Need Help / Off Grid Battery Balancing

[DougL]

I appreciate the information. I guess the core issue or question is, is it normal for 2-48v batteries in parallel to discharge in the manner I described at the end of my previous post? And is it normal for 2 48 volt batteries to require a minimum power draw of, (as in my case 8 amps) for the 2 batteries to remain operational or remain awake ( through the BMS)? I am the first to admit I have a lot to learn and I am hoping that members of this forum will cut a less knowledgeable member a little slack. Thank you for for your advice!

It all depends on the BMS. Most all the 'standard' BMSs are well documented but since you're unaware of what the cells are doing it appears there's either no access to the BMS or you've chosen not to gain access. Personally, I would not own any 48V battery pack without some way of seeing what the BMS is doing and seeing all the cell voltages. A 'black box' of this size and power should have the ability to be monitored better.
And if they created a custom BMS who knows what 'features' they chose to implement.

 

[LightZone]

It all depends on the BMS. Most all the 'standard' BMSs are well documented but since you're unaware of what the cells are doing it appears there's either no access to the BMS or you've chosen not to gain access. Personally, I would not own any 48V battery pack without some way of seeing what the BMS is doing and seeing all the cell voltages. A 'black box' of this size and power should have the ability to be monitored better.
And if they created a custom BMS who knows what 'features' they chose to implement.

It all depends on the BMS. Most all the 'standard' BMSs are well documented but since you're unaware of what the cells are doing it appears there's either no access to the BMS or you've chosen not to gain access. Personally, I would not own any 48V battery pack without some way of seeing what the BMS is doing and seeing all the cell voltages. A 'black box' of this size and power should have the ability to be monitored better.
And if they created a custom BMS who knows what 'features' they chose to implement.

Just to clarify, here are photos of the 3 screens available on each of the 48 v batteries and the information they display.

 

[MakeMePower]

does discharge show disabled on the second battery when under 8a?

 

[DougL]

Good that you can at least see the cell voltages but wow, 0mV difference between 16 battery cells seems amazing.
As for the BMS turning off output if below 8A goes that doesn't make sense because it means you could never use the battery.
An inverter won't pull 8A from a battery with no load and if the BMS output turns off at less than 8A, how would it even turn back on.
I think it's hogwash.

Are you 2 batteries connected together with bus bars or equal size/length cables?
Are your 2 batteries then connected to your inverter with the same size/length cables?
Are your 2 batteries cables connected to the Positive terminal of one battery and the other cable connected to the Negative terminal of the other battery?

If not, fix it as described above and see if your batteries charge and discharge more evenly.

 

[LightZone]

does discharge show disabled on the second battery when under 8a?

No. What will happen is it will read DISCHARGING (where IDLE is seen in the photo) and the other will read IDLE ( like in the pic. On the top of the display are actually 2 switches with the ability to turn charging and discharging off for each battery. Doing so will force the other battery to take over the load.

 

[LightZone]

Good that you can at least see the cell voltages but wow, 0mV difference between 16 battery cells seems amazing.
As for the BMS turning off output if below 8A goes that doesn't make sense because it means you could never use the battery.
An inverter won't pull 8A from a battery with no load and if the BMS output turns off at less than 8A, how would it even turn back on.
I think it's hogwash.

Are you 2 batteries connected together with bus bars or equal size/length cables?
Are your 2 batteries then connected to your inverter with the same size/length cables?
Are your 2 batteries cables connected to the Positive terminal of one battery and the other cable connected to the Negative terminal of the other battery?

If not, fix it as described above and see if your batteries charge and discharge more evenly.

I may not have been absolutely clear about the issue. Each battery will handle loads smaller than 8 amps without any problem. But according to Big Battery, a minimum load of 8 amps is needed for the BMS to allow/wake up the second battery (in parallel) to share the load
The batteries are connected to a bus bar with cables of equal size and length.
The batteries DO charge equally. It is only when a load of less than 8 amps is applied that they DISCHARGE asymmetrically because one battery will remain in IDLE status while the other with show DISCHARGING status. And this remains as long as the power draw remains under 8 amps eg. from the time we go to bed and wake up the next morning. Once charging commenses, both batteries will equalize.

 

[MakeMePower]

I may not have been absolutely clear about the issue. Each battery will handle loads smaller than 8 amps without any problem. But according to Big Battery, a minimum load of 8 amps is needed for the BMS to allow/wake up the second battery (in parallel) to share the load
The batteries are connected to a bus bar with cables of equal size and length.
The batteries DO charge equally. It is only when a load of less than 8 amps is applied that they DISCHARGE asymmetrically because one battery will remain in IDLE status while the other with show DISCHARGING status. And this remains as long as the power draw remains under 8 amps eg. from the time we go to bed and wake up the next morning. Once charging commenses, both batteries will equalize.

i wonder if actually is discharging and its a display issue, that said 8A is very low for a house

 

[LightZone]

i wonder if actually is discharging and its a display issue, that said 8A is very low for a house

According to the seller Big Battery, it's not a display issue but rather just how they work (for whatever reason). You're right, I suppose 8 amps is very little for a typical house. We live in an 800 square foot strawbale house (we built ourselves) that was designed and built to be extremely energy efficient. We do come close to the 8 amps (overnight usage) typically 6 amps. This will of course change once we get into the fall and winter seasons. It'll actually go even lower than 6 amps.

 

[DougL]

According to the seller Big Battery, it's not a display issue but rather just how they work (for whatever reason). You're right, I suppose 8 amps is very little for a typical house. We live in an 800 square foot strawbale house (we built ourselves) that was designed and built to be extremely energy efficient. We do come close to the 8 amps (overnight usage) typically 6 amps. This will of course change once we get into the fall and winter seasons. It'll actually go even lower than 6 amps.

At night and when nobody is home our house load is under 500W also and since we have more battery than we need most of the time I'm leaving computers on not worrying about turning things off or unplugging.

As far as the outputs being turned on and off by the BMS depending on the load that is an odd one but lets try to come up with why they would implement this. Are they trying to limit really small current loads in hopes of keeping the SOC algorithm closer to accurate. It can't be to limit total output current because they do it only under 8A output.
I would look for a setting to see if you can disable the BMS-to-BMS communications and also ask them if it's ok to disable that characteristic because you want both batteries operating in a parallel( 2P1S ) configuration.