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Old 09-25-2021, 06:11 PM   #1
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Battery monitor/charger question

I am curious about the monitor readings I get.

I have two Battleborne 100AH batteries, and a Victron BMV-712 battery monitor. I charge the batteries with a Victron Multiplus hybrid inverter. I have a 45-watt solar panel also.

Both the monitor and the inverter report the rate of charge, but those rates do not match. At one point the inverter was reporting 82 amps to the battery, while the Monitor was reporting 75 amps.

Why do you suppose these values are so different?
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Old 09-25-2021, 06:35 PM   #2
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The inverter/charger output is also powering anything running in the RV.

If you have 10 amps of stuff running that power is not going to the batteries.
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Old 09-25-2021, 07:16 PM   #3
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I have nothing on in the RV, so there should only be the 1 1/2 amp parasitic draw. Certainly not 10 amps on the DC side.

I sent the same question to Battleborn, as they are the Victron dealer I bought the gear from. Perhaps they will have some ideas.

It is not a crucial issue, I am just curious why the two monitors report different things. If the devices are not that precise I will be disappointed.
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Old 09-25-2021, 08:26 PM   #4
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The BMV-712 has a zero current calibration procedure but I think not one that affects higher readings (beyond whatever offset the zero current error is). It wouldn't hurt to check that it reads zero when the charge/load current is really zero.

The accuracy of the 712 should be excellent. See the second page at this link.

The charger manual may list the same accuracy specs.

I have no experience with the inverter charger. If you are sure that you have accounted for all loads between the charger and the BMV712 shunt, it's time for a call to Victron.
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Old 09-25-2021, 09:45 PM   #5
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Thanks, hclarkx.

No comparable accuracy info for the inverter.

Victron will not deal with end users or installers. We have to work through the dealers, who are supposed to be trained. Hopefully Battleborn can answer my question. They have been pretty helpful in the past.
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Old 09-25-2021, 11:24 PM   #6
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Just curious.....
  • which VIctron Inverter/Charger are you using?
  • What size cables do you have cabling the batteries together?
  • what size cable do you have going from battery bank to inverter?
  • how long is the cable run from battery bank to inverter?
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Old 09-26-2021, 07:44 AM   #7
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Good morning, Paul.

I have a Victron Multiplus 12/3000/120-50. All of the cabling is 1/0, with about 5 feet (if I recall correctly) from inverter to the busbars at the battery.

Apparently using larger cable requires at least one more battery (I have only two) as the inverter inrush current trips the BMS. As one forum contributor put it, I am using cables as resistors.
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Old 09-26-2021, 10:28 AM   #8
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Quote:
Originally Posted by JHinman View Post
Good morning, Paul.

I have a Victron Multiplus 12/3000/120-50. All of the cabling is 1/0, with about 5 feet (if I recall correctly) from inverter to the busbars at the battery.

Apparently using larger cable requires at least one more battery (I have only two) as the inverter inrush current trips the BMS. As one forum contributor put it, I am using cables as resistors.
well I'm not an expert, I just pretend to be one on the internet..............but based on your cable choices and adding resistance I would pontificate that this is making the difference in the voltage readouts you are seeing.

BTW I have not heard of that issue and was always of the belief that bigger cables (Properly fused) never hurt anything.

For instance, my 24V system only really needs 2/0 cables but then again I upgraded from a 12V system and just kept the cables based on my theory above...... no issues on our side with 10 LiFePO4 batteries in 5 sets of 2.

Cheers!
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Old 09-26-2021, 10:50 AM   #9
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It is apparently an issue with this combination. Battleborne advised me not to use 4/0 cables for that reason, and other folks have said they had a problem.

It is an amperage issue that caught my eye, not voltage.
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Old 09-26-2021, 01:29 PM   #10
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To me it’s also strange that you’re picking up 2 different temperatures. Is everything still on the the same network?
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Old 09-26-2021, 01:49 PM   #11
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Precharge your inverter capacitors when you power it up and it won't trip your BMS. Then you can move to properly-sized cables. You're generating 10amps of heat in there. Undersized cables is a cludgy way to handle the BMS issue.
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Old 09-26-2021, 03:23 PM   #12
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Seems like you are losing 135W between the two units. 13.78x84.1-13.63x75.1
84.1 = 135. P=RxIxI then the run has a resistance of about .023 Ohms. Normally, I would think of that resistance as the same as zero but the current is so high that any small resistance turns into a measurable power loss and heat. One thing I would do is check all the terminals to make sure one isn't bad and adding resistance.
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Old 09-26-2021, 04:25 PM   #13
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How does a person pre-charge capacitors in an inverter? I do not see that in the user manual.

I can see how the readings would be different if I was wasting power by heating cables. That is something to look into. 10 amps at 14 volts is 140 watts, which seems like a lot. I don’t feel the cables getting warm, but I have not actually measured the temperature.

I am planning on getting another battery and re-wiring with 4/0 cable. What I have seems to work, but if I can make it “better” I will do so just because I can.

I really appreciate all of these comments. It is really educational!

Perhaps Battleborne will get back to me tomorrow with their thoughts.
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Old 09-26-2021, 06:03 PM   #14
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This video may help about precharging.
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