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Using Pylontech US3000 batteries efficiently


Pren
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I have two US3000 Pylontechs (3.5kw each).  My Deye 8kw inverter was default programmed to run down to 80% DOD then moves to grid power.  I get maybe 2/3 hours of battery use after sunset before switching to grid.  Questions:

- What DoD can I safely set the system to without damaging the Pylontech batteries - would like it to last 10 years!

- Lowest DOD that is recommended for these batteries?

- The Deye BMS is configured for Pylon Li batteries - does it automatically choose the rate of charge at the optimum level - the inverter settings only ask for the max charging amps

 

 

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11 minutes ago, Pren said:

Lowest DOD that is recommended for these batteries

I would say never go below 20% for LiFePO4

But keep it above that if you can. The less you test the limits the longer the battery will last. It's like brake disks: They can last a long time if you are an easy driver, or fail fast if you are a race driver. :) 

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Yes, the alarm is set to scream if it goes below 20%, but this only happens when no grid power.  On a normal evening, it goes to grid at 80% DOD which is probably wasting the very expensive battery capacity and potential.  Correct?

 

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2 hours ago, Pren said:

The Deye BMS is configured for Pylon Li batteries - does it automatically choose the rate of charge at the optimum level - the inverter settings only ask for the max charging amps

Not sure what settings the Deye automatically chooses, but I would recommend checking against / adjusting to the settings recommended here

2 hours ago, Pren said:

On a normal evening, it goes to grid at 80% DOD which is probably wasting the very expensive battery capacity and potential.  Correct?

Agreed. I am running my Pylons down to around 23% every night

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3 hours ago, Pren said:

it goes to grid at 80% DOD which is probably wasting the very expensive battery capacity and potential.  Correct?

Are you sure that you are not confusing DoD with the SoC?

80%DoD means that there's roughly just 20% of energy left in the battery. Such a deep everyday cycling cannot be called "wasting of a potential" as consuming that last remaining 1.4kWh won't make any change but it will kill the batteries for sure.

In the US3000 BMS the 9%SoC is hardcoded as a lower limit. Once you discharge US3000 to this limit it will auto-shutdown and log an error to its NVRAM.

Edited by Youda
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7 hours ago, Youda said:

Are you sure that you are not confusing DoD with the SoC?

YES, my bad.  SoC is at 80% when the inverter switches to grid.

Guess what I am trying to understand is what is a level to take SoC down to and keep battery life to ten years +.  Someone mentioned that if I draw power to SoC of 50% then the life of the battery drops exponentially!  

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Personally, I'm cycling my US3000 stack between 100% SoC and 40%SoC. Once discharged to 40% SoC, my system automatically switches to the grid. Since manufacturer's specs allow cycling between 100% and 10%, I have a lot of margin here.

BTW:
Technically, it would be better to cycle these between 40% SoC and 90% SoC daily, while allowing a full charge (100%SoC) only once a week, to let BMS reset amphours counter and balance the cells. But as far as I know, no inverter on the market has ability to set such a complex rule.

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11 hours ago, Pren said:

I have two US3000 Pylontechs (3.5kw each).  My Deye 8kw inverter was default programmed to run down to 80% DOD then moves to grid power.  I get maybe 2/3 hours of battery use after sunset before switching to grid.  Qs

What sort of loads are you running to have such a short period out of two Pylons? My single pylon does 2-3hrs. But loads are under 1kw constant. 

My pylon gets discharged to roughly 35% SoC as I only start discharging them from 5am when I switch from grid to Solar / Battery. But 7:30 the solar has picked up enough to carry the loads and the battery is at 35%  

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1 minute ago, Youda said:

@Rclegg

just check the whole thread you will notice the reason for that short period:
@Pren mismatched DoD and SoC in his description. In reality, he's using just 1.4kWh out of 2xUS3000 and then he's switching to grid. Basically treating 7kWh of lithium like 7kWh of lead-acid :lol:

Ooooooh yeah okay that makes more sense! 😅

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59 minutes ago, Youda said:

@Rclegg

just check the whole thread you will notice the reason for that short period:
@Pren mismatched DoD and SoC in his description. In reality, he's using just 1.4kWh out of 2xUS3000 and then he's switching to grid. Basically treating 7kWh of lithium like 7kWh of lead-acid :lol:

Too many acronyms for a noobie....

Going to reset the SoC limit to 50% before going to grid today... that should balance using the batteries more efficiently, allowing some leeway if there is power outage, and preserving the battery life!

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1 hour ago, Pren said:

Going to reset the SoC limit to 50% before going to grid today... that should balance using the batteries more efficiently, allowing some leeway if there is power outage, and preserving the battery life!

I have a 4kWh battery. My SOC limit is set at 40% and when we had load shedding the other day it went down to 30% using about 400W during that time. If you don't have mega loads (resistive loads) then 50% might be too conservative still for a 7kWh battery. 

But look at your loads that you plan to use and check the values during the next loadshedding and then adjust accordingly for your setup.

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What I would like to see is the option to limit charging to 90% of capacity..

The DoD one can manage, its the charging that is the problem.

3 hours ago, Youda said:

BTW:
Technically, it would be better to cycle these between 40% SoC and 90% SoC daily, while allowing a full charge (100%SoC) only once a week, to let BMS reset amphours counter and balance the cells. But as far as I know, no inverter on the market has ability to set such a complex rule.

 

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3 hours ago, Youda said:

Technically, it would be better to cycle these between 40% SoC and 90% SoC daily, while allowing a full charge (100%SoC) only once a week, to let BMS reset amphours counter and balance the cells. But as far as I know, no inverter on the market has ability to set such a complex rule.

Well I guess an option would be to have a big bank that won't be able to charge from 20% SoC to 100% SoC with the PV available in which case Victron's BatteryLife (if enabled) will kick in. The way I understand it (pretty much pasting from there) is that it will raise the Low SOC limit by 5% each day until it reaches a 85% SoC. It will then stay on that level, but if it reaches 95% then it will lower the Low SOC limit by 5% again.

It's not exactly the "ensure 100% once a week" state, but it ensures that it reaches 85% to 100% each day.

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1 hour ago, FixAMess said:

What I would like to see is the option to limit charging to 90% of capacity..

The DoD one can manage, its the charging that is the problem.

 

@FixAMess the Deye seems to hover around 99% SoC most of the time, going to 100% but not staying there for long.  wonder if this helps??

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ck the 3 dots on top right and select device details, then go to inverter and stats. 

12 minutes ago, Vassen said:

anyway, both apps should give you the soc info. Have you set the grid type to storage system 

 

Have set it to storage and see the batteries now - thanks this was very useful

Do you know how to reduce the SoC limit on the Deye - not obvious on the menus I went through

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17 minutes ago, Vassen said:

Not sure what the deye interface looks like but on the sunsynk I have the following where I can Choose how much to charge batteries from either generator input or grid. 
 

but as I said, excess solar will still charge batteries to 100%. 

 

Deye has a similar page setup except it just shows "Batt" and not SoC %

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20 hours ago, FixAMess said:

What I would like to see is the option to limit charging to 90% of capacity..

The DoD one can manage, its the charging that is the problem.

 

Would it not be possible to just check what voltage your battery has at 90% SoC and then just set that as the voltage to which it should be charged?

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This is not my understanding of how a Victron would charge a battery. I am able to set the voltage to which it needs charge the battery and the Venus effectively overrides what the BMS wants to do.

For example, I am only charging my Pylons to 52V at the moment. I understand this is enough for the balancers to kick in and get reasonably close to 100% SoC.

The only problem I think there is to charge to less than 100% SoC is because you run the risk of not getting the cell balancers to activate (which is a big problem if you intend on never taking it to 100%, because my understanding is that the cells would become unbalanced over time).

Edited by jykenmynie
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12 hours ago, Vassen said:

Can you post a picture of what it looks like. 
 

im pretty sure it should be configured the same way. Sunsynk has been releasing lots of updates recently and with some of the updates, they change the descriptions from  bad chinglish to not-so-bad chinglish.  😂

 

deye.jpg

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2 hours ago, jykenmynie said:

Would it not be possible to just check what voltage your battery has at 90% SoC and then just set that as the voltage to which it should be charged?

For a shame, it's not that simple.

The SoC/Voltage curve of the LiFePO4 batteries is so flat that there's almost the same voltage anywhere in the range of 20-80% SoC.
Even in the 80-100% range you cannot say what's the SoC based on the voltage. It's like behaving like 80%...80%... and now suddenly 99% and 100%.
 

Technically, the feasible way is when the inverter actively talks to the battery BMS and once the BMS reports 90% SOC the inverter should stop charging it. But, like I said before, I don't know any inverter that is able to use such a rule.

For the high-voltage batteries (200-400V) the common practice is that the inverter charges to 90%, then stops charging. For example SolaX X3 Hybrid operates like that. But in the 48V world, if you want to have this feature, you have to implement your own monitoring solution that will communicate with the battery via CAN or RS485 and then instruct the inverter to start/stop charging. It's doable even in my current homebrew monitoring, but I'm too lazy to code it...

Edited by Youda
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5 minutes ago, Youda said:

For a shame, it's not that simple.

The SoC/Voltage curve of the LiFePO4 batteries is so flat that there's almost the same voltage anywhere in the range of 20-80% SoC.
Even in the 80-100% range you cannot say what's the SoC based on the voltage. It's like behaving like 80%...80%... and now suddenly 99% and 100%.
 

Technically, the feasible way is when the inverter actively talks to the battery BMS and once the BMS reports 90% SOC the inverter should stop charging it. But, like I said before, I don't know any inverter that is able to use such a rule.

For the high-voltage batteries (200-400V) the common practice is that the inverter charges to 90%, then stops charging. For example SolaX X3 Hybrid operates like that. But in the 48V world, if you want to have this feature, you have to implement your own monitoring solution that will communicate with the battery via CAN or RS485 and then instruct the inverter to start/stop charging. It's doable even in my current homebrew monitoring, but I'm too lazy to code it...

So wouldn’t the same voltage relate to the same SoC always? Even if the voltage curve is quite flat, after you have established what voltage relates to 90%, can’t you use that? You can establish it with communication with the BMS originally, and then only charge to that voltage from that point onwards. Only problem I see is sell balancing?

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Trust me, it's not possible for LiFePo4. With some other Liion yes, but not here.
The only way to determine the correct SoC is counting in/out amphours. So Victron BMV700, shunt, coloumbmeter. And since BMS already has this circuit implemented, the most obvious way is to communicate with the BMS.

But don't worry. I would say that within 5yrs this feature (custom top-bottom charge/discharge based on the true SoC and BMS communication) will make it to the inverters ;)
 

In the meantime, you have to code your own middleware, if you want this.

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