question

janickspielmann avatar image
janickspielmann asked

Setting 0% charge on a BMV-700

I have already posted this in May but the answer I got there did not help me. I get, that Voltage is not really a good way to measure battery state but still my devices shut of at curtain voltages, and it's the easiest way for me to show the behaviour.

I am using the 700 for 3 months now together with a Raspberry Pi gateway. Everything works fine and I am happy. There is just one problem. How can I set the 0%.? The 14.4V is my 100% and if I use my 160ah battery to 9V It still showed 40% charge. Of course everything else (fridge, GPS...) has shut of at around 11V so that should be my 0% How can I change that? I don't have the BT dongle but can control everything else from the display or the Pi.

My setup


160Ah 12V lithium battery

MPPT 75/15 with 2x120W solar Panels -> connected to pi via Ve.direct USB cable.

230V battery charger

Relay to the camper's starter battery 12V, so I can charge while driving.

BMV 700, the shout is connected between the battery and everything else -> connected to pi via Ve.direct USB cable.


Pi has a backup battery so it does lose power last.


BMV Battery Monitor
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6 Answers
Jaco Reinecke avatar image
Jaco Reinecke answered ·

Hi,

What do you mean by asking how to set the 0%?

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jwfrary avatar image
jwfrary answered ·

Hi, doesn’t quite work like that.

So it will use 14.4 or whatever it’s set to as the 100 % mark and then count the amps out of the battery to give you SOC and the % is based on the the programmed capacity you enter into the BMV

So the 0% is a calculated number, equally this changes due to pukerts law. So it’s not as easy to say or change.

Ps if your running the bank down to 11v regularly then it’s not going to last long!

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dazey77 avatar image
dazey77 answered ·

To me something seems wrong with your setup. It may be that there is some current draw thats not going through the shunt as otherwise your battery does not seem to be any where near specs for a lithium. I have a Valence lithium (I am using as an example as I have the sheets for it). It allows you to discharge down to 10v but that is 100% of its rated capacity. The voltage vs capacity holds over 12.5v until 90% discharged (20˚C) and then it falls off a cliff. For you to be at 60% discharge and massivly low volgages isn't right (and 9v is not healthy for a lithium, mine disconnects its cells at 9.8v). Either your drawing more than your shunt knows about or your battery isn't performing like a 160AH lithium.

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dazey77 avatar image dazey77 commented ·

Example LiFePO4 discharge curves


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screenshot-2020-06-23-at-230745.png (267.0 KiB)
dazey77 avatar image
dazey77 answered ·

I have a theory that its not a 160AH battery but a 90 or 100AH battery with usable 80AH (with discharge to 10 or 20%). An 80AH usable battery might be described as 90-100AH (160AH lead acid equivalent) or similar sales blurb that takes into acount the 50% capacity of lead batteries. Guess work of course.

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irwinr avatar image
irwinr answered ·

EDIT: I missed the part where you specified this is a Lithium Pack. The 160AH lithium pack should not have lost this much capacity (They should last 10+ years easily)

If your pack showed 40% at 9v then that means either you don't have a 160 AH pack (Because it was damaged somehow) or you have a load somewhere that's not being picked up by the BMV.


How often do you discharge it down to 9v? That's a dangerously low voltage for a lithium 12v battery, low enough to damage it. Is this battery protected by a BMS? What make/model/brand of battery is it?

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rbreding avatar image
rbreding answered ·

Sounds like a load test needs to be performed on this battery pack to see exactly how many AH it has.

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