Multiplus-Inverter switches on and off again after approx. 2 seconds

Q&A and troubleshooting
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1

Hello,

I have installed a new LiFepO4 battery (24V, 200Ah) in a motorhome (expedition vehicle) that already had a LiFepO4 battery before.Current battery status: SOC=94%, 26.66 volts.

I am using a Multiplus 24|3000|70-16|230/240V

The Multiplus charger continues to work without any problems.
However, now when the Multiplus is powered on (exclusively from the LiFePO4) the inverter no longer works correctly:

It switches on and then switches off again after approx. 2 seconds, then switches on again briefly for 2 seconds and switches off again, etc, etc…
No error messages are shown on the LEDs.

My inverter settings
3.1. PowerAssist . . . . . . . . . . . . . . . unchecked
3.2. Inverter output . . . . . . . . . . . . . 230 V
3.3. Ground relay. . . . . . . . . . . . . . . checked
3.4. Inverter DC shut-down voltage . 24.20 V
3.5. Inverter DC restart voltage . . . . 25.50 V
3.6. Low DC alarm level . . . . . . . . . . .25.50 V
3.7. Do not restart after short-circuit . . . . unchecked
3.8. Enable AES . . . . . . . . . . . . . . . . checked
3.9. Load for starting AES mode . . .69 W
3.10. Offset for ending AES mode . .30 W
3.11. AES type . . . . . . . . . . . . . . . . search mode

Any idea what to do/change to get the inverter working properly again?

Kind regards,
Hilmar

2

3.8 tried to switch this off and test?

3.4 isnt this a bit high for the 24v LFP bank?

1 Like
3

We need more info. What is the load at that moment? Does the voltage on the MP’s terminals change during the flipping when measuring with a multimeter? Have you charged the battery first and synchronized the smart shunt? Smartshunts remember the SOC incrementally by currents so that 94% could be the old value from the old battery.

4

First of all, thank you both very much. I am not near my motorhome at the moment.

There is currently no load connected.
After changing the battery I charged it up to 100% in order to balance the cells.

Unfortunately, I also changed the following settings in the inverter:

  • inverter DC shut-down voltage from 23.5 V to 24 V
  • inverter DC restart voltage from 25.5 V to 26.0 V
  • low DC alarm level from 25.5 V to 26.0 V
    When I get back to the vehicle, I will reset the values.

I did not synchronize the smart shunt. How do I do that?
the SOC=94% comes from the the battery´s BMS

Thanks Hilmar

5

Manual synchronization.

It happens automatically too.

The on off (or power pulse) may be the AES setting.

6

“It happens automatically too”
That was the case for me (displayed by the battery monitor)

7

Thank you all for the helpful tips.

  • with Inverter DC shut-down voltage=23.5 V
  • Inverter DC restart voltage =25.5V
  • low DC alarm level=25.5V
  • enable AES=unchecked

It is working again!

8

Set DC low-shutdown at 22.0V, DC restart at 23.2V. Lithiums can take it, as long as this doesn’t happen daily.

9

This seems to me quite low. Why would I do that. Is there any advantage?

What do you mean by “Lithiums can take it, as long as this doesn’t happen daily”

10

Do you have a pre-charge circuit? It sounds like your battery can’t supply the high start-up current demanded by the inverter.

11

What is a pre-charge circuit?

12

Basically, the big inverter capacitors are charged before normal operation. Some batteries can do that automatically, others (most?) can’t. There are pre-charge devices on the market. Or you can do it more manually. Or use the MPPT to pre-charge. Others use AC but that is not supported.

13

DC shut-down at 23.5V is 98% usable capacity vs 99% at 22.0V. You are correct, it’s almost exactly the same.

However, if your battery empties and you try to charge it up using another one in parallel, a restart voltage of 25.5V (as per your setting) will probably make you wait for far too long, since there are 24V nominal LFP battery packs that get their voltage over 25.5V at over 80% charged. So yes, it does make sense to adjust cut-off and restart voltages as low as possible on LFPs.

14

Where do these values come from? The tables I find give quite different values.

15

Thanks for explaining. I hadn’t realized that.

16

Unfortunately, every LFP battery out there has indeed different values. Additionally, FLP voltage tends to be lower while discharging and higher while charging, which makes the whole thing even more complicated. Super low DC disconnect and DC restart values are my personal preference for the solar parks I do O&M for, since these backup systems need to be as quick as possible when it comes to getting them back online, after a long-duration power failure.

17

Thank you, yes, now I understand.