Multi RS doesn't stop charging LiFePo4 batteries at SOC=100%

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libor asked Jun 15, '24

Multi RS doesn't stop charging LiFePo4 batteries at SOC=100%

Hello Victron community,

I am using BMS mode (hopefully only until another set of CAN terminators arrive) and charging doesn't stop at 100%.

First, I set everything according to the manufacturer's documentation; as meaningfully as possible. I.e. "Bulk Voltage" is nowhere to be found in the Multi RS. It's mentioned in Multi RS documentation as a description of charge mode, in description of built-in BMS, but I don't see anything about setting it.

I may have confused the BMS management by initially entering 9600 Ah (which is KWh) instead of 200 Ah. Of course I closely monitored charging and stopped it by turning off solar switch, when it did not stop upon reaching 100% SOC on the battery (higher SOC on the two batteries, to be exact). The SOC indicator in VC was evidently off by a big margin. This was just a brief state, since the batteries were nearly full, when I turned Multi RS on.

I found the mistake and set 200 Ah, but it did not change anything. I consulted documentation and used Settings -> Battery -> Expert Mode -> "Sychnronize" button for "Synchronize SOC to 100%" (when 2nd battery charged to 100%) and after that the SOC on VC perfectly matched what the battery shows (in range circa 68-100%, that I could test Today).

However, the charging still didn't stop at 100%. I didn't try to let it charge beyond 100%, so I'm not sure if "synchronizing" to lower value would help, or if it is because I missed some setting, or if I misunderstood the manual about built-in BMS capabilities with LiFePo4 batteries.

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Bellow are VC settings I am using and Manufacturer's recommended settings.

Battery charging range:

It doesn't seem to be accurate for two parallel batteries, though.

Voltages:

Bulk = 54.0 V
Absorbtion = 53.4 V
Float = 51.7 V

This is recommended specifically for Victron Energy invertors:

• DC input low shut down: 37,5V

• DC input low restart: 46,5V

• DC input low pre-alarm: 43,5V

• Shutdown Voltage:

--> 0.005C 47,4V

--> 0.25C 48,2V

--> 0.7C 48,3V

• Sustain Voltage: 38,0V

However, those such (exact) settings aren't present in Victron Connect, I think. And they seem to be all related to the low state of charge.

battery charging SOC Multi RS

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vc2.png (15.3 KiB)

voltage-lifepo4-battery.png (27.1 KiB)

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Answer 1 · 2024-06-17T18:35:47Z

libor answered · Jun 17 at 06:35 PM

It does work, probably as intended, but I had to do several adjustments (using Synchronize button) so it switches to Float just bellow 100% and last iteration I'm testing is with 95% on battery 1 and 96.5% on battery 2. Still, it seems the batteries get still charged on Float at least some time, so I wonder if that means I should reduce the float voltage by a tiny little bit...

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Answer 2 · 2024-06-18T02:21:23Z

kevgermany answered · Jun 18 at 02:21 AM

Bulk charging is current limited. Output Voltage of the charger stays below absorption voltage setting.

Once absorption voltage is reached charger output voltage doesn't go higher. And charger switches to absorption mode.

Your table for the batteries shows much lower charging voltages than you are setting as absorption. This will cause the BMS to cut charging to prevent overcharging/damage. It looks as if your batteries are 15S, not 16S. Your settings are more appropriate for 16S batteries.

Batteries should be at the same state of charge before being connected. The BMS may correct this over time, but much better to completely charge them individually before going further.

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libor commented · Jun 19 at 05:21 AM

Thank you for your insight.

Very good point about 16s. I'll have to focus how many cells I have. I remember the distributor's support said there was a switch in the production line (I believe from 15s to 16s) and that I have the newer (16s) batteries. So I wonder, if they have old manual on the product's web site, or what.

As for difference in SOC, the batteries are not the same age, one is about twice as old. Before I purchased the 2nd, I consulted it with the tech support and they told me it's OK, that the BMSes will handle different charging profiles of the batteries. I guess it's true, but the difference between them is 5-10% ocassionally getting to nearly 0 or as much as 15%. I am a bit suspicious about the grade and quality of the cells, but I bought the solar set in attack-on-Ukraine hype, so there weren't many choices being in stock...

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kevgermany ♦♦ libor commented · Jun 19 at 04:28 PM

If these are Pylontechs, make sure the newest battery is master.

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libor kevgermany ♦♦ commented · Jun 19 at 05:07 PM

They're SunStone and I have the older as "1" and newer as "2". I'll switch it over, I think I understand your point about having the "stronger" battery as "1". I also set AC input assist to quite high SOC to accommodate for weaker battery draining faster.

I've never seen a documentation, that would explain how multiple batteries are handled and as for Victron, I think I haven't seen any schematics for such case. Which is strange, I'd expect this would be one of the most common use case.

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Answer 3 · 2024-06-19T17:15:52Z

Alexandra answered · Jun 19 at 05:15 PM

@Libor

Out of curiosity how did you determine 54v for bulk? The table shows 51.89v for 100% SOC?

The battery is full the inverter doesn't know that as its target voltage is much higher. So it isn't stopping at battery 100% since according to the settings it hasn't reached the target voltage you set.

The voltage for charge shouldn't / wont change no matter how many batteries you have in parrallel. If charged is 51.89v then that is is for al batteries in the bank.

Voltages suggest a 15s battery.

Sustain should be much higher, closer to 48v.

Your low cut off is 47.5v (0%) under load.

Leave equalisation at absorption/bulk voltage.

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libor commented · Jun 19 at 06:07 PM

I did not determine bulk voltage, just copy & pasted what the manual says. The battery has a printed table in the front, that also shows "Max. charging battery voltage: 54V". I was always puzzled about that, but since I haven't found a settings for bulk voltage anywhere in Victron Connect for Multi RS or the manual, I didn't have to take any decision about reducing it to 100%.

As for low cutoff, I was also worried about that and set AC to kick in at an SOC a high number, 35%. The batteries don't get emptied bellow 50% now anyway (during summer).

I verified in the BMS UI, that the batteries are in fact 15 cell packs.

Today 2nd set of terminators came and I've just put one into the BMS-can and reconnected the BMS data cable to the 2nd BMS-can port.

The Network details show no lost or dropped packets but I think "ERROR-ACTIVE" means it's not working:

I couldn't find any documentation (so far) for this screen, so I can only guess it's purpose.

On the battery side, the documentation says it's capable of multiple protocols (including CAN), so I wonder if I have to switch something.

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