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Tail current on Blue Solar MPPT 100/50

My configuration:

Fixed absorption time: 1 hour

Tail current: 6 Amps

I thought / have learnt that the MPPT should only svitch from absorption to float before the 1 hour fixed absorption time if the charge current dropped below the tail current setting BECAUSE THE BATTERY IS ALMOST FULL, not because of a cloud/low in-Power to the MPPT

In other Words: I thought the effect of setting a tail current only "kicked in" due to battery conditions, not PV conditions

But yesterday I experienced something different:

The MPPT was i absorption, still charging With about 18 Amps into the battery. Suddenly the sun conditions got worse, so the charge current dropped below 6 Amps beacuse of that.

- Then the MPPT switched to float, even if the 1 hour fixed absorption time was not yet reached, and even if a full battery was not the reason for charge drop below 6 amps.

Is this normal? This is not how I thought it should be...

MPPT - Solar Charge ControllerBMV Battery Monitor
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It has the latest one, V1.46

The absorb voltage is allowed to drop by 0.2v, before the timer stops.

Could this be the situation for yesterday?


If the voltage drops by 0,2V (0,4V i my 24V configuration), the absorption time will be EXTENDED, not shortened.

The point with setting a tail current is of course that the MPPT should switch from absorption to float when the charge current drops below the tail current setting.

But I thought the MPPT was "smart enough" to NOT do it when it was a drop because of bad sun condition and not because of a full battery…

Maybe a Victron staff could reply if I have misunderstood the feature? Or if what I experienced is something un-intende / a bug…?

When your controller wakes up, post a snap of your history, eg.

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1 Answer
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spirou answered ·

This is the reason why I turned off tail current setting and adaptive absorption as my van is parked under semi transparent roof so solar will rarely, especially in winter, deliver much more than a fraction of full power. It doesn't need much to cover a tiny standby drain but if it switches to float almost immediately it's constantly undercharging. You can clearly see it when looking at slowly declining minimum daily voltage. Each day it's 0.03V lower.

I can always turn it back on in summer or when van is in use.

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Your experience seems to confirm my suspision:

If the charge current falls below the set tail current, the MPPT switches to float even if the current drop is due to low PV input (a cloud or similar) and not just due to a full battery.

It's not a very big deal for me. I do not have to use tail current. I can turn off the tail current and just use pure fixed absorption time.

- But I was curious about what Victron intends the MPPT to do in these circumstances.

the solution for the victron technicians would be very simple: to relate a set voltage to the tail current. For example the mppt controller could switch from absorption to float when the tail current is reached, but at the same time an absorption voltage is maintained. In this way a cloud or a sudden load would not have the effect of stopping the absorption prematurely. In fact, the smartshunt works JUST THIS. it synchronizes 100% when the current falls below the tail current and together the voltage stays above a certain "charged voltage" value. It would be enough to copy this system also for bluesolar smartsolar. Meet 2 conditions (V> "charged voltage" and I <"tail current"), instead of just one condition (I <"tail current").