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bcowley avatar image
bcowley asked

Turn off AC input to cycle Pylontech batteries in UPS role

We have recently setup a MultiPlus II, with 2 Pylontech US3000C batteries, plus Cerbo GX, for backup power at our (remote) community radio station. We understand the batteries should not be left 100% charged, but partially discharged maybe twice per week. The Multi uses the Pylon BMS and DVCC mode is enabled.

So far I've set up root access to the Cerbo. Can a script write to a register in order to control the AC Input relay? Please let us know the register details to achieve this, or if there is a better method.

Thanks.


Pylontech
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Alexandra avatar image Alexandra ♦ commented ·

@bcowley

I have a tack on question, where does the understanding of not keeping the Pylontec at 100% SOC come from?

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bcowley avatar image bcowley Alexandra ♦ commented ·

Thanks for that question! The company that sold us this equipment said we should partially discharge these batteries on a regular basis. However there's relatively little on the internet about optimum use of LiFePo4 batteries in a UPS application. The VRM portal shows that the BMS is doing minimal cycling - but the SOC is always reported as 100%. Maybe it's enough?

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Alexandra avatar image Alexandra ♦ bcowley commented ·
On all my keep charged systems (they have ess and solar though) go up to 52v in bulk (during solar hours) back down to float every day. So technically not staying at the same voltage or full. So the SOC hovers between 99 and 100%.

Not heard of Pylontec not being kept at 100 to be honest. And their requested CVL is 53v so really they are not being taken up to the full full mark.


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bcowley avatar image bcowley Alexandra ♦ commented ·
I'm a newbie in this area so please forgive any dumb questions. At present our cell voltages range from 3.324 to 3.327 Volts. Looking at the battery voltage and current plots over time, the voltage cycles a couple of times per day from about 52.4 to 49.9 Volts, with small amounts of current injected to keep them topped up. Maybe the BMS is smart enough to know it's operating in a UPS capacity, so it can keep the cell voltages a little lower? I guess you don't think it necessary to periodically discharge, say to 70% SOC? Thanks!
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hominidae avatar image hominidae bcowley commented ·

I don't think so. They are designed for a full cycle on a daily basis, with an ESS scenario.

I have seen appliances with Lithium based batteries, that do not start charging immediately when supply gets connected, but rather keep a certain level below 100% - so these appear to have a smarter BMS (premium line of laptops, for example) but also their day-to-day use case is very different.

To be honest, for a UPS use case, Lead-Acid based batteries would have been a better choice in terms of how much care these would need.

What you should at least do, give them some full cycles, so the BMS can learn the lower and upper balance and also for the master BMS to balance the complete stack of modules. For SoH, I'd consider to charge only to 50-60% and for maintaining SoC measurement correctly do a full cycle at least once every 2 weeks or so.

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bcowley avatar image bcowley hominidae commented ·
Thanks for your comments on the need for cycling and also for your suggested answer below. Re using lead acid batteries in UPS mode: agreed, but we want to add PV + more batteries in future, in which case this should not be a problem. Re your first suggested approach, we aren't allowed grid export. Re NodeRed: possibly but (as mentioned in original posting) I was assuming one could simply write to a register that controlled the Multi AC Input relay.
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hominidae avatar image hominidae Alexandra ♦ commented ·
Based on the technical parameters, I do agree. But since you do have hands on experience, may I ask if you could spare some more details?

Like how long are these system in deployment/production and did you measure the health of the batteries on a regular basis, like every 6-12 month (IMHO the SOH reported by the BMS is not to be trusted).

I am not a technician in that field but from my real world experience with EVs over the years, SOH drops faster when users are tying to keep the batteries at full charge instead of topping them up only before driving a longer leg.

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nickdb avatar image nickdb ♦♦ hominidae commented ·
How an EV manufacturer architects their packs and chooses how they are charged/discharged is somewhat different to a pack designed for long life in an ESS system.

The commercial packs have generally been tuned/derated to perform optimally and to give you your warranty.

100% SOC is a relative measurement of what the bms has been set to use.

If you built your own battery with cheap cells bought online, they often have used an unrealistic charge/discharge cycle to determine their capacity, charging higher and discharging lower than any supported battery you can buy.

In this case they state that caveats apply to the cells use if you want them to last.

It really isn't necessary to worry about this, nor to draw a comparison to EV's.

If your 10 year+ warranty needed this it would be stated in the ops manual, and it isn't.




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hominidae avatar image hominidae bcowley commented ·
I also heard this, not only for pylontech but for all lithium based batteries, including the manufacturers of my EVs, suppliers of PowerBanks, etcpp.


Behind that, I think, is a general communicated knowledge, that batteries based on lithium technology do have an ageing process, where parameters like temperature and SoC have an influence. They age faster when SoC is high as well as when when temperature is high.Both parameters combined have a higher effect on age/detoriation.


Pylontech battery packs are, when being fresh from the shelf, not fully charged but come with an SoC around 50%. Which can be considered the sweet spot for storage mode, can't it?

On the other side, these batteries need to be fully charged on a regular basis, in order for the BMS to balance the individual cells in the package and determine the SoC correctly.


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Alexandra avatar image Alexandra ♦ hominidae commented ·

[Edited: sorry to anyone who read this earlier, grammer and spelling was utter garbage]

There is a law about how much energy can be shipped. So the SOC when shipped is governed by that.


Pylontec in their manual recommend 90% charge when putting the battery into storage (so not in use on a system), and every 6 months or so recharged. It is in their manual.

When you have a systen with absorption and float voltages, then the rebulk off set and also the re-absorption intervals. The battery is never kept at one point anyway.

So it is never truly kept on a system at a constant voltage. In theory also not at the same SOC continuously.

So far the systems I have been personally involved with are about 4 years old. Still at 100% SOHealth.

Some are cycled heavy some kept as backups almost never used.

The only time I have seen damaged pylons is on under-batteried systems, and systems who weren't programmed correctly and were overcharging in voltage settings. Stick to the 52v 51v.

I guess you could try float lower to increase how it cycles, but if you use the comms you will find the battery does its own thing anyway.

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2 Answers
hominidae avatar image
hominidae answered ·

..if you are running this setup in ESS mode, you could simply force a discharge of the batteries by setting the Grid-Point to a negative value, resulting in a Grid-export (if allowed by your network operator).

If you want to do this programmatically, I suggest to look into Node-Red. It is part of the VenusOS-Large Image and can be installed on the Cerbo.

You can as well run NodeRed on a separate RPi or in a Container/VM on a NAS, in your network...also nice for testing before changing a production system.

There are separate, special Nodes available for integrating with a Victron System, but it can be done with just using MQTT as well (enable Modbus and MQTT Services on your Cerbo).


Have a look here: https://www.victronenergy.com/live/venus-os:large

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wkirby avatar image
wkirby answered ·

If you are not using any assistants then you could use this method:
https://www.victronenergy.com/live/drafts:acin_ignore_timer_using_ccgx_gen_start_stop_example

Then use register 806 or 807 to write to a CerboGX relay.

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