Circuit Breaker Sizing and maybe BMS Bypass?

[Oldtoad]

3 circuit breakers planned. Sizing is 125% of design max.
1. Coach battery circuit 70Amp
2. Solar controller 70Amp
3. This one is where my question begins.
DC fuse load and 1500w Inverter. What size circuit breaker?
The battery is 280AH, has a 120Amp BMS. Isn't 1500w 125Amps @12v? Do I need to bypass the BMS to run the inverter?

 

[John Frum]

1500 ac watts * 1.15 inverter efficiency / 12 volts = 143.75 dc amps
143.75 dc amps * 1.25 fuse headroom = 179.6875 fuse amps.

dc amps > 120 amps so yes the inverter should should bypass the bms.
You need to have the bms "signal" the inverter to shut down on bms trips.

If you have an inverter/charger then things get more complicated.

 

[Oldtoad]

1500 ac watts * 1.15 inverter efficiency / 12 volts = 143.75 dc amps
143.75 dc amps * 1.25 fuse headroom = 179.6875 fuse amps.

dc amps > 120 amps so yes the inverter should should bypass the bms.
You need to have the bms "signal" the inverter to shut down on bms trips.

If you have an inverter/charger then things get more complicated.

No inverter charger.
Thanks.

 

[John Frum]

Coach battery circuit 70Amp

Hold the phone.
You need a 180 amp fuse on the coach battery circuit as close to the battery positive as possible.
What size wire are you planning for the main circuit?

 

[Oldtoad]

Will likely go 200A fuse on the inverter
The MPPT Renogy 50A DC_DC battery charger with MPPT is 50A max, 80A might be a little big.

RV Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

 

[John Frum]

Will likely go 200A fuse on the inverter
The MPPT Renogy 50A DC_DC battery charger with MPPT is 50A max, 80A might be a little big.

RV Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

The fuse's primary job is to protect the wire.
4 awg 105 celcius pure copper wire is rated to 160 amps so you have plenty of breathing room.
50 amps output * 1.1 conversion efficiency = 55 amps
55 amps * 1.25 fuse headroom = 68.75 fuse amps

 

[Zil]

I have an low voltage disconnect for my inverter. The inverter by-passes the negative BMS. The BMS controls the LVD but I don't expect the BMS to ever disconnect. BMS is there as emergency cell level voltage disconnect. Every thing from loads to charging is adjusted to protect the battery.
"...charger with MPPT is 50A max, 80A might be a little big." And then there is Rule #1; Fuses Protect Wires.

 

[Oldtoad]

I have an low voltage disconnect for my inverter. The inverter by-passes the negative BMS. The BMS controls the LVD but I don't expect the BMS to ever disconnect. BMS is there as emergency cell level voltage disconnect. Every thing from loads to charging is adjusted to protect the battery.
"...charger with MPPT is 50A max, 80A might be a little big." And then there is Rule #1; Fuses Protect Wires.

LVD, is that Low Voltage Discharge?
So you are saying if I use 4 gauge wire, use a circuit breaker to protect the wire?
Thinking more of a balancing act, protect everything....

 

[Zil]

I don't recall that. I would say; If you use marine grade, 4 awg cable, use a 150 amp fuse at the point the cable ties into power. Be it buss bar or battery. I don't like DC circuit breakers. It is not a balancing act. Fuses protect wire from short circuits. Devices may or may not have internal fuses to protect themselves.

 

[Oldtoad]

Thought I saw a schematic drawing of the BMS and inverter with and without relays.
Where might that be?

 

[Zil]

There are BMS that have relays. I know little about these, but they have relays. Also called contactors. https://www.electriccarpartscompany.com/ecpc404-meter-assembly-complete-low-and-high-voltage-12v

 

[Oldtoad]

Thanks, a real help.