I had a system installed Feb 2023, and am concerned about a number of aspects relating to the batteries. 

1. V system pushes float voltage to 53v

2. Batteries only seem to deliver about 80% of rated output before tripping.

System spec:

1. AXpert Max-E 11kw inverter

2. Rentech 150AH 7.62Kwh batteries x2 (15.2w2 total)

3. 12 x Canadian Solar 550w solar panels

 

Problem one: When the system is running and the solar panels are not active, the batteries are charged until just over 56 volts, then settles on a float of 55 volts. When the solar kicks in, the battery voltage drops to 53 volts and stays there. I checked voltage on the panel and on the batteries themselves. Always within 0.2volts. This means the batteries are being kept at a float voltage well below the recommended and in fact are never being properly charged.

Problem two: When solar is active, and the batteries have been used, the system does not charge the batteries at all, despite the system being set to charge from Utility & Solar. The other settings are Solar only (No!) and Solar first, which does not change the behaviour... no charging at all while solar is active.

Problem three: The battery spec for 100 amps each maximum discharge. That gives 200A (With a set 10% overload = 220Amps absolute max) = 200 x 50 volts = 10KW. Taking the inverter conversion efficiency into account (at worst 90%) that should give me 8.1KW discharge rate (measured at the inverter) without problems. Because my house could potentially draw a high current, I've installed a load management system to keep total demand to 8KW. The problem is that the batteries trip out at about 160Amps draw... maybe 170 max, but not for long. This is giving me a 15 to 20% shortfall, even taking conversion and cable efficiency into account. The Rentach guys claim it has to be able to handle 200Amps, but that it might be resistive load in the cabling. I tested the amp draw using a clamp meter and had the cables checked - all good and the actual amperage measured in the cables never made it above 175amps. So, I bought a 10kw system, which will have to be limited to just 7kw.. I really feel cheated.

  

 

 

 

On 2023/03/29 at 8:17 PM, CesareC said:

I had a system installed Feb 2023, and am concerned about a number of aspects relating to the batteries. 

1. V system pushes float voltage to 53v

2. Batteries only seem to deliver about 80% of rated output before tripping.

System spec:

1. AXpert Max-E 11kw inverter

2. Rentech 150AH 7.62Kwh batteries x2 (15.2w2 total)

3. 12 x Canadian Solar 550w solar panels

 

Problem one: When the system is running and the solar panels are not active, the batteries are charged until just over 56 volts, then settles on a float of 55 volts. When the solar kicks in, the battery voltage drops to 53 volts and stays there. I checked voltage on the panel and on the batteries themselves. Always within 0.2volts. This means the batteries are being kept at a float voltage well below the recommended and in fact are never being properly charged.

Problem two: When solar is active, and the batteries have been used, the system does not charge the batteries at all, despite the system being set to charge from Utility & Solar. The other settings are Solar only (No!) and Solar first, which does not change the behaviour... no charging at all while solar is active.

Problem three: The battery spec for 100 amps each maximum discharge. That gives 200A (With a set 10% overload = 220Amps absolute max) = 200 x 50 volts = 10KW. Taking the inverter conversion efficiency into account (at worst 90%) that should give me 8.1KW discharge rate (measured at the inverter) without problems. Because my house could potentially draw a high current, I've installed a load management system to keep total demand to 8KW. The problem is that the batteries trip out at about 160Amps draw... maybe 170 max, but not for long. This is giving me a 15 to 20% shortfall, even taking conversion and cable efficiency into account. The Rentach guys claim it has to be able to handle 200Amps, but that it might be resistive load in the cabling. I tested the amp draw using a clamp meter and had the cables checked - all good and the actual amperage measured in the cables never made it above 175amps. So, I bought a 10kw system, which will have to be limited to just 7kw.. I really feel cheated.

  

 

 

 

Get PBmodbustools and connect to the battery BMS using a RS458 cable to see what is actually happening in the BMS. This will tell you the exact amps provided by each battery. If you ask Rentech to test the batteries they will send someone out to your premises who can do all the testing. Ate you in Cape Town?

Rentech batteries (Shenzhen BAK) do require a high voltage (compared to other batteries) to charge full. As soon as they reach 100% SOC the voltage should drop to a resting voltage (while the cells are balancing). The difference can easily be about 3v between resting voltage and voltage just after reaching 100% SOC.

While the SOC is at 100% and the cells are balancing the batteries do not care about the float or charge voltage supplied by the inverter (you can set it to 58v by example, the battery won't accept further charge). You will obviously measure 58V on the battery terminals but this voltage will never reach the cells.

Also note that you have to charge these batteries to 100% SOC for cell balancing to work correctly.

Are you using BMS communication between inverter and batteries?

The BMS on these batteries will not allow a discharge below 20% SOC when using BMS communication between batteries and Inverter. The BMS will override any setting in the inverter and will force a charge. If you want to use the available capacity below 20% SOC you will have to use an AGM or similar setting on your inverter that does not require BMS communication.  

This might also cause the tripping issue when exceeding load or 170A (this one is just a guess).

 

Hi I84RiS,

Thanks for the tip on the correct software to use. Rentech are understandably cagey about the software. They instructed the installer to double up the DC cabling, but I suspect that is not the issue. We will wait and see. I still have the Solar issue where is stops charging the batteries if solar becomes active IE if we had load shedding at 04h00 to 06h00 and Solar becomes active at 06h30 then the AC charging stops. If I disable the Solar, then it completes the charging using Utility. 

I will give feedback, when the cable upgrade is done, but I suspect the compatibility between the Inverter and batteries is not good. Installer recommended not to create a coms link between the batteries and inverter, saying the batteries have a more advanced and accurate BMS vs using the inverter to manage them. I thought that was a fair comment and Battery is set to USR on the inverter. Lets see what happens after the cable upgrade.    

PS: The tests I did were at 100% charged (or very close). I have the cut off at 46v. Batteries never been below 50% SOC, even after 4 hours of load shedding. Mostly I'm getting down to 80% SOC, so only using 20% of capacity.

  

On 2023/04/19 at 3:42 PM, CesareC said:

and Solar becomes active at 06h30 then the AC charging stops. If I disable the Solar, then it completes the charging using Utility. 

You can solve this problem with setting 16, "charge source priority" to SNU. It will then charge from utility and together from PVs when available. That's what I did, it works fine. If you have 2 units in parallel you must set it at each unit individually.

By the way: There are no issues of battery/inverter compatibility. There are only issues of BMS/inverter compatibility. Running the charge management solely with the correct voltage settings on the inverter omits those issues.

Edited April 21, 20231 yr by Beat

Good day

Sorry to hijack the post as I'm in the exact same situation,

Please advise what FW are your currently running on the invertor and also what charge management do you have installed?

Tanx