question

Tom Ranson avatar image
Tom Ranson asked

LiFePo4 batteries not reaching full charge voltage, but are still fully charged

Hi all,


Grid parallel ESS

24V system

SmartShunt 500A

2x MPPT 100/30

Multiplus ii

Cerbo Gx

4x UltraMax LiFePo4 102Ah 24V "drop in" batteries with internal BMS in parallel in a busbar configuration (identical cable lengths)


I have 4x UltraMax LiFePo4 102Ah 24V "drop in" batteries with internal BMS (not connected/BMS compatible with Victron). These are in a parallel configuration for 408Ah. The system appears to be working well, with the batteries charging to 100% SoC according to the SmartShunt, and this 100% charge being reflected on all of the batteries internal BMS's (Bluetooth enabled) also. So the Victron SoC tallies up with the internal BMS SoC perfectly. This is good.


However, these batteries call for a 28.8V bulk/absorption charge voltage, yet they are reaching 100% charge according to their internal BMS's (and the SmartShunt) at a much lower voltage, around 27.6V. The MPPT's and MultiPlus ii are configured for bulk/absorption 28.8V and float 27.6V. I have noticed that the chargers are only ever in bulk mode, and never seen to transition to absorption or float, either by charging from the MultiPlus ii or the MPPT's.


Please find below attached screenshots of my MPPT's charger settings, MP ii settings and SmartShunt settings and also the specification sheets for the batteries.


MPPT charger settings page 1 of 2 Screenshot_2023-02-16-18-41-00-70_30b6efbd53acd6f273baafa7ca03da38.jpg

MPPT charger settings page 2 of 2 Screenshot_2023-02-16-18-41-05-60_30b6efbd53acd6f273baafa7ca03da38.jpg

SmartShunt settings Screenshot_2023-02-17-19-12-10-73_30b6efbd53acd6f273baafa7ca03da38.jpg

MultiPlus ii settings General tab MultiPlus ii General tab.png

MultiPlus ii settings Battery tab MultiPlus ii Charger tab.png

Battery specification sheet 1 of 2 Screenshot_2023-02-16-18-46-49-50_260528048de7f2f358f0056f785be619.jpg

Battery specification sheet 2 of 2 Screenshot_2023-02-16-18-46-55-72_260528048de7f2f358f0056f785be619.jpg

Battery manufacturer recommended charging parameters Screenshot_2023-02-16-18-47-17-72_260528048de7f2f358f0056f785be619.jpg


Please can someone help explain why this is happening, and if I should be concerned?


Many thanks.

battery chargingLithium Battery
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screenshot-2023-02-17-19-12-10-73-30b6efbd53acd6f2.jpg (339.0 KiB)
multiplus-ii-general-tab.png (156.3 KiB)
multiplus-ii-charger-tab.png (107.8 KiB)
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klim8skeptic avatar image klim8skeptic ♦ commented ·

@Tom Ranson you forgot to post screenshots of your SmartShunt settings, and multi charge settings..

Victron has a really useful BMV Youtube tutorial. to help set up a BMV / SmartShunt.

I have noticed that the chargers are only ever in bulk mode, and never seen to transition to absorption or float, either by charging from the MultiPlus ii or the MPPT's.

The only time you can consider your Li batts fully charged is then they are held at the bulk / absorp voltage, and the charge current drops to 2 - 4% of the Ah capacity of your batt bank. That's 8 - 16a for your battery bank.

Your batt manufacturer has fairly wide range of charging voltages. 28 - 29.2v for bulk / absorp. And fairly miserly "absorp time settings. (this is where cell balancing occurs).

28.8v for bulk / absorp and 13.6v for float seems ok.

Aborp time really depends on how often the batteries get fully charged. Cells need balancing.

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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic many thanks for the advice.


So, even though my batteries open BMS's are showing 100%, they're not reaching the point of being able to cell balance properly? Why would a BMS tell me they're 100% when they're not actually? What's the harm to be done here by then not reaching balancing voltage?


I will respond with SmartShunt and MultiPlus screenshots later when home.

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Alexandra avatar image Alexandra ♦ Tom Ranson commented ·
Do the batteries stop taking in amps at the 27.6v? It is possible 100% SOC on both monitors are triggering early.

The datasheet shows to charge to 28.8v then have a of 0.02c or 2A per battery for balancing/absorp.

The chargers wont move out of bulk until you reach the target voltage which as you say you are not.

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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic I've editied the original post above to include the MP ii and SmartShunt settings screenshots also. I look forward to hearing your thoughts.
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klim8skeptic answered ·

@Tom Ranson Your battery bank has not reached absorp voltage and is still taking current? It's not fully charged yet, give it more time on charge.


Have a look the the Volt / charge plot of your battery. Only at the very end of the charge cycle does the voltage rise rapidly. Once the absorp voltage has been reached, the current required the keep the voltage at this level plummets. At this stage your battery is probably at better than 98% Soc. A bit of time at the absorp voltage finishes topping the battery up, and gives the cell balancers time to work.

battery-soc.png

You didn't watch the bmv video? Set the charged voltage as 28.5v in the SmartShunt for now.

The battery monitor in the Multi is not good if you have DC charging sources. Un-tick "enable battery monitor". Put your effort into the SmartShunt for good Soc readings.

Did you fully charge the batteries before paralleling them?


battery-soc.png (269.7 KiB)
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Tom Ranson avatar image Tom Ranson commented ·

@klim8skeptic thank you, I see the graph and understand what you are saying about it.


So yes it appears as they are still taking full current at the end of an overnight charge cycle, they are not fully charged because they're not reaching approx 28.8V. Last night they reached approx 27.4, and the SmartShunt claimed 98% SoC. I wasn't able to check the individual BMS SoC's via Bluetooth as it was overnight, however given the behaviour I have already witnessed, I would hazard that they were also reading approx 98% SoC, as they appear to track quite accurately with the SmartShunt SoC.






screenshot-2023-02-18-10-14-23-45-bc2aea2f1887607a.jpg


I did watch the BMV video before writing this post, however I still found a few bits ambiguous, hence my settings probably aren't perfect. I will set the Shunt charged voltage to 28.5V as you suggest and disable the MP battery monitor. Yes I did individually charge the batteries before paralleling, until their internal BMS's read 100% SoC and all were at 27.7V resting (after several hours idle). However it seems they may not have been fully charged?

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Tom Ranson avatar image Tom Ranson commented ·

@klim8skeptic it's looking much better now. The MP ii has completed bulk and moved to absorption, sitting at 28.8V nicely and now with very low charging amps.

Is my Shunt tail current config correct at 4%?

Do I need tail current configured on the MPPTs?

Can you observe anything else I should adjust? Many thanks.

screenshot-2023-02-18-14-01-21-88-bc2aea2f1887607a.jpg

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klim8skeptic avatar image klim8skeptic ♦ Tom Ranson commented ·

The MP ii has completed bulk and moved to absorption, sitting at 28.8V nicely and now with very low charging amps.

Much better. Notice the voltage wriggling a bit at absorp voltage? I suspect that some of the batteries BMS's have disconnected their batt due to cells going overvolt. What does the BMS app show?

If that ^ is the case, the solution is to have the batts spend more time at the absorp voltage, that will give the BMS time to balance the offending cells.

Regarding time spent at absorp voltage. If your batts get fully charged a couple of times a week, absorp times can be shorter. If you only fully charge your batts a couple of times a month, absorp times should be longer.

Is my Shunt tail current config correct at 4%?

Leave it at 4% for now. The SmartShunt tail current is 1 of the values needed for the SmartShunt to auto sync the Soc to 100%. Not going to affect how the batts get charged.

Do I need tail current configured on the MPPTs?

More of a lead acid thing. Li need fixed time at aborp voltage.

Have you checked that your batts are equally sharing the charge / discharge currents? A semi decent clamp meter would be useful.


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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic yes I see the wriggling. I'll take a closer look at the battery BMS's next time an absorbtion charge is running. Unfortunately the BMS app doesn't have an event history, only detailing what's happening in real time.


I think it's likely that I will achieve a full charge once per week (over winter), as I can achieve that with an extended scheduled charge on one day. The present charging schedule is for a 4 hour window each night when we benefit from cheap-rate electricity. In the sunnier months the batteries will likely reach a full charge every day by my rough solar production calculations.


What sort of duration should my absorbtion phase be do you think?


Noted on the Shunt tail current.


Yes I have a decent clamp meter and have verified that the batteries are sharing load/ charge evenly and proportionally.

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klim8skeptic avatar image klim8skeptic ♦ Tom Ranson commented ·

@Tom Ranson What sort of duration should my absorbtion phase be do you think?

I think observation comes into this.

You need to hold the batt at absorp voltage until the tail current falls to the desired level. Then add the necessary number of minutes required by the SmartShunt "Charged Detection Time" for the Soc to auto sync to 100%.

Your batt manufacturer recommends 6 mins per charge. That's probably ok if you fully charge the batt every day.

If you fully charge the batt every 7 days, maybe 42 mins might be ok.

If you only charge your batts once a month, maybe 180 mins would be appropriate?

Remember that the time spent in absorp is mainly to give the BMS time to balance the cells.


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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic ok all understood. I think I'll leave the settings as they are for the moment and continue to monitor, as I need to see how effectively my nightly 4 hour charge is vs. the average daily discharge.


Did you note and other settings that I should be adjusting?

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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic and I think the minimum absorbtion time on the MP is 1 hour? Why is this not more granular like the MPPT's where I can set by the minute? Is this a concern?
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klim8skeptic avatar image klim8skeptic ♦ Tom Ranson commented ·

@Tom Ranson Did you note and other settings that I should be adjusting?

I would be setting the float voltage to the lower value of 13.6v, as shown in the big table.

I think the minimum absorbtion time on the MP is 1 hour?

An hour is probably an appropriate amount of time if you are only going to charge once a week, or when required after a discharge.

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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic sorry, where should I be seeing the float voltage of 27.2V (13.6)? I thought it was referenced on the manufacturer recommended charge settings as 27.6 +/- 0.2V.
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klim8skeptic avatar image klim8skeptic ♦ Tom Ranson commented ·

@Tom Ranson Your right, the 12v batt shows 13.6, the 24v batt shows 13.7v and the 48v batt as 13.75v as lower float settings.

float.png

Might be worth checking with Ultramax if 27.2v is ok.

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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic ok noted. What's the advantage of lowering the float voltage? Just to be more gentle on the cells?
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klim8skeptic avatar image klim8skeptic ♦ Tom Ranson commented ·
@Tom Ranson Yes. Not every last Ah has to be squeezed into the batt.
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Tom Ranson avatar image Tom Ranson klim8skeptic ♦ commented ·
@klim8skeptic Every day is a school day for me :-)


I'll set it to 27.4 for now as that's the documented lower limit, but I'll check with the manufacturer also.


Many thanks as always for your excellent help and advice.

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Tom Ranson avatar image Tom Ranson Tom Ranson commented ·

@klim8skeptic I'm seeing the shunt reporting 20% SoC, but the battery BMS's are all reporting 35%. Why am I seeing this discrepancy? I thought the shunt just counted consumed Ah, so they should marry up? At the moment I'm hitting 20% cut off on the Victron system when the batteries still have plenty of charge in them.

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