abauti avatar image
abauti asked

BMV-712 not showing actual current

I saw a few post regarding inaccurate readings with the BMV-712; my issue is with current. I have a aims AC-DC battery charger set up to charge (2) 48v Lifepo4 batteries. My particular set up has the charger hooked up to a power strip rated for 12amps max. While adjusting the current output on the charger to make sure not to go over 12amps, I noticed my clamp meter on the circuit for that outlet was showing a different current than what the BMV shows while charging. Clamp meter shows 6.5amps, while the BMV app shows 11.6amps. I have the charger negative connected to the main lug on the load side, so not sure what else could be throwing off the current reading. I have nothing else charging to the batteries and the the inverter is off. Same goes with when there is a load, the BMV always shows more current being drawn than what actually is shown on the clamp meter. Any ideas?

BMV Battery Monitor
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Kevin Windrem avatar image Kevin Windrem commented ·
Are you confusing AC current into the charger and battery DC current?
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abauti avatar image abauti Kevin Windrem commented ·

Are you saying the DC current into batteries will always be different? How about when there is a load being drawn? Is the victron app always going to show higher amps then what the appliances are rated?

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kevgermany avatar image kevgermany ♦♦ abauti commented ·
You don't say what you AC voltage is, but assuming 120V, whatever current you measure in the AC side will be multiplied by the voltage conversion to the batteries. For example 10A AC into your batteries (let's assume 50V charging) will be (120/50)*10=24A out of the charger and into the BMV. If you have a load between charger and BMV, then that will reduce the current seen by the BMV.

These examples approximate and take no account of efficiency losses etc

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Kevin Windrem avatar image Kevin Windrem Kevin Windrem commented ·
The current INTO a charger is at the AC line voltage. The current from the charger to the batteries is at battery voltage. What will be consistent is the power (voltage * current), less the loss in the charger. Lets say that you are on 120 volt AC power. Then the power into the charger is 120 * 6.5 amps or about 780 watts. The power into the battery is 48 volts * 11.6 amps or about 560 watts. Note that the voltage * current calculation does not include power factor so power on the AC side would be somewhat less. Inexpensive meters are available to accurately measure power rather than volt-amps.

When looking at situations like this, it's best to use a power value rather than current so you take the voltage differences out of the equation.

The BMV measures the total current into/out of the battery. So loads will show as negative, charging sources show as positive. So if you are charging with say 10 amps and you have a load of say 5 amps, the BMV will indicate 5 amps of current into the battery. If the load increases to 20 amps then the BMV will show -10 amps (out of the battery).

Now, add in an inverter and that shifts voltage again. So if your load is an AC load of say 5 amps at 120 volts or 600 watts, you'll see a little over 600 watts from the battery (with no charging sources or other loads). That's about 12.5 amps out of the battery.

The same would be true if your DC loads were fed from the battery through a DC-DC converter. Let's say you have a load of 10 amps at 12 volts. That's 120 watts. That's 120 watts out of the battery or about 2.5 amps.

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abauti avatar image abauti Kevin Windrem commented ·

Ok, I think I understand. I'm still new to this, so I was treating the Victron BMV display as if it were a typical plug in power meter for an AC outlet. I just have to get the DC and AC parts sorted out in my head. So would the power in watts be what is similar going from AC to DC and visa versa?

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Mike Dorsett avatar image Mike Dorsett abauti commented ·
power in watts = voltage multiplied by current.

yes, power at ac will be approximately equal to that at dc, (less ~ 5% for conversion loss).

so 10A at 12V dc is 120W, or 1A at 120V (ac).

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