculate that the BMS is using AWM rules because...
bms rating / 2 + fuse headroom + asymmetry headroom far exceeds the 90C rating of 55 amps.

 

[Brewman]

Ok now I understand