question

thomassolar avatar image
thomassolar asked

Syncronizing the BMV

Imagine the following scenario. How will the BMV battery monitor react?

MPPT regulator absorption setting: 28,70V

MPPT regulator float setting: 28,20V

MPPT fixed absorption time: 1 hour


BMV charged voltage: 28,50V

BMV tail current: 2%

Detection time: 3 minutes


After one hour absorption time at 28,70V, the MPPT switches to float. Since a lithium battery has almost the same voltage as the charge voltage, the charge current drops to zero and reminds at zero for several hours.


Which of the following scenarios will the BMV do??:


1. Syncronize after 3 minutes because the voltage is above 28,50V and the charge current is zero, hence less than 2% of battery capasity

(this is what I hope will happen)


2. Not syncronize because the charge current has to be more than zero in order to syncronize


3. Not syncronize because the voltage is not exactly 28,50V


4. Syncronize after 3 minutes, but then keep on syncronizing several times until the voltage has dropped down to 28,50V

BMV Battery Monitor
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JohnC avatar image
JohnC answered ·

Hi @ThomasSolar

It won't sync at all because mppt Vfloat is lower than BMV Charged V.

For it to sync the BMV must see more than Charged V, and less A than the 2% tail, for longer than 3 minutes

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thomassolar avatar image thomassolar commented ·

Thank you for responding, but I respectfully do not think you are right.


As you say, the BMV has to see a battery voltage higher than the set "charged voltage", and it certainly DOES!


Because this is a Li-ion battery pack, the battery voltage will remain at 28,60-28,70 for HOURS after the absorption charging ends. It does NOT drop to the MPPT float voltage settings right away.


So there will be plenty of time for the BMV to detect at voltage higher than the "charged voltage" 28,50V at the same time as the current is zero or close to zero.


My concern is more if it will syncronize multiple times all the way until the battery voltage has finally dropped to 28,50V


And also what happens since the charge current at detecting point will be much smaller than 2%, it will be zero or very close to zero.

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JohnC avatar image JohnC ♦ thomassolar commented ·

Yeh ok, I see what you're getting at.

What I can't get beyond is that your Float V is lower than the Batt V, which you say can stay above it for hours.

In that scenario, Float does no charging, until your batts are well down. How does this work?

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thomassolar avatar image thomassolar JohnC ♦ commented ·

Yes, you are right. The float does no charging.


These battery celles (from an Electric car) do not need float charging. So the float is set kind of low in order to give the batteries a "break"


The battery is charged With constant current in Bulk until it reaches the absorption voltage, 28,70V


The battery then charges With constant voltage and a decreasing current in Absorption for 1 hour fixed time


The MPPT then switches to float and the battery will "rest" (no charging) until sunrise the next day. (Or if I use a load, the voltage will eventually drop to set float voltage and the MPPT will keep the battery at 28,20V as long as the sun is strong enough)


I hope this explained what you wondered about, JohnC :-)


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JohnC avatar image JohnC ♦ thomassolar commented ·

Yeh ok.

As I understand it, your BMV will sync when you tell it to.

Then once discharge overpowers charge, it will countdown the A and SOC from 100%. It shouldn't resync until it sees charge return to the parameters you've set.

That may well mean you may need to tune it more finely to make sense, if you have frequently alternating charge/discharge situations.

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