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bms bypass vs equipment ground

John Frum

Tell me your problems
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Nov 30, 2019
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Assumitions
280ah LFP cells and 100amp overkill bms.
We are ignoring the dc load center for load calculations.
Finally we assume Samlex PST series inverters which support external switching and have a conversion efficiency of ~.9.

1st topology can support a 2000 watt inverter.
The constraint is the bms.
2000 ac watts / .9 conversion factor / 24 volts low cutoff = 92.592592593 service amps

2nd topology(Same as the first except the inverter bypasses the bms) can support a 3000 watt inverter
3000 ac watts / .9 conversion factor / 24 volts low cutoff = 138.888888889 service amps
The constraint is a policy decision to limit discharge to .5c.
A side benefit is no pre-charge resistor is required.

q: What is your opinion on having the equipment ground go through the bms but the inverter neg conductor bypassing the bms?

Other observations are also welcome.

Topo diagrams in subsequent message.
 
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If the inverter negative is isolated from the case ground and neutral/ground I can't see an issue. Haven't looked into that aspect of inverter wiring though.....

Quick test here-1 brand seems isolated and another not so much....
 
If the inverter negative is isolated from the case ground and neutral/ground I can't see an issue. Haven't looked into that aspect of inverter wiring though.....

Quick test here-1 brand seems isolated and another not so much....
By case ground you mean equipment ground?
 
From memory their is continuity between the negative current carrying conductor and the equipment ground.
 
Bottom lug between the fans...
pst-2000-back-670x496-1.jpg

If there is continuity it would essentially bypass the BMS. Xantrex Prowatt 2000 seemed isolated, Renogy (gasp...) seemed connected-not a real good test though...
 
Bottom lug between the fans...
pst-2000-back-670x496-1.jpg

If there is continuity it would essentially bypass the BMS. Xantrex Prowatt 2000 seemed isolated, Renogy (gasp...) seemed connected-not a real good test though...
The current carrying negative connection bypasses the bms.
The equipment ground(case ground) is connected to the negative busbar.
I just checked my own system which includes a Samlex pst inverter.
The current carrying negative and the equipment ground both are connected to the negative busbar.
The current carrying negative is passing ~10 amps and the equipment ground is passing ~0 amps.

The intent of the system in the second drawing to make a system that supports a 3000 watt 24 volt inverter with a 100 amp bms.
 
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