the_colorist
"Move over... let me fix it" Installer/Engineer
As an installer/engineer, I'm usually explaining this once a week or so (sometimes more) so I felt I would write up a quick post on it. Perhaps it has been covered elsewhere (I know I've written about it a few times) but I didn't bother to dig around.
I'm not going to get technical here, this is a very high-level post. Intended for the benefit of everyone. There are a lot of guys here who will instantly pick up on what I'm talking about and help to flesh things out for anyone coming along with questions. Don't get me wrong, I'm glad to help with explanations but my time these days is unfortunately extremely limited.
I get asked regularly: What is the benefit of some of the more expensive BMS units (Orion, REC, etc) over the cheaper ones?
Well, there are a lot of advantages but I'm only going to mention one in this post.
BALANCING
At first, this may seem odd as it doesn't appear that those units on average have any higher current balancing capabilities or really anything else in the "specs" that would lead one to believe that they are more advanced in any way, there is however one extremely important difference that sets them apart.
COMMUNICATION
What I'm referring to here is the CAN bus communication link between the BMS and the system (Victron/SMA/Growatt etc) that is charging/discharging the bank.
I find in talking with people/clients that this specific feature seems like a black box and even appears to possibly be unnecessary/optional but in fact it serves a very, very useful purpose with regards to balancing.
There has been plenty of discussion of active balancing vs passive balancing and how passive balancing doesn't appear to do anything in some cases and yet we see balancing readouts on BMS units such as Orion that look like this (@cinergi I hope you don't mind I stole this):
So what's the diff here? Does passive balancing work or not? Do I care about communication or do I not?
Here is how it works:
During the charging phase, the BMS monitors the per-cell voltage among many other things (nothing strange here). At the same time, it uses the per-cell information to make calculations about the most optimal charging current/voltage for the bank and then it uses the communication link to send the following commands/information to the system the battery is powering. It varies some across the market but here are the basic commands.
Bear in mind these commands are sent every few hundred milliseconds, sometimes even faster. And the commands (Max Charging Current for example) will vary based on the state of the cells. Lines in italics are information messages.
1. Max Charging Voltage (Pack Full Charge Voltage Target)
2. Max Charging Current (DC charge current limitation)
3. Max Discharging Current (DC discharge current limitation)
4. State-of-Charge (SOC value)
5. State-of-Heath (SOH value)
6. Battery Pack Voltage (Total pack voltage as read by the BMS)
7. Battery Pack Current (Pack charge/discharging current as read by the BMS)
8. Alarms (cell over-voltage etc)
During the charge phase, as the pack voltage rises, there is usually a target voltage per cell when the passive balancing resistors come on.
Given enough time, even the lowest current balancers could balance a bank the amount of time it would take would be ridiculous depending on the size of the bank. Assuming a properly sized BMS/balancer vs bank capacity, it could actually achieve it in a reasonable time. The key is in communication.
As the first cell begins to reach the balance start voltage, the balancer kicks on. Nothing strange here.
As the first cell reaches the per-cell full charge target voltage (3.45VPC etc) however, this is where the magic happens.
The BMS uses the communication link to instruct the inverters/charge controllers/etc to begin to dynamically back down their charging current to the point where:
The balancer is able to keep up with burning off the power from the cells that already at the per-cell full charge voltage target (preventing a rise to cell over-voltage) while the low voltage cells are still charging.
The BMS at this point could be requesting a pack charging current of 15.6A, 10A, 2A or even 0.5A. It all depends on how many cells are at or near their full charge target voltage and how many are still needing to charge.
This phase at the end of every charge cycle will last as long as it takes (or as short as it takes) to get all of the cells to reach the full charge voltage target (per cell).
This could be 3.45VPC etc. It will depend on the targets programming into the BMS.
On the Orion, there is an extra special feature.
Victron explains it like this:
Adaptive Maximum Charge Voltage Set-point – the value transmitted varies from 54.8V to 55.8V depending on the state of balancing on the particular ****. Typically a well-balanced **** battery will request a maximum charge voltage of between 55.0V and 55.8V. This advanced feature is only available from ***** and allows superior system control and optimal battery management. The GX-device uses this set-point to control the real time operating or target voltage of the inverter/charger devices and the MPPTs."
That comment is associated with a specific brand of commercial battery banks that use Orion BMS units internally. I'll let you figure out who it is.
REC explains their charging algorithm like this:
************
The communication between the REC BMS and the Victron CCGX is established through the CAN bus. All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to the CCGX unit in every measurement cycle.
The charging current is controlled by the Maximum charging current parameter. It’s calculated as Charging Coefficient ('C','H','A',’C') x Battery capacity.
The parameter has an upper limit which is defined as Maximum Charging Current per Device ('M','A','X',’C') x Number of Devices ('S','I','S',’N').
When any cell reaches the voltage interval between Balance Voltages Start and Balance Voltage End, the charging current starts to ramp down to 1.1A x Number of Devices until the last cell rises to the End of Charge Voltage. At that point, the Maximum charging voltage is set to Number of cells x (End of Charge Voltage per cell –0.5 x end of charge hysteresis per cell) and the charger is disabled also via the BMS I/O interface.
End of Charge, SOC hysteresis, and End of charging cell voltage hysteresis prevent unwanted switching. SOC is calibrated to 100 % and Power LED lights ON 100 %. Charger turn-off can also be caused by some of the system's errors (See System Errors indication chapter). SOC is calibrated to 96% at the 0.502 x value between Balance Voltages Start and Balance Voltage End.
************
So active balancing? Does it solve the issue?
Well, we using them with "Smart BMS" units with the type of communication/algorithms mentioned above and they do help. They aren't always needed BUT with A- cells that have varying IR and are powering systems with large loads with high startup currents pulling the cells out of balance, I feel they can be a big plus.
In the future I feel "Smart Active Balancing" in a single BMS unit would be even better but for now, communication backed by a great charging algorithm and appropriate cell voltage targets will get us where we want to go IMHO. There are a couple units on the market that I'm aware of with "Smart Active Balancing" such as Autarctech's BMS units and REC's Active BMS but unfortunately one is over $1K last I checked and the other is 4S so not appropriate for budget 15/16s banks. I'm sure we'll get there though. Even some commercial companies are realizing the usefulness of active balancing. Tesvolt APU is a great example of this.
That said, nothing helps to get things rolling in the right direction like a great top-balance...
I'm not going to get technical here, this is a very high-level post. Intended for the benefit of everyone. There are a lot of guys here who will instantly pick up on what I'm talking about and help to flesh things out for anyone coming along with questions. Don't get me wrong, I'm glad to help with explanations but my time these days is unfortunately extremely limited.
I get asked regularly: What is the benefit of some of the more expensive BMS units (Orion, REC, etc) over the cheaper ones?
Well, there are a lot of advantages but I'm only going to mention one in this post.
BALANCING
At first, this may seem odd as it doesn't appear that those units on average have any higher current balancing capabilities or really anything else in the "specs" that would lead one to believe that they are more advanced in any way, there is however one extremely important difference that sets them apart.
COMMUNICATION
What I'm referring to here is the CAN bus communication link between the BMS and the system (Victron/SMA/Growatt etc) that is charging/discharging the bank.
I find in talking with people/clients that this specific feature seems like a black box and even appears to possibly be unnecessary/optional but in fact it serves a very, very useful purpose with regards to balancing.
There has been plenty of discussion of active balancing vs passive balancing and how passive balancing doesn't appear to do anything in some cases and yet we see balancing readouts on BMS units such as Orion that look like this (@cinergi I hope you don't mind I stole this):
So what's the diff here? Does passive balancing work or not? Do I care about communication or do I not?
Here is how it works:
During the charging phase, the BMS monitors the per-cell voltage among many other things (nothing strange here). At the same time, it uses the per-cell information to make calculations about the most optimal charging current/voltage for the bank and then it uses the communication link to send the following commands/information to the system the battery is powering. It varies some across the market but here are the basic commands.
Bear in mind these commands are sent every few hundred milliseconds, sometimes even faster. And the commands (Max Charging Current for example) will vary based on the state of the cells. Lines in italics are information messages.
1. Max Charging Voltage (Pack Full Charge Voltage Target)
2. Max Charging Current (DC charge current limitation)
3. Max Discharging Current (DC discharge current limitation)
4. State-of-Charge (SOC value)
5. State-of-Heath (SOH value)
6. Battery Pack Voltage (Total pack voltage as read by the BMS)
7. Battery Pack Current (Pack charge/discharging current as read by the BMS)
8. Alarms (cell over-voltage etc)
During the charge phase, as the pack voltage rises, there is usually a target voltage per cell when the passive balancing resistors come on.
Given enough time, even the lowest current balancers could balance a bank the amount of time it would take would be ridiculous depending on the size of the bank. Assuming a properly sized BMS/balancer vs bank capacity, it could actually achieve it in a reasonable time. The key is in communication.
As the first cell begins to reach the balance start voltage, the balancer kicks on. Nothing strange here.
As the first cell reaches the per-cell full charge target voltage (3.45VPC etc) however, this is where the magic happens.
The BMS uses the communication link to instruct the inverters/charge controllers/etc to begin to dynamically back down their charging current to the point where:
The balancer is able to keep up with burning off the power from the cells that already at the per-cell full charge voltage target (preventing a rise to cell over-voltage) while the low voltage cells are still charging.
The BMS at this point could be requesting a pack charging current of 15.6A, 10A, 2A or even 0.5A. It all depends on how many cells are at or near their full charge target voltage and how many are still needing to charge.
This phase at the end of every charge cycle will last as long as it takes (or as short as it takes) to get all of the cells to reach the full charge voltage target (per cell).
This could be 3.45VPC etc. It will depend on the targets programming into the BMS.
On the Orion, there is an extra special feature.
Victron explains it like this:
Adaptive Maximum Charge Voltage Set-point – the value transmitted varies from 54.8V to 55.8V depending on the state of balancing on the particular ****. Typically a well-balanced **** battery will request a maximum charge voltage of between 55.0V and 55.8V. This advanced feature is only available from ***** and allows superior system control and optimal battery management. The GX-device uses this set-point to control the real time operating or target voltage of the inverter/charger devices and the MPPTs."
That comment is associated with a specific brand of commercial battery banks that use Orion BMS units internally. I'll let you figure out who it is.
REC explains their charging algorithm like this:
************
The communication between the REC BMS and the Victron CCGX is established through the CAN bus. All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to the CCGX unit in every measurement cycle.
The charging current is controlled by the Maximum charging current parameter. It’s calculated as Charging Coefficient ('C','H','A',’C') x Battery capacity.
The parameter has an upper limit which is defined as Maximum Charging Current per Device ('M','A','X',’C') x Number of Devices ('S','I','S',’N').
When any cell reaches the voltage interval between Balance Voltages Start and Balance Voltage End, the charging current starts to ramp down to 1.1A x Number of Devices until the last cell rises to the End of Charge Voltage. At that point, the Maximum charging voltage is set to Number of cells x (End of Charge Voltage per cell –0.5 x end of charge hysteresis per cell) and the charger is disabled also via the BMS I/O interface.
End of Charge, SOC hysteresis, and End of charging cell voltage hysteresis prevent unwanted switching. SOC is calibrated to 100 % and Power LED lights ON 100 %. Charger turn-off can also be caused by some of the system's errors (See System Errors indication chapter). SOC is calibrated to 96% at the 0.502 x value between Balance Voltages Start and Balance Voltage End.
************
So active balancing? Does it solve the issue?
Well, we using them with "Smart BMS" units with the type of communication/algorithms mentioned above and they do help. They aren't always needed BUT with A- cells that have varying IR and are powering systems with large loads with high startup currents pulling the cells out of balance, I feel they can be a big plus.
In the future I feel "Smart Active Balancing" in a single BMS unit would be even better but for now, communication backed by a great charging algorithm and appropriate cell voltage targets will get us where we want to go IMHO. There are a couple units on the market that I'm aware of with "Smart Active Balancing" such as Autarctech's BMS units and REC's Active BMS but unfortunately one is over $1K last I checked and the other is 4S so not appropriate for budget 15/16s banks. I'm sure we'll get there though. Even some commercial companies are realizing the usefulness of active balancing. Tesvolt APU is a great example of this.
That said, nothing helps to get things rolling in the right direction like a great top-balance...