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Trev

Lithium-Ion & Axpert Maximum Discharge Currents

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Hello All,

I am in the process of upgrading my battery bank from AGM to Lithium-Ion and when consulting the datasheets for each product I am mindful of the Maximum Discharge Current detailed by most of the Lithium-Ion batteries. For example, the Pylontech 2.4kWh battery (US2000) has a maximum discharge current of 50A (or 100A for 15-sec). This is in contrast to the Axpert MKII 5kVA Inverter which maximum discharge output at rated capacity of 5kVA is 104A. 

I was about to now state that according to the Axpert datasheet, the unit has the ability of withstanding 2 x rated capacity (i.e. 10kVA) for 10-min. I was quite certain of this feature as i recall it being one of the reasons for buying the unit in 2015 when I was comparing to other brands. But I have just downloaded an Axpert user manual to check myself and I see that it states  'Surge Capacity: 2x rated capacity for 5-sec' (a significant difference). Practically however, what I do know for certain which is a common daily occurrence is that the ladies in the household run the inverter up to 7-8kVA with some appliances (typically the kettle, microwave and too many lights etc) where the Axpert starts warning with beeps and a red flashing light. The observation is that it holds this out for the duration of the kettle being boiled which is typically 3-4 min. The inverter does not trip on overload protection so I'm thinking that my original specification of 2 x rated capacity for 10-min may have been valid unless i have bypassed this with my settings 🙈

Anyway, the question i wanted to ask about this common occurrence in my household with respect to the Lithium-Ion battery is:

Q1:  At 7-8kVA the Axpert would be drawing up to 167A for this period of time. How does this now work with the maximum discharge current specifications outlined by the Pylontech or any other battery for that matter?

This has been my biggest procrastination point that has stalled me from moving ahead with this and i would like to get your expert input to this matter please?

Please note that I have tried to browse and search for previous discussion on this topic. I do recollect a conversation on this in one post where someone (I recall either @Chris Hobson or @plonkster) was alerting everyone to the limited discharge current capacity of the Pylontech battery. Please excuse me if this is covered elsewhere. If it is, kindly point me to the informative post?

Getting back to my current set-up and now pondering over the details of the Axpert datasheet, the reason for upgrading my batteries is because my bank (9.6kWh: 2 x 4x100Ah Renesola 6GFMJ-100 - see attached for ease of reference) no longer has the capacity to hold-out for the design duration of 4-hrs at 50% DOD. It was doing so in its early life and held out for an entire 11-hrs to 100% DOD on 2-3 occasions. Now, when entering load shedding, my charge capacity drops drastically and any heavy loads such as a microwave or kettle causes it to shutdown. Reading previous posts I thought it may be due to imbalance and a damaged cell so I tested my batteries which all checked okay according to the guidance provided in those posts, but for good measure i purchased and installed two HA02 Battery Balancers from @ChrisB along with a Victron BMS-712 which shows me trends on the App of the drastic roll off in charge capacity (Note: i'm behind on installing the midpoint cable for whatever reason at the time of installation so cannot confirm the BMS warnings... I know, this defeats the reason for getting it in the first place... Rand out of time...)

Q2:   Could the frequent (daily) demand surge at for extended durations of 3-4 min at a time have prematurely damaged or aged my Renesola AGM batteries? 

Consulting the Renesola data sheet, I note that the 'Constant Current' table has a 10h rating of ~10.3A per cell which should give ~120A for the 2x (4x100Ah) set-up. However, I also note that this table shows capacity ratings down to 15-min where it holds out at higher currents of 128-166A per cell implying a massive capability of delivering very high currents far exceeding the cable capacity and hence the need for inline fuses or fast tripping DC breakers at 125A etc...

Q3:   Given the fact that Lead Acid / AGM / Gel  batteries are supplied without overload protection when compared with the Lithium-Ion packs that are supplied with all and sundry, can I deduce that unless you do not implement the necessary protection mechanisms, the hammering of my AGM batteries on a daily basis would have resulted in a general degeneration of the batteries faster than the specified 1000 cycles at 50% DOD? Hence the system is typically undersized for the connected loads to start with and it's probably advisable to parallel a second Axpert (which i have available) should i wish to maintain the current load profile?

This talks to my first question about the need for 'matching' the maximum discharge current of the battery bank and the inverter. Could the AGM's have quietly taken he abuse at the cost of a shortened life span given their ability to deliver on high currents? When switching to Lithium-Ion, can I anticipate that the batteries will trip if i breach the maximum discharge specifications given the built in circuit breaker? 

Thank you,

Trev

 

 

Renesola 6GFMJ-100 TDS v3.pdf

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I didn't read with too much attention, sorry... long day 🙂

Try to size the battery so that the capacity of the battery (in kWh) is the same as the peak of the inverter (on kW). So for a 5kW inverter, pair it with a 5kWh battery.

What this means is that at full power, the inverter will drain the battery in one hour, or in other words, you're highest discharge rate is 1C. LiFePO4 can handle 1C for several minutes, technically you can abuse them all the way to empty at that rate (but your warranty might be affected). If you size it this way, then you cannot really overdo the discharge, and for the average daytime loads (which will be less than half the inverter capacity for most people) you are then way under C/2, which is the Pylontech recommended max discharge current.

So for a 5kW inverter, you want at least two Pylontech modules (either US2500 or US3000).

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Posted (edited)

Hi @plonkster

Thanks for this, but my main concern is the ability for Lithium-Ion batteries to handle Surge Currents in excess of their rated Maximum Discharge Currents. So with 2x US2500's connected in parallel this would provide 5.1kWh delivering a maximum 112A (continuous) or  224A (15-sec). So what happens during short term over current situations drawing say 7kVA of power which should be pulling 145A from the batteries for 3-4 minutes? How do the Pylontech's and similar Lithium-Ion batteries handle this? Do they trip or just keep push current like my flogged AGM's?

Thanks,

Trev

Edited by Trev
Clarified the 'what-if scenario to include the time interval of 3-4 min at 145A'.

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2 hours ago, Trev said:

Q1:  At 7-8kVA the Axpert would be drawing up to 167A for this period of time. How does this now work with the maximum discharge current specifications outlined by the Pylontech or any other battery for that matter?

This has been my biggest procrastination point that has stalled me from moving ahead with this and i would like to get your expert input to this matter please?

I’m in a similar situation, I’ve been looking at upgrading from gel to pylontech’s but have a family who has no mercy for a battery bank. So i’m half thinking to just go gel again as the price also play a big role if one wants to get similar Amps from the pylontech’s as the gel.

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2 hours ago, Trev said:

Do they trip or just keep push current like my flogged AGM's?

If you overdo it, the BMS will disconnect the battery. But in my experience, they are quite forgiving. A bit of surge is not going to cause a disconnect. Several minutes might.

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With the Goodwe and pylontechs, the BMS determines the discharge rate so it will take what it wants.

I've seen 87A from my 3 x us3000's. Yes, it really grinds ones soul...Trouble is, if you try train your family and become a battery Nazi you're wife will eventually throw you and the inverter out.

The only solution is to get more batteries.

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4 minutes ago, FixAMess said:

I've seen 87A from my 3 x us3000's. Yes, it really grinds ones soul...

87A is still well within the comfort zone for 3x Pylon US3000. Each is fine for 37A, so 3x 37A = 111A

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Yes, the point is the BMS manages the discharge rate, so one needs to have the appropriate sized battery bank that matches the inverter power output.

I prefer to use/reduce the batteries SoC gradually, but constantly, so the SoC time curve is shallow so as not to stress the batteries. 

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32 minutes ago, FixAMess said:

Yes, the point is the BMS manages the discharge rate, so one needs to have the appropriate sized battery bank that matches the inverter power output.

I prefer to use/reduce the batteries SoC gradually, but constantly, so the SoC time curve is shallow so as not to stress the batteries. 

Looking at the data sheet it’s saying that Recommended is 37A but Max is 74A. So surely for the instance I’m explaining about 3x 74A = 222A which is even better! We’re talking about an event that happens 2-3 times per day for 3-4min. 

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48 minutes ago, Trev said:

Looking at the data sheet it’s saying that Recommended is 37A but Max is 74A. So surely for the instance I’m explaining about 3x 74A = 222A which is even better! We’re talking about an event that happens 2-3 times per day for 3-4min. 

Are you sure that your inverter is actually inverting 1.5x its rated capacity for 3-4 minutes? There have been people on the forum struggling just to get their inverters to do that for more than a few seconds. Couldn't it maybe be that your grid was on and it was just busy transferring for that time?

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Posted (edited)

This is the beauty of having an inverter that talks to a battery BMS. Like Goodwe and Pylontech, one does not need to set any parameters re charge / discharge, size etc. You just tell the inverter you have e.g. 3 X us3000 pylontechs and it managers everything.

On SEMS portal the max discharge rate is set to 111A, so I assume that the BMS will limit it to 111A and not 222A as per the max burst.

I'd be interested to know what percentage of all the posts on the forum have to do with battery settings and inverter compatibility issues and settings. There are some people who have to constantly over time make adjustments etc...Painfull!

Edited by FixAMess

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8 minutes ago, FixAMess said:

I'd be interested to know what percentage of all the posts on the forum have to do with battery settings and inverter compatibility issues and settings. There are some people who have to constantly over time make adjustments etc...Painfull!

That is why, for a layperson like myself, getting an inverter/control device that communicates with the BMS is so important. I'm sure those more knowledgeable can sort out issues with this manually, over time, I don't have time for that.

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