BMV 712 % Charge and Midpoint

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icemanxicd asked Mar 17, '19

BMV 712 % Charge and Midpoint

I have 4 6v batteries connected in Series/Paralell to create 2 12V battery banks. Batteries are brand New. I have a couple of questions as i am new to the battery monitoring, and victron.

1. My positive connection goes to one of the 12V battery banks. If i want to read the second battery bank, would i put the second cable on the positive of the second battery bank and put it into B2 utilizing the Midpoint?

2. I did not zero anything, but after a few hours of being plugged in and a light on just to get data,

I had 12.46 volts but 100% Charge. It is my understanding that

100% Charge - 12.7 Volts +

75% Charge - 12.4 Volts

50% Charge - 12.2 Volts

25% Charge - 12.0 Volts

0% Charge - 11.8 Volts

Was the Victron Just reading my first battery, first battery bank, or all 4 of my batteries.

BMV Battery Monitor installation

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Answer 1 · 2019-03-18T02:26:04Z

Mark answered · Mar 18, 2019 at 02:26 AM

You essentially have 2 x 12V battery banks connected in parallel (with each bank consisting of 2 x 6v batteries)

So I assume that you already have a link cable between the +12v positive & the negative between the 2 banks.

Accordingly the voltage & current readings reflect BOTH banks / all batteries together.

In reality there would be a slight variance between each bank (if you were to disconnect the link cables & measure each bank separately), but this delta would be small unless a significant imbalance exists.

For the 'mid point' measurement to work you will also need to link the mid points of the 2 battery banks - this point should be at +6v.

Ensure that you use a sufficiently large sized cable to make the mid-point link (to protect for a worst case imbalance condition/failure when significant current could flow across this cable), unless you add a fuse in between.

Then you can connect the B2 lead from the shunt to either of the center battery terminals.

See extract from the Victron quick connect guide (which is the same setup as yours except this example has 3 banks in parallel instead of 2);

Another option (which is what I do) is to have a 4 position battery switch (off, 1+2, 1 only, 2 only) and connect the output to B2, & inputs to the mid point of each battery bank. The normal operating position for the switch is 1+2 - so the switch acts as the link cable.

With this switch you can momentarily 'disconnect' the mid-point link & INDEPENDENTLY check the mid-point reading for each bank.

This is a very handy feature & not hard to achieve. I then fuse both connections to the battery switch separately at the battery.

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icemanxicd commented · Mar 18, 2019 at 02:49 AM

My batteries are connected like the diagram below.

Thank you for the explanation about the midpoint, that was extremely helpful.

I am a bit confused on how to read this Monitor when it comes to the information it provides.

The Amp Hours were only a couple used at the time, so i know not much was out of the battery bank, but its my understanding that.

100% Charge - 12.7 Volts +

75% Charge - 12.4 Volts

50% Charge - 12.2 Volts

25% Charge - 12.0 Volts

0% Charge - 11.8 Volts

I am assuming by the way i have wired it and your explanation that its reading 12.46 Volts of the Entire bank of 4 batteries Correct? So which is correct, i know that i have 4 brand new 6V 225AH batteries which when wired below give me 450 AH total, 50% =225. By using just a couple of AH (3 i think it was) i could see where the monitor would read 99% or even 100% charge left if thats what it goes by, however the voltage was 12.46 which should have been 80% ish. I guess What do i go by to see how much i have left in my batteries, because the voltage and the % left didn't match.

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Mark ♦♦ icemanxicd commented · Mar 18, 2019 at 09:09 AM

The first point is just because your batteries are new doesn't mean that they are fully charged to begin with.

Also, when you connect a new BMV the default is to turn on with the assumption that the batteries are 100% full. This may or may not be correct.

So in summary you can't trust the BMV SOC % readings until you complete one full charge cycle and the BMV is synchronised.

Now once this occurs and you have correctly setup the BMV using the battery bank C20 Ah rating, then you should treat the BMV SOC% as master over the voltage reading as it should be correct.

The voltage vs SOC % will vary between different types / brands / models of batteries. I don't know if the numbers you have provided are specific for your batteries or generic...

Also during use (discharge & charge) the battery voltage is artificially reduced or elevated from its true voltage in a 'rested' state.

So if you want to use voltage as a measure of battery SOC % then you really need to have the battery disconnected & resting for ~6 hours to give it time to stabilise & provide an accurate/true reading.

Finally, the BMV reading always represents ALL batteries that it is connected to - so in your case it's all 4. It has no way to tell the makeup of the battery bank & sees it all as 1 big battery.

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