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diy solar

Cannot control SOC based on voltage

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gdb0701

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Sep 10, 2021
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Hello members...

I have a REVO III 8kw with a Sorotec 10kw Lithium battery. Between the inverter and battery I have a RS485 cable, which is working since I can
see the battery information on my inverter (voltage, charge/discharge, SOC, etc...)

When I set the inverter on Lithium, only float and bulk will be set automatically to 53.2V, the other parameters like cut-off, back to grid, back to battery mode, I still have to set manually... anyway, USE or LITHIUM doesnt matter, i kept it on Lithium...

current parameters:
set Low Vol Disconnect = 48V (back to grid)
set Shutdown Volume= 47.5V (cut-off)
set High Volume Reconnect= 49V (back to battery mode)

The problem I have is that I cannot control the SOC based on the battery Voltage...

For example: During the night I have a load of 800W, battery voltage of 48.9V, SOC 23 %, after some time (maybe 1 hour) I have a load of 300W, battery voltage 48.9V and SOC 22%, after some time ( discharging 5-6 hours later) using a load of 60W, the battery voltage is still 48.9V and SOC is 13%
(I once reached 48.7V and a soc of 4%... with a very low load of 40W)

With a heavy load, I can have a battery voltage of 48.9V and a SOC of 40% ... so when I set the parameter "set Low Vol Disconnect = 49V" then I would go back to grid at a SOC of 40%.....

how can I ever control the SOC based on the voltage? because I have 3 variables here VOLTAGE-SOC-LOAD ...

I have seen some inverters that can set the "back to grid / battery mode" based on the SOC %, however I don't want to invest in another inverter again...

THANKS
 
Victron makes full featured, well regarded battery monitors. You program it with the capacity of your battery bank and it will track the watts in and out and calculate the SOC. The key is to start with a fully charged battery.

I make do with a much cheaper AiLi brand monitor which works well for my needs, but it only supports a maximum of 350 amps in or out which may not enough for your system.


 
Hello Camelcase, thanks for the good information!

I'm looking more into a device that can protect my battery from discharging below a SOC of 20% .... is there any device on the market like this?
(obviously the inverter cannot, based on the voltage :)

I guess it would be easy to read the SOC% on the battery via RS485 and based on that info allow the battery to charge or discharge ...
 
Hi Don,

Thank you for the info ! it points me in the right direction... :)
I am looking for device that will work bi-directional between battery and inverter (charge and discharge)

Actually, read the SOC value from the battery and if lower than a set value, disconnect the battery, or better just stop the flow from battery to
inverter and still allow the flow from inverter to battery. (a kind of "diode simulation system")

I don't know and have to check how the inverter will react when the battery suddenly gets disconnected / connected... especially when the
battery provides power to the load....

According to the Victron manual, this device is unidirectional and will be damaged when the inverter wants to charge the battery again...

The BP is designed to allow current to flow from IN (battery) to
OUT (load) terminals only.
Reverse currents from OUT to IN terminals are strictly forbidden,
and will damage the device. If you wish to use the BP as a
disconnection for a charge source, you must orient the unit in the
system so that the current is flowing in the intended direction, IN
to OUT...

Great forum, great people :)
 
From what you write I gather your inverter is also a charger. From PV only or also from grid?
Still, I don't know, really, I've never experimented with automatic... anythings, really, I prefer to do things myself and tend to distrust automatisms.
The only thing I (have to) to trust is for my SCCs to stop charging when the battery reaches a certain voltage - which they do (I check).

I have separate controllers from the inverter - which has multiple advantages - so if I wanted to disconnect the battery at a certain voltage I could use the Battery Protect... except I would never want to be in a position to do so.
You can probably tell your BMS to do that.
Voltage controlled switches on individual loads after the inverter would probably be best... but really, I would prefer to have enough battery - and recharging means - not to have to do that.
Which I don't, I draw very little and my battery just about never goes under 75% SOC.

Maybe someone else has a better idea on how to address your problem, really not my thing.
-
 
look at solar assistant.
it can xonnect to your batteries and your inverter and play "middleman"
you'll get sone pretty good monitoring too
 
My inverter is also a charger, Axpert MAX II 10kw.

I was trying to connect the battery to the Axpert MAX II 10kw
the cable I made is: 3-5-8 to 4-5-8, but cannot make the inverter talk to the battery.

Sorotec engineers are not helpful, I'm trying to get the information for the battery, which protocols it talks and the pin layout of all connectors (rs232, CAN, RS485). I'm waiting for 4 weeks now, they answer my email sometimes, but avoid my questions. NOT happy about SOROTEC!

The Solar Assistant looks interesting, I will experiment with this :) thanks for the tip!
 
gdb0701 said:
My inverter is also a charger, Axpert MAX II 10kw.

I was trying to connect the battery to the Axpert MAX II 10kw
the cable I made is: 3-5-8 to 4-5-8, but cannot make the inverter talk to the battery.

Sorotec engineers are not helpful, I'm trying to get the information for the battery, which protocols it talks and the pin layout of all connectors (rs232, CAN, RS485). I'm waiting for 4 weeks now, they answer my email sometimes, but avoid my questions. NOT happy about SOROTEC!

The Solar Assistant looks interesting, I will experiment with this :) thanks for the tip!
Click to expand...
are you sure your battery "talks" pylontec over rs485 ?
 
FEEDBACK

I got it working now, RS485 communication between Axpert II max 10kw and a battery using the PACE BMS inside.
1) the RS485 cable uses: pin 3 and 5 (inverter side) to battery pin1 and pin2 (this depends of the battery of course)
2) I used PbmsTools and changed the "Inverter Set" to LuxPower 485 (no need to restart the battery after choosing this setting)
3) on the Axpert inverter, choose option 05: Pylontech
 

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I think that I am looking for a similar solution with an Axpert max ii 8kW.

I would like to be able to use the grid like a generator, only to top up the batteries if they threaten to run empty.
Typically there is some risk for that in the morning. So IF the battery goes under let's say 15% then I want the grid to kick in and top up the battery to lat's say 30%. That should be enough, because soon after the sun will start charging the batteries.

Setting 12 & 13 allow me to set the values, but based on Voltage. Does anybody know how to achieve this with more accuracy? (My Victrons allow it based on SOC, which is quite accurate), but the Axpert does not seem to be capable to do that.

So, if any body knows how, I would be most grateful.
PS: I am not an expert, nor a technician.

Greetings, Kurt
 
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