230ah battery w/200BMS and 2000w inverter question

[smieglitz]

I'm trying to put together a portable DIY power bank system for camping and I'm confused about the maximum amperage I have to plan for going through the cables. I've read various posts about sizing cables based on various ampacity charts and formulas, but many of them give conflicting results. I've also read a couple books on DIY PV systems and watched many YouTube videos on the subject, but still have some gaps in my understanding. So, I'm hoping this one question might clear my confusion:

If I have a LiTime 12w 230ah LiFePO4 battery with internal 200BMS and I wire it to a BougeRV 2000w inverter, does the BMS limit the draw to 200a (max continuous current discharge from the battery spec sheet, 5 second max is 600a) or do I have to plan for higher current?

I've seen recommendations to figure the ampacity based on a formula that divides the total watts of the inverter by the lowest voltage draw of the inverter divided by an inverter efficiency factor and finally multiplied by a 1.25 safety factor. For my situation that gives (2000w/9.5v/.85) x 1.25 = 309.6 amps. Other formulas and recommendations just suggest 167amps (=2000w/12v). That's quite a range! I've also seen calculations where the efficiency factor is a multiplier rather than divisor and the would result in a significantly lower amperage (223.7amps). So, I'm very confused as to how to proceed.

I'll also mention that I never plan to draw more than 1500w from the inverter, but I bought it slightly higher than needed since its footprint matches my build better. I'm planning to use 2/0 welding cable from the battery to inverter (<5' total circuit length) and probably 4AWG from the busbar connecting those to a DC fuse box on the DC side of the build. There I have a Redodo 14.4v 40a DC-DC charger and will be purchasing a Victron 100|50 MPPT to charge from 2 (or 3 in the future) Ecoflow 220w bifacial panels (21.8VOC, 13.0 ISC). I'll also be using a 12v Redodo 40a DC-DC (and solar MPPT) charger for hooking into my vehicle alternator system and plan to connect the chargers via SB50 Anderson connectors back to the battery side. Planning on Blue Sea fuses and circuit breakers, LiTime or Blue Sea busbars and welding cable throughout and also have a Victron 500a shunt with 712BMV monitor. I'm building this in 2 modular toolbox enclosures and plan to use an Anderson SB175 connection between the lower battery/invertor box and a separate upper box holding all the chargers and most of the DC stuff. 50amp breakers on the chargers and for a pv cutoff switch, and 120a DC breaker before the 100a fuse panel as the box connecting cable enters the DC side (depending on any feedback I get back about the ampacity values). Probably go with a 300a ANL fuse on the positive coming from the battery unless I hear otherwise. As a complete newbie, my head is spinning from all the specifications and sometimes conflicting equations and online calculators.

So again, can this all be simplified to the battery BMS limiting the amperage to 200amps? If so, maybe my brain can stop hurting...

Thanks in advance for any clarification you can provide. If you see something suspect in my plans, I'd appreciate any additional tips. I'll be creating a diagram to post later once I get the info on max amperage to plan for.

Joe

 

[hwy17]

There are complexities and ways to bend the rules, but you can simplify it by matching your fuse and your wire amperage. Fuses and wires are sized for continuous current, and both allow peaks above that.

E.g. if you use 1/0 and a 150 amp fuse or 2/0 and a 200 amp fuse, you can trust the fuse to protect the wire. Both the fuse and the wire will allow peaks above those amperages, but the fuse will blow if the peak lasts too long for the wire.

 

[smieglitz]

There are complexities and ways to bend the rules, but you can simplify it by matching your fuse and your wire amperage. Fuses and wires are sized for continuous current, and both allow peaks above that.

E.g. if you use 1/0 and a 150 amp fuse or 2/0 and a 200 amp fuse, you can trust the fuse to protect the wire. Both the fuse and the wire will allow peaks above those amperages, but the fuse will blow if the peak lasts too long for the wire.

Understood regarding sizing the fuse to the wire but does the BMS number always indicate the maximum continuous current to plan for or is it just a coincidental match (200 BMS and 200a continuous current even though the battery is rated 230ah) in the battery I purchased?

IOW, do I never need to plan for a current in excess of the BMS rating because the BMS isn't going to allow it? I realize factors such as wire length, voltage drop, type (e.g., Cu vs Cu-plated Al) and stranding of the cable, temperature, etc., may increase the resistance and thus require a greater thickness of wire, but for short runs, is the BMS number indicative of the amperage I need to account for hen choosing the wire?

 

[hwy17]

Understood regarding sizing the fuse to the wire but does the BMS number always indicate the maximum continuous current to plan for or is it just a coincidental match (200 BMS and 200a continuous current even though the battery is rated 230ah) in the battery I purchased?

IOW, do I never need to plan for a current in excess of the BMS rating because the BMS isn't going to allow it? I realize factors such as wire length, voltage drop, type (e.g., Cu vs Cu-plated Al) and stranding of the cable, temperature, etc., may increase the resistance and thus require a greater thickness of wire, but for short runs, is the BMS number indicative of the amperage I need to account for hen choosing the wire?

I would say it like this: If a BMS claims a 200A rating, that indicates that you can use it in a system where you are intending to draw up to 200A and it is not supposed to crap out, shut down, or fail on you.

But I would not consider it an over current protection device. A 200A BMS will usually interrupt the load if you try to draw 250A from it, but if you accidentally short 1000A through it then it might fail to interrupt, or it might fail closed one day for no reason and allow all current through.

Basically the BMS is not trustworthy like the fuse is.

Others might disagree with me.

At the same time, the usually "fault" of a cheap BMS is that it will interrupt current at below it's rating, like you'll find it's shutting down at 175 when it's supposed to be rated for 200A. This is the more common problem, but not the dangerous one.

 

[smieglitz]

Got it!

Thanks for the explanation. I appreciate the time you took to answer my question.