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blackmarlin avatar image
blackmarlin asked

Battery power not stable when battery is full and excess power is fed-in

Hello,

I have a MP2 and a MPPT 150/35 with 2kWp solar power. I have connected a 14,3kWh battery with Seplos BMS. I charge the battery to 55.2 V and then after about an hour at 55.2V, I bring it down to 54V for the rest of the day. As soon as the battery is full, the excess solar power is fed-in to the grid.

I recognized that if the power is fed-in, the battery power is often not 0W as I would have expected, but alternates between charging / discharing with power up to +-/ 200W instead (charge / discharge cycles).

I use DVCC with voltage sense and already tried to use the MultiPlus instead the BMS as battery monitor.

Are there any suggestions or tipps to become the system more stable if the battery is full?

Thanks and regards

Michael

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Multiplus-IIBMSfeed-in limit
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3 Answers
Alexandra avatar image
Alexandra answered ·

@blackmarlin

Use a bmv or other shunt system from Victron for SOC.

Also you might need to clarify here...

I charge the battery to 52.2 V and then after about an hour at 52.2V, I bring it down to 54V

Since 54 is higher than 52.2. To feedback to grid the mppt voltage is raised by 0.4v so you might find there is a trickle in and then a discharge as the system tries to keep the battery voltage down.

Eeited with another thought.

Could be a battery/cell balancing issue as well or at least i have seen similar behaviour in systems with that issue.

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blackmarlin avatar image
blackmarlin answered ·

@Alexandra thanks, I corrected the voltage to 55.2V, this was a typo.

Cells are in balance without about 10mV difference.

So you mean the battery power (current) is more stable if I use a victron shunt instead of the BMS current (via DVCC)? In this case the BMS communication would be more ore less obsolete

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Alexandra avatar image Alexandra ♦ commented ·
@blackmarlin

Since you know what you batteries are doing in balance then I guess that was not the right track.

The Selphos is not officially supported so unexpected behaviours are always possible. What is the CCL and DCL doing at this time in the system? In installs with not officially supported BMS or batteries just in my experience, i have seen the system stability improve with the addition of the shunt.

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blackmarlin avatar image
blackmarlin answered ·

Thanks, I have ordered now a Smart Shunt and will test it.

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