question

Normally such a big voltage difference between a Multi and battery will be due to some wiring problem. Voltage drop along excessively long or thin cables. Maybe additional voltage drop on crimps or connections. Or even a bad circuit breaker or fuse.

Normally multi's above 3000w will have a voltage sense cable connected to the battery to compensate for small differences in voltage readings between the multi and battery.

Is your v-sense wire connected to the battery? Is there a fuse inline between the battery and multi v-sense wire? Is the fuse ok?

Have you got Battery Monitor and Low SOC shutdown enabled in the Multi?

I am Fairly sure that when the 'system' is using something other than the Multi to determine battery state the Multi should have those features disabled. I am set up that way and it seems to work just fine. To be fair I am using a SmartShunt, not a BMV, and my system is anything but standard so I suggest you check what I am suggesting, if it may apply.

I also agree with the post above, that is a BIG difference. The currents don't match and neither W figure is correct but my own experience with battery measurements makes me think that measuring low currents in the Multi or the Shunt just isn't that accurate.

Is the discrepancy worse at higher currents? With that voltage drop those currents suggest a resistance between 0.7 and 0.9 Ohms, depending on which current you pick.

If the BMV is right, so circa 0.7, you would be dropping around 14V at 1kW... That you would notice in a hurry! I am assuming that isn't the case.

I don't have a good answer for you right now save to say something isn't right... I would be checking voltage at the Multi and the Bat terminals with a known good meter, in a range of conditions, as well as measuring the voltage drop of both the Neg and Pos sides of the circuit, as a whole, and across every unindividual item. That will tell you both If, and if so, where, you have a volt drop.

N.B. USE PPE and If you are not 100% sure you have a reliable test meter, in good condition, preferably with fused probes, and know exactly what you are doing... DO NOT mess with a high energy battery circuit at all! Those batteries could vaporise a spanner, let alone faulty meter or lead, and face full of molten metal is a life changing event. 50V is relatively safe but several thousand amps of fault current could set you on fire and blind you in an instant.

(Sorry if you know all this but some folks may not!)