MultiPlus II Low Battery and Overload alarms on system startup

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bradleyasmith asked May 7, '24

MultiPlus II Low Battery and Overload alarms on system startup

I have a new system with the following components:

VE Cerbo GX, Lynx Smart BMS, 4x 200ah lithium batteries, Smart Solar Charger 30/100, 2x 50a Orion Chargers, and the MultiPlus II 12/3000/120-50 120v

When I started the system today, by turning on the BMS and then turning on the solar disconnect, the MultiPlus II showed "low battery" and "overload" alarms and the system was beeping. I turned the MultiPlus II off and, shortly thereafter, turned it back on and it went into normal inverter status.

I had a similar problem a day or two ago when I shut the system down, by turning off the BMS and leaving the solar disconnect in the on position. The next day, I turned the system off by turning off the solar disconnect and, shortly thereafter, turning off the BMS, and the entire system shut down cleanly.

I have updated all firmware in the system devices, including the MultiPlus II.

I don't know what to do to avoid these low battery and overload alarms on the MultiPlus II, and I don't know why they are occurring.

The system seems to be working as expected in all other respects.

Any help would be greatly appreciated.

Best regards,

Brad

Multiplus-II low battery warning

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phoenix commented · May 07 at 08:27 PM

Sounds like you are not pre-charging the MP II caps. They will take an enormous chunk out of the battery when you close the disconnect, so the MP goes into low battery. Your bms hasn't shut down, so the capacitors get charged. Then the MP switches on ok.

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snoobler answered · May 07 at 08:43 PM

This is likely a user issue. Your system has a proper startup and shutdown sequence.

When you first connect a battery to an inverter, there is a massive surge of current as the capacitors charge up.

The BMS in LFP batteries do not tolerate high current surge well. When they see it, they assume there is a short circuit, and they cut discharge.

This is also more complicated if the inverter is in the ON position. Inverter should be OFF when battery is connected.

When PV power is present (or DC-DC charging is active), recommend you reverse the order - inverter OFF, turn on solar disconnect, wait about 10 seconds and then turn on BMS, inverter ON. The MPPT will feed the caps, so there is no surge when the BMS is activated.

Alternatively, turning off the BMS is not best practice for routine operations (fine when placing a system in storage). Recommend you never do this without a compelling reason to do so. If you don't want to run the inverter, just turn it off.

Routine ops (BMS always on):

Startup: Inverter on, PV disconnect ON (order not important)

Shutdown: PV disconnect OFF (so PV doesn't power inverter), Inverter OFF

Storage:

Placing in: PV disconnect OFF (so PV doesn't power inverter), Inverter OFF, BMS OFF.

Removing from: PV disconnect ON (with PV present or DC-DC charging active), 10 second wait, BMS ON, Inverter ON.

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bradleyasmith commented · May 07 at 10:54 PM

Thank you!

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snoobler bradleyasmith commented · May 07 at 11:06 PM

Something I don't think I made clear - If you leave the BMS on, even with the inverter off, the battery will keep the caps fully charged and avoid the whole surge current/short circuit protection thing, and you can turn everything else on/off almost willy-nilly.

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bradleyasmith snoobler commented · May 08 at 02:46 PM

Thanks again for your assistance! Your response has been very helpful. I appreciate you taking the time to lay it out for me. That's exactly what I needed.

Best,

Brad

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snoobler bradleyasmith commented · May 08 at 02:53 PM

Glad to hear it! Thanks for the feedback.

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klim8skeptic answered · May 08 at 05:59 PM

@Phoenix One thing of note about the Lynx Smart BMS is that it has an inbuilt Built-in pre-charge circuit .

It would be a good idea to read the Commissioning and Operation of the Lynx Smart BMS section of the manual.

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snoobler commented · May 08 at 06:05 PM

@klim8skeptic Thanks! I read that as a Lynx Shunt. Good to know the BMS has pre-charge built in.

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