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abegweit avatar image
abegweit asked

MPPT Completes Bulk and Absorption with Zero Charge Current. Is this normal?

Hi my MPPT went through bulk and absorption cycles today while disconnected from the batteries. I am trying to determine if this is normal behaviour. I would have thought that once the batteries were reconnected bulk charging would resume.

System set up

  • Two 12.8V lithium smart in series feeding 24/3000 Multi-plus operating off grid
  • 1.2kW solar PV fed through a 100/50 MPPT
  • Battery protect between the MPPT and the batteries
  • VE Bus BMS charge disconnect wired to battery protect to prevent charging due to high cell voltage or low temperature.
  • Raspberry PI using Venus SW is logging data to the VRM.

Temperature dropped to -2C overnight for the first time but warmed up to a high of +14C by 2pm

The site is 200+ km away but looking at the VRM data, I noticed the MPPT battery current remained zero throughout the morning presumably due to low temperature shutdown.

I have the MPPT absorption voltage = 28.4V and fixed absorption period of 15min.

You can see in the chart that the MPPT battery voltage reached 28.4 at 7:12am and stayed there for 15min, then dropped at 7:27am. The MPPT battery current was zero during this time. (Lower right corner of the chart below)

screen-shot-2022-09-29-at-42139-pm.png

Here is a zoomed view of the absorption cycle:

screen-shot-2022-09-29-at-42208-pm.png

It seems the MPPT completed its absorption cycle at 7:27am and then switched to float, without actually feeding any current to the batteries.

At 13:04 the batteries had warmed up enough for the BMS to trigger the battery protect to close the circuit and you can see the MPPT battery current begin flow, but the MPPT remained in float.

Is this normal behaviour in this situation? I would have thought that once the batteries were reconnected, bulk charging would resume.

Thanks



BMSBattery Protectmultiplus ve.bussmart mpptsmart lithium
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3 Answers
kevgermany avatar image
kevgermany answered ·

Take a look at rebulk voltage offset, lowering it may help

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ludo avatar image
ludo answered ·

It would be better to interrupt the solarvoltage to the mppt instead of the charger output, then everything works without problems.


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kevgermany avatar image kevgermany ♦♦ commented ·
Seems to be a remote site. Manual switching probably not an option.

What seems to be happening is that the charger does not restart the charge algorithm once the battery becomes available. That needs to be triggered, hence rebulk... May be other options, ideally with canbus control from the battery. But probably not in this case.


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abegweit avatar image abegweit kevgermany ♦♦ commented ·

Thanks for the feedback @Ludo I was following the Victron schematics which alway show the battery protect interrupting the output of the MPPT, but moving the battery protect to interrupt the PV input to the MPPT would be a really simple solution. @kevgermany I'll also look into the re-bulk voltage. Maybe I can try that remotely. Thanks again.

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ludo avatar image ludo abegweit commented ·
Pls check if the battery protect can be used at solarvoltage!



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abegweit avatar image abegweit ludo commented ·
Good point. Will do!
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abegweit avatar image abegweit ludo commented ·

Glad you mentioned that! I checked the spec sheet. The battery protect is only rated to 35V. So it will need to stay where it is, between the MPPT and the battery.

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ludo avatar image
ludo answered ·

Why don't you take a standard relais with the fitting contact specification instead ?

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abegweit avatar image abegweit commented ·

Thanks, I could try that but I would think that others using Smart Lithium in cold temperatures must be having the same issue


It was cold again last night and I see the same thing today, the MPPT cycled through bulk, absorption, and then to float while not connected to the battery. A bit later, after the battery warmed up, the VE.Bus BMS closes the battery protect and the MPPT stays in float.


At float voltage, the MPPT is handling the load (a fridge) without dropping <0.2V (the re-bulk voltage offset).


So the SOC is maintained during the day but drops lower each night because there is no charging happening. I'm guessing I have 24hrs before the Multiplus triggers a low voltage shutdown.


Thanks again



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