janickspielmann avatar image
janickspielmann asked

BMV 700 % makes no sense


I have a BMV 700 hooked up to a 160ah battery that is charged by a 200W Solar Panel. Everything works ok. The only problem I have is that the percentage dose not work.

The Display shows 100% at everything higher than arround 13.5. That's okay. But when I drain my battery down to 10v it still shows like 50%. When it reaches 9.5v the system shuts of and the last known percentage is like 45%. How can I set the 0% Mark?

I have a RaspPi connected and can check the values constantly.

BMV Battery Monitor
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1 Answer
john-hagtharp avatar image
john-hagtharp answered ·

I think this post shoud be moved to the main area.

I helped someone recently in this post regarding setup of a BMV. I think the problem arises due to people not realising what an imprecise and strange behaving 160 yr old science experiment a lead acid battery really is. What you have is some lead plates, half of which are in the form of a grid caked in lead peroxide, dunked in a bucket of sulphuric acid.

Assuming you're charging it correctly (a big topic on it's own), it takes work to get a battery monitor closely reflecting what's happening in your bucket of lead and acid. The main parameters to get correct are:


Use the rated value if new, otherwise the only real way is to measure how much you get out of it from fully charged to a safe low voltage cutoff point. This is best done at your C20 or C10 rate.


Use the battery manufacturer's figure if available. If not, derive one using an online calulator like this.

Charged Voltage / Tail Current

At some point in the charging, the voltage should be constant at the float voltage or absorption voltage and the current going to the battery will hardly be dropping any more. If you're looking at a graph of current, it will have flattened out (within reason). This is your indicator that the battery is fully charged and setting these two values will enable the montor to sync at 100% here. The monitor's logic is that if the voltage remains above the charged voltage and the current below the tail current for at least the charged detection time, it will sync to 100%

Charge efficiency

Everything above should make the monitor pretty close to accurate under discharge down from 100%. The charge efficiency is about trying to get it accurate on the way back up again. Unfortunately, the reality is not very linear. The battery will charge very efficiently at lower states of charge and very inefficiently at high states of charge. Just tweak the value until, under your typical usage, the monitor is reaching 100% indicated SOC about the same time your sync point above is reached.

Two particular points from re-reading your post.

  1. Voltage is a very poor way to measure state of charge. The basic functionality of the BMV is to indicate SOC based on monitoring everything that flows into and out of the battery.
  2. It's the 100% mark you want to be setting, not 0%.

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