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Victron vs BMS amperage displays differ

Awsmits

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Jul 11, 2021
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I am trying to sort out a mystery in charging displays in my marine DIY system. Here’s my setup:

- I have four 12V batteries, each made with the 280AH EVE cells. Each battery has it’s own 4S BMS.
- The four 12V batteries are connected in a 2S2P arrangement, that is, each pair is connected in series for 24V, and the pairs are connected in parallel for 24V/560AH.
- The batteries are connected to a Growatt 24V 3000A “all-in-one”. I have shore power charging and solar charging. A Victron Smart Shunt monitors the charging as well as the Growatt inverter output.

The issue, maybe simply explained, is that the Victron Smart Shunt always displays about 2X the amperage in or out of the batteries compared to the individual BMS display. Not exactly 2X, but in the ballpark. For example, today I used the batteries through the inverter to run the microwave…the Victron Smart Shunt displayed 44A (which seems high for a microwave), but each of the four BMS units displayed about 26A.

Is this wrong, or what am I missing?

Allan
 
Each parallel pair would be splitting the current so it makes sense for each BMS to show half of the charge or discharge load.
 
Of course, that seems logical. I didn’t really understand how the BMS units would report the amperage flow when connected in that battery arrangement.

Also, still learning to keep amps at 24V separate from amps at 120V. I was surprised at the 44A the microwave was drawing, until I realized that was 44A at 24V!

Thanks!
 
This is why it's better to think in watts. The 1000W being used by the microwave are watts being provided by the inverter and are the same watts being drawn from the battery (not really since the inverter isn't 100% efficient).

1000W microwave at the inverter is 120VAC at 8.3A
1000W microwave at the battery is 1000W / 90% = 1111W at current battery voltage (say 26.6V at the moment) which is 41.8A.

Plug in action numbers to get actual results.
 
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