AGM voltage drop under load with high SOC - Multiplus 2

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ralphd asked Jun 7, '23

AGM voltage drop under load with high SOC - Multiplus 2

I have a Multiplus 2 with a smart shunt and five 100AH AGM batteries connected in parallel. I also have solar panels with two MPPTs that charge the batteries when not on shore power.

The problem - the batteries can be at a full state of charge with a voltage of 12.8V. If I run a heavy load like the microwave, the voltage will drop instantaneously to ~12.1 volts which will cause a low voltage alarm to trigger. When the heavy load is removed the voltage returns to 12.8 volts.

I measured the voltage at the batteries and confirmed that there is less than 0.1V drop between the batteries and Multiplus 2.

Is there some way to dynamically set the low voltage cut off based on the battery state of charge to prevent this from happening?

I don’t want to just lower the threshold since under a light load and low SOC the batteries would be over discharged.

I appreciate and help that you can provide.

Thank you,

Ralph

Multiplus-II AGM Battery low battery warning shutdown voltage drop

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Node Red Space

Answer 1 · 2023-06-07T02:23:17Z

mvas answered · Jun 07, 2023 at 02:23 AM

I think, the MultiPlus has a "Disconnect battery based on Low SOC" setting.

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Answer 2 · 2023-06-07T05:26:44Z

kevgermany answered · Jun 07, 2023 at 05:26 AM

It's annoying, but normal behaviour from the AGMs. It might be an indication that the batteries are getting tired.

Had the same problem.

The low voltage alarm in the Multiplus is set using an MK3 to USB adapter and VE.config software. I couldn't find another way.

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pwfarnell commented · Jun 07, 2023 at 06:42 AM

If the Multiplus is connected to a GX device then you can change the settings via VRM if you have VE Configure 3 on a PC / laptop. You can download the Multiplus settings, edit them in VE Configure 3 and then upload the new settings. The Cerbo GX may need to have certain specific settings enabled such as 2-way communication. I have successfully used this method through a Cerbo GX. You do not need the Mk3 to USB cable this way.

https://www.victronenergy.com/live/vrm_portal:remote-veconfigure

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Answer 3 · 2023-06-07T07:39:31Z

marc-heide answered · Jun 07, 2023 at 07:39 AM

Maybe this helps?

When the voltage drops too much the battery's are probably damaged.

1c would be 100A for a 100ah battery.

0.5 would be 50A for 100ah battery.

These are third party charistics.

1686123344213.png (293.8 KiB)

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Answer 4 · 2023-06-07T16:19:20Z

ralphd answered · Jun 07, 2023 at 04:19 PM

Thanks for your input. My thoughts are this - the AGM batteries are only a year old and I have load tested each one individually with good results. I believe that the internal series resistance of the AGM batteries are causing the low terminal voltage when under a heavy load. When under a heavy load, knowing that the batteries are close to 100% state of charge, it would be nice to have a dynamic alarm and voltage threshold which would not trigger. For example, if the SOC >75% under a heavy load the Multiplus 2, or cerbo gx, would use a lower alarm and shut down threshold. Is anyone aware of a workaround using either the Cerbo gx or Multiplus 2?

Thank you,

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derrick thomas commented · Jun 07, 2023 at 05:07 PM

You could cook something up to make it behave just about any way you want using node red

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