Proper process for balancing two Dyness batteries

[Hanno Labuschagne]

Hey everyone

We have two Dyness 5.12kWh DL5.0C batteries in our solar system. One was installed back in March 2024 while the other was added roughly a year later.

There are so many conflicting views about how to properly balance batteries but I think whatever our installer did was incorrect.

Although many online claim that the BMS on the two batteries will sort out the balancing automatically, others say they MUST be connected initially at the same level of charge.

I am now often finding that the newer battery has one fewer battery level indicator light on than the older battery.

For example, when the total SoC is 29%, the older battery is still at 2 lights while the new one drops down to 1 light.

This has me especially worried that I am dropping below the minimum rated SoC on the new battery when discharging to as low as 20%.

Some have recommended that I should charge the batteries to 100%, one at a time. Would it be possible to do this just by switching off one of the batteries with its toggle and changing the necessary amp ratings on the Deye inverter and then moving over to the next one? Or are more steps necessary?

 

[wingnut771]

Hey everyone

We have two Dyness 5.12kWh DL5.0C batteries in our solar system. One was installed back in March 2024 while the other was added roughly a year later.

There are so many conflicting views about how to properly balance batteries but I think whatever our installer did was incorrect.

Although many online claim that the BMS on the two batteries will sort out the balancing automatically, others say they MUST be connected initially at the same level of charge.

I am now often finding that the newer battery has one fewer battery level indicator light on than the older battery.

For example, when the total SoC is 29%, the older battery is still at 2 lights while the new one drops down to 1 light.

This has me especially worried that I am dropping below the minimum rated SoC on the new battery when discharging to as low as 20%.

Some have recommended that I should charge the batteries to 100%, one at a time. Would it be possible to do this just by switching off one of the batteries with its toggle and changing the necessary amp ratings on the Deye inverter and then moving over to the next one? Or are more steps necessary?

I wouldn't believe the SOC counter. Look at the voltage. Batteries in parallel will always be in balance voltage wise. I doubt there is an issue here, just the SOC counter being notoriously unreliable with LFP flat voltage curves and should be taken with a bag of salt.

I would drain them completely flat then recharge them, maybe the SOC counters will reset? Maybe use AGM mode on inverter to disable the coms.

 

[wingnut771]

 

[wingnut771]

You also find batteries have slightly different internal resistance so one might work slightly more than the other but they both get to the same place in the end.

 

[wingnut771]

 

[DrJohnZoidberg]

This is why I'm glad I did the full battery monitoring setup with Solar Assistant. I also initially had 2 batteries and added a third later, on my system there is a slight difference with the newer battery but it's very small and probably just has to do with the voltage difference.

I can monitor my battery SOC individually and have it logged in Home Assistant, this is what my SOC looks like for the 3 batteries, you can see the on curve is slightly offset from the other two but they all stay quite close together.

 

[wingnut771]

This is why I'm glad I did the full battery monitoring setup with Solar Assistant. I also initially had 2 batteries and added a third later, on my system there is a slight difference with the newer battery but it's very small and probably just has to do with the voltage difference.

I can monitor my battery SOC individually and have it logged in Home Assistant, this is what my SOC looks like for the 3 batteries, you can see the on curve is slightly offset from the other two but they all stay quite close together.

View attachment 1861728

Due to slightly different internal resistances, but they all end up the same in the end. Thats how you can parallel a 100Ah with a 5Ah in the extreme example above. The 100Ah has lower resistance so takes most of the load.

 

[Hanno Labuschagne]

Cannot seem to find a way to view the individual voltages on the batteries on the Deye, but I may also just be stupid. Will post some photos for reference.

 

[B-1]

The right way is to charge the batteries to full one at a time and then connect them together. You also should have them on the same firmware ideally but if not have the newer firmware as the primary.

As things stand now switch your inverter to keep the batteries full charging off the grid for 2 days and then switch back to your regular settings.

That being said its been cloudy for a while are they charging up to 100% and staying there for a few hours to equalise every couple of days?
Which is the problematic battery? Old or new?

 

[wingnut771]

The right way is to charge the batteries to full one at a time and then connect them together. You also should have them on the same firmware ideally but if not have the newer firmware as the primary.

As things stand now switch your inverter to keep the batteries full off the grid for 2 days and then switch back to your regular settings.

That being said its been cloudy for a while are they charging up to 100% and staying there for a few hours to equalise every couple of days?
Which is the problematic battery? Old or new?

IMHO, thety only say this so that you dont connect them at different SOC which would cause large current draw if one battery is flat for example. As long as voltage is similar then you can carry on. Lets say one is at 60% and the other at 80%. The 80% battery will hardly charge while the 60% battery catches up due to internal resistance.

 

[wingnut771]

 

[B-1]

IMHO, thety only say this so that you dont connect them at different SOC which would cause large current draw if one battery is flat for example. As long as voltage is similar then you can carry on. Lets say one is at 60% and the other at 80%. The 80% battery will hardly charge while the 60% battery catches up due to internal resistance.

100% agree its just easier to do it that way. If you have 2 batteries getting them to a similar voltage is relatively easy but the more there are the harder it starts becoming to get them all in the same ball park because you would have to charge, let them soak, measure, repeat.
Just charge them all, connect and done.

 

[Hanno Labuschagne]

Something definitely seems off. The second (newer) battery now only has two lights (between 20% and 40%) and the older one four (between 60% and 80%).

 

[wingnut771]

100% agree its just easier to do it that way. If you have 2 batteries getting them to a similar voltage is relatively easy but the more there are the harder it starts becoming to get them all in the same ball park because you would have to charge, let them soak, measure, repeat.
Just charge them all, connect and done.

Even a 4V difference wont do much. Let me try find a video showing the amps.

 

[wingnut771]

Something definitely seems off. The second (newer) battery now only has two lights (between 20% and 40%) and the older one four (between 60% and 80%).

Keep discharging down to 10%. SOC counter probably lying as its physically impossible to have different voltages.

 

[Hanno Labuschagne]

The right way is to charge the batteries to full one at a time and then connect them together. You also should have them on the same firmware ideally but if not have the newer firmware as the primary.

As things stand now switch your inverter to keep the batteries full charging off the grid for 2 days and then switch back to your regular settings.

That being said its been cloudy for a while are they charging up to 100% and staying there for a few hours to equalise every couple of days?
Which is the problematic battery? Old or new?

They normally reach 100% and stay there for about 2 hours per day. I once did do the 100% fixed level for two days...did not seem to have the desired effect.

 

[Hanno Labuschagne]

They normally reach 100% and stay there for about 2 hours per day. I once did do the 100% fixed level for two days...did not seem to have the desired effect.

Will give that a try. Does the cabling look correct there?

 

[wingnut771]

They normally reach 100% and stay there for about 2 hours per day. I once did do the 100% fixed level for two days...did not seem to have the desired effect.

You need to empty them so SOC counter can reset. How low do they normally go?

 

[Koosvanwyk]

The reality is there is not much you can do about it. Batteries in parallelle will balance themselves, irrespective of whether one was at 20% SOC and the other at 60% SOC when you connected them together (not recommended to do though due to other reasons not associated with balancing them)

Make sure your cables are the same length and guage, luggs are the same size and material, botls torgued to the same level, and you should be good.

As already stated the SOC counters are not very accurate, your best defense here is to charge to the point where the BMS resets the counter to 100% (also vital for cell balancing)

You would need to understand what this threshold is for your specific BMS, can by example be a certain cell voltage (most common 3.65V for any given cell), pack voltage (most common being around 58V for a 16 cell pack), or the best method is a combination of charge amps below a certain rate and pack voltage above a certain volts).

The longer you do not charge to a point where the SOC counters resets the worse it gets. Using my batteries as an example, if I do not charge to that point for 5 consecutive days this delta can be as much as 15% to 20% on the SOC readings but the pack voltages are usually exactly the same.

I often discharge by batteries to 0% SOC and the BMS allows discharging to continue way past the point where the the SOC is shown to be 0% as long as the voltage is still above the low voltage disconnect point. In other words the BMS does not care about the SOC readings, it is all about the voltage.

 

[wingnut771]

The reality is there is not much you can do about it. Batteries in parallelle will balance themselves, irrespective of whether one was at 20% SOC and the other at 60% SOC when you connected them together (not recommended to do though due to other reasons not associated with balancing them)

Make sure your cables are the same length and guage, luggs are the same size and material, botls torgued to the same level, and you should be good.

As already stated the SOC counters are not very accurate, your best defense here is to charge to the point where the BMS resets the counter to 100% (also vital for cell balancing)

You would need to understand what this threshold is for your specific BMS, can by example be a certain cell voltage (most common 3.65V for any given cell), pack voltage (most common being around 58V for a 16 cell pack), or the best method is a combination of charge amps below a certain rate and pack voltage above a certain volts).

The longer you do not charge to a point where the SOC counters resets the worse it gets. Using my batteries as an example, if I do not charge to that point for 5 consecutive days this delta can be as much as 15% to 20% on the SOC readings but the pack voltages are usually exactly the same.

I often discharge by batteries to 0% SOC and the BMS allows discharging to continue way past the point where the the SOC is shown to be 0% as long as the voltage is still above the low voltage disconnect point. In other words the BMS does not care about the SOC readings, it is all about the voltage.

Yeah, its hard to say how to reset SOC counter as different BMS's have different strategies they adopt.