Not going into absorption mode after second MPPT installed

DIY
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1

Hello, I’m trying to chase this fault where since I added a second solar array and MPPT (150/70 Tr Ve can) to my existing system, both chargers go straight to float mode and not absorption first. Prior to adding the extra solar the system worked perfectly for over 3 years.
The system consists of: 7 x 3kwh lithium batteries (Genz)
Cerbo GX, MPPT 250/100 connected to 4kw solar array
New array - MPPT 150/70 with 6 x 475w Jinko Tiger Neo panels (2 strings of 3)
Smart Shunt
Victron Mulitplus 5kw inverter charger (charger not used)

I connected the 2 MPPTs via Ve Can with terminators at both ends and updated to latest 3.16 firmware on both including the shunt. Both chargers are linked and communicate together with the original being the master unit according to the Cerbo GX.
I’ve gone through all the settings and mirrored the charge setting from the original MPPT to the new one, see below. I tried reducing tail current to zero but no difference. After it bypassed the absorption stage yesterday, I shut down the solar and 48 volt supply to both MPPT chargers during the day and fired them back up again after battery capacity dropped to 95%, the system went from bulk to absorption without any problems for the 30 minutes I set it to and then to float as it should. It wasn’t until the next day that it skipped the absorption phase again. So clearly it will work, but it only seems to do it correctly the one time after the battery is connected to the MPPT input. I thought perhaps the MPPT is remaining in bulk mode overnight. When I checked both MPPT’s at night they were off (through the victron connect app via bluetooth where is says ‘state’) with the blue flashing light flickering in unison once every 5 or so seconds, which makes be think they are asleep and should look for another absorption cycle the following day. But this didn’t happen the following day and it went straight to float from bulk. Below are some screen shots to hopefully provide more information.

The graphs show the moment it changed from bulk mode to float mode, bypassing the absorption mode.

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Cerbo GX has DVCC enabled with:
Limit charge current - OFF
Limit managed battery charge voltage - OFF
SVS - Shared voltage sense - ON
STS - Shared temperature sense - ON

The settings for both MPPTs:

Battery voltage - 48v
Max charge current - 70A
Charger enabled - ON
Battery preset - user defined
Absorption voltage - 57.60V
Float voltage - 55.4V
Equalization voltage - Disabled
Re-bulk voltage offset - 0.40V
Absorption duration - Fixed
Absorption time - 30 m
Tail current - disabled
Equalization current percentage - 0%
Automatic equalization - Disabled
Equalization stop mode - Fixed time
Temperature compensation - Disabled
Low Temp cutoff - 5 deg Celsius

Shunt settings:

Battery capacity - 403Ah
Charged voltage 55.4
Discharge floor - 50%
Tail current - 4.00%
Charged detection time 3m
Peukert exponent - 1.02

Thank you so much for any assistance in advance.

26/2/25 - Update. Have been playing around with the parameters and seem to have exhausted every possible setting scenario with the issue still persisting. I will try and swap it from VE Can connections to VE Direct and see how it goes.
If anyone else has encountered this I am still very keen to hear from you…

2

Charged voltage in shunt should be set to 57.4
Tail current 0.5 or 1 %

3

Thank you for that.
Do you think by changing these values it will rectify the issue of not going into absorption mode?

4

It’s worth a try.

5

Thanks anyway. I tried but it skipped straight past the absorption. I’ll leave the settings you recommended though as they are more suited to my off-grid set up

6

There are a few numbers different to mine.
Mine is set up for 24v so have doubled the numbers.
I’m also offgrid and only using solar

My discharge floor is 30%

Float 54.4v
Charged 56v. (Shunt)
Absorption 56.6v

But…I dont think lithium really hangs around absorption voltage. It goes straight from bulk to float.

7

Its interesting how different manufacturers provide varying recommended settings for the same chemistry battery. The Genz batteries I have specified all the settings that I used.
I did change the Charged Voltage to just below the Absorption as that is recommended for solar feeds and also the discharge floor as it’s more for lead acid I think.
I’m pretty convinced this isn’t a settings issue though as it works fine for the first time after power removed to the MPPT and then refuses to go into absorption mode after that.

8

Your current is dropping off very quickly - I would be looking at the BMS software to see what is happening to the cell and battery voltage on each battery as you transition from Bulk to Absorb or Float

Ideally, if you can’t see the issue, run from one battery B1 until it floats, then change to B2 and see if it jumps to float, then B3 - i suspect that two of them will cause the MPPT to Absorb for a while, but one will make it jump to Float, in which case you have your suspect

9

I appreciate your advice. That might be beyond my abilities though. The 7 Genz batteries all have internal BMS monitoring and are standalone batteries with no comms option. Given both MPPT chargers go to float after being reset each time, but only the one time, do you think the issue could be within the batteries?

10

Good point.
Going to float the first time, but not again, does point to more of a GX issue.
I’ll think some more …

11

You might try setting absorption time to 8h in the mppt’s, and i set float to 56.6, that still leaves me at 100%.

That’s whay i am sure my batteries are at 100% when the sun goes down.

12

Thanks. Its a strange one. Especially when the issue only presented itself when the second MPPT was added to the existing system.

13

This sounds like the same problem I’m having, that I posted about in the thread below.

My only fix so far has been to revert back to v3.13.

14

Set the tail current in the mppt to 2x of the shunt.