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kryten avatar image
kryten asked

Multiplus DC current drop-off when Pylontech batteries at 55%

Hi all,

I have 2 x Multiplus ii 48/5000/70 configured as master/slave connected to 8 x Pylontech US5000 batteries. Everything is working ok and I have no alarms but im interested to know if its normal to see a drop-off in AC & DC current when charging the batteries and they get to 55% SOC. I am charging them from the grid and up to 55% I get 140A DC charge (2 x 70A) so split across 8 batteries thats about 17.5A each. At 55% to 100% this drops to around 125A so 15A per battery. Is this normal? I do expect there to be a drop of in charge rate as the battery approaches 100% but expected this to be more between say 80 and 100%. Attached are some graphs showing the drop, interested to know if this is normal based on the Amps. Thanks!

screenshot-2022-11-16-at-130944.png

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Multiplus-IIPylontech
screenshot-2022-11-16-at-130944.png (362.3 KiB)
screenshot-2022-11-16-at-131012.png (365.5 KiB)
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Alexandra avatar image Alexandra ♦ commented ·

@Kryten

There are a couple of other factors at play as well.

They include battery temperature. Are they new? Inverter temperature as well. As cells charge their internal resistance changes as well. That can affect charge. If battery cables are getting warm their resistance changes as well, this can also impede current.

And also how the bank is wired affects the charge as well. While in theory for example 200 ÷ 8 is 25A each, if they are daisy chained it is not actually like that in reality.

The Pylontecs are controlling the system so is could be them doing it as well.

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kryten avatar image kryten Alexandra ♦ commented ·
Thanks for the info. Battery temp ranges from minimum 18 deg C to max 24 deg C. They are about 3 weeks old and they are cabled in pairs using the standard pylontech cables rated at 120A through DC breakers for overcurrent protection then connect to lynx DC where there are 70mm sq cable to the inverters with class T fuse. All cable lengths are exactly equal at all points. Cables don’t even feel warm. Inverters are in a garage that’s about 10 deg C so don’t think there overheating
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4 Answers
nickdb avatar image
nickdb answered ·

Have you charted the BMS's CVL limits? They may be limiting charge, you can also see this in the pylontech parameters tab on the GX, but easier to chart or do a custom widget that overlays all this data on one chart.

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kryten avatar image kryten commented ·

Just charted the Min/Max Cell voltage together with the current, voltage and state of charge. I have the CVL set to 53.2 and at 98% SOC it hits 52.25v. I have only ever had one over voltage alarm and that was the very first time I charged them to 100% Ever since its been fine. It's weird the drop in charge current seems to always be about 2 hours into a 4 hour charge period, so irrelevant of the SOC or battery voltage at the time always around 2 hours in it reduces the current. Would this be the Multiplus throttling back a bit say having run at max for two hours? Ambient temperature is 10deg C where they are so thought they could do 2 x 70A continuous. Have to say the battery charge graph is very linear even when the charge current drops off.

screenshot-2022-11-16-at-145034.png

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kryten avatar image kryten kryten commented ·

Just to add some background im just trying to make sure I am gettingt the maximum charge into the batteries in a 4 hour window which is my off-peak rate. Thanks

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bbmz avatar image bbmz kryten commented ·
Your screenshot (from VRM - BMS min/max - via BMS.CAN?) clearly shows the readon for your problem. The current reduction is caused by the pylontech BMS. A single cell should never reach more than 3.6 v and somewhere around 3.5 v it starts to slow diwn the charging. You have a severe imbalance and/or problem with one of your batteries. Are all batteries connected to each others and on the same SoC? Can you monitor the number/index of the „high-voltage“ cell?


Normally the pylontechs (at least the us2000) reduce the charge-current at 90% SoC gradually (with each SoC-% even more). The reduction via single-cell voltage should be never noticeable (and especially not with SoCs lower 95%).


At least from my point this seems not like a temperature issue of your MP-IIs.
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kryten avatar image
kryten answered ·

I think this is just the inverters throttling back due to heat. While the air temperature is only 10 deg C, I measured the temperature of the inverters after 1 hour of running at full charge capacity. temp at bottom of the inverters was around 18 deg C and at the top was 30 deg C so I'm guessing internally they will be hitting 30 deg C so perhaps derating the charge current slightly. Does this seem plausible? Thanks

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Alexandra avatar image Alexandra ♦ commented ·
@Kryten

Plausible. Yes.

Do the internal fan run?

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raymiller avatar image raymiller commented ·
If you download your spreadsheet data from VRM and have a look at what is going on would be the first step over your charge period. It does appear as if you have worked out that the inverters are reducing the power output. But at 10C air temp, I would expect your inverters to deliver 100% charge for days on end.

From a battery life perspective, a lower charge rate is always preferable, and the slower the better, yes you need to understand the limits of your system.

As a trial try reducing your battery charge limit by 10% and 20% on successive days to see if the inverters take longer to reach the current reduction threshold. This should prove your overheating theory.


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fish avatar image
fish answered ·

Thats the same as I was getting on my two multis.. I've now put a 15kva Quattro in and it will nail 200a all day long when set up. However I am getting the split in min max cell voltage which I think is possibly a cell which has slightly lower capacity. I'm getting a battery view cable and will be following up incase it is a bad cell.

Generally I think you are fine..


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Paul Beskeen avatar image
Paul Beskeen answered ·

Hi Kryten,

Did you ever find a solution to this issue? I have a very similar setup - 2x MPII 5000/70 and 6x US5000. I also see the drop off in charging current past the two hour / ~50% SoC mark - see attachment.

Once the SoC gets past 95%, then as you might reasonably expect, charging gets very slow. I never get beyond 97% in my 4 hour cheap rate window.

If I am down to 10-11% DoD, then it can be challenging to get back up to 97% within the charge window - so I would like to see a more sustained charge rate.

It's also somewhat annoying that the Pylontechs may be speced at 95% DoD, but I cannot get them beyond 90% DoD. If I set ESS Minimum SoC below 10% then it gets to 9% and then goes into an automatic sustain and starts to charge them again immediately.

Cheers, Paul.


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