lithium & solar power LiFePO4

Steca XPC 2200-24 integrated charger details – FAQ part 2

The XP-Compact (XPC) model offers integrated automatic battery charger.  See some more FAQs for Steca XPC 2200-24:

Question: What is the MIN voltage for Steca XPC charger?
Answer:
 The battery is disconnected when 2.9V per cell is reached and “Batt. Undervolt.” LED goes on.

Question: Can I control the charging current?
Answer:
 The Steca XPC allows user to adjust charging current with the turning the knob 22 – charger current adjustment. Please keep the maximum current within the declared continuous charging current limit of your LiFePO4 batteries.

Question: How do I find out how much charged my battery is?
Answer: SOC can be checked with Steca RCC-01 remote control, which is not supplied together with Steca XPC. With RCC-01 you can monitor state of charge of your batteries in four levels, you can change factory predefined voltage levels for MIN (low battery), MAX (absorption voltage) and FLOAT (maintained voltage at full charge). Without Steca RCC-01 remote controller only MAX and EMPTY levels can be observed at the Steca XPC itself. Additionally the Steca RCC-01 is designed for lead-acid batteries, it doe not support LiFePO4. When detailed monitoring for LiFePO4 pack is needed, we suggest using the BMS for this purpose

http://www.ev-power.eu/Inverters-DC-AC/

Steca XPC 2200-24 integrated charger details – FAQ part 1

The XP-Compact (XPC) model offers integrated automatic battery charger. It is designed to work with lead acid or gel batteries, but it can also work with LiFePo4 perfectly, when set up properly for the LFP voltage levels.  See some FAQs for Steca XPC 2200-24:

Question: How to turn on the integrated charger?
Answer:
The Charger is fully automatic. It switches on itself if minimum alternating voltage is detected at the AC input. During the charging phase the appliances at the outlet AC OUT are continually supplied with AC power.

Question: Can I leave LiFePo4 batteries connected to Steca XPC permanently?
Answer:
Yes, internal float charge system allows you to connect LiFePo4 batteries and keep them connected. Equalization charge option must be switched off! There is small switch at the bottom of Steca XPC called “equalize.” This must be switched OFF!

Question: What is the MAX voltage for Steca XPC charger:
Answer: Integrated automatic charger stops charging when 3.6V per cell absorption charge is reached and 3.37V is maintained (float charge). Green LED “Batt 100%” goes ON.

More details: http://www.ev-power.eu/Inverters-DC-AC/ 

The Steca Solarix PI 1100 with LiFePO4 - FAQ

Q: Can the Steca PI 1100 inverters with LiFePo4 batteries?
Yes, the Steca PI1100 inverters were tested with the LiFePo4. The 24V models are designed to be used with a pack of 8 cells (24V).

Q: If batteries are getting empty, when is the load disconnected?The “Safety Low voltage disconnect function” is will disconnect the load when the battery voltage drops down (usually because of discharge). The disconnect level depends on the amount of current. If a high current is being taken from the batteries, the safety disconnect is done later than if the low currents are taken.

Q: What is the disconnecting voltage?
Our tests show following:  with high currents the load is being disconnected at 20.8V per pack, with small currents the low voltages disconnect is between 21.6V to 23.2V,

Q: Should I use the BMS?
Yes. We strongly recommend using the BMS to monitor and protect the cells on the individual cell level. We suggest always to use the BMS as additional safety disconnect at low voltage. In case of failure of the Steca inverter, the BMS would disconnect the battery pack as a protection. See additional information here: why to install a BMS and understanding the BMS operation.

Q: Does the Steca PI 1100 need restarting manually after low voltage fail-safe was activated?
No, the PI1100 inverter does not need a manual restart after the low voltage disconnect. When the voltage at the batteries returns back to the normal level, the inverter will start working again automatically.

FAQ: Battery Overcharged – What to do? 

In case of failure to charge properly, the overcharge will result in damage of the internal cell structure. In base of mild overcharge, there is some chance to recover the function of the cell.  

If overcharge happens, follow these guide lines:

1) Remove the charger and stop charging immediately.
2) Discharge the cell slowly with reasonable currents (bellow 0.1C, bellow 10A for 100Ah cell). The discharge should last until the voltage drops to normal level (e.g. 3.2V for LFP cells)
3) While discharging, open the safety valve to allow the internal pressure to release. (Not all cells have the safety valve designed to be opened. Please contact us for specific instructions.)
4) If the cells are swollen, we suggest to follow the instruction to press the cells back to their original size (see details here).

After the cells have been restored, we suggest to make some testing cycles to see the performance of the cell. (E.g. 5 to 10 cycles). For cells that were overcharged by mistake, we strongly suggest to avoid repeated charging them to full voltage. Charging to 80% ~ 90% level of nominal capacity is a good way to avoid further degradation of the cells that were already overcharged.

FAQ: Battery Overcharged – What to do?

In case of failure to charge properly, the overcharge will result in damage of the internal cell structure. In base of mild overcharge, there is some chance to recover the function of the cell.

If overcharge happens, follow these guide lines:

1) Remove the charger and stop charging immediately.
2) Discharge the cell slowly with reasonable currents (bellow 0.1C, bellow 10A for 100Ah cell). The discharge should last until the voltage drops to normal level (e.g. 3.2V for LFP cells)
3) While discharging, open the safety valve to allow the internal pressure to release. (Not all cells have the safety valve designed to be opened. Please contact us for specific instructions.)
4) If the cells are swollen, we suggest to follow the instruction to press the cells back to their original size (see details here).

After the cells have been restored, we suggest to make some testing cycles to see the performance of the cell. (E.g. 5 to 10 cycles). For cells that were overcharged by mistake, we strongly suggest to avoid repeated charging them to full voltage. Charging to 80% ~ 90% level of nominal capacity is a good way to avoid further degradation of the cells that were already overcharged.

FAQ: Low temperature operation: What is the capacity of the LiFePO4 battery at different temperatures? (at 25°C,  at 0°C,  at -20°C ?)

1) Battery capacity

For 10*C to 40*C the battery nominal capacity is 100%. For temperatures bellow 10*C the internal resistance of the battery increases and as a result the total capacity decreases. The information is published here at GWL/Blog.

at 0°C:  we estimate some 90% of available capacity.at -20°C: we estimate some 60% of available capacity.

2) Low voltage disconnect (under load)

he discharge voltage level changes (from suggested 2.8V) with the change of temperature.

The information is published here at GWL/Blog:

at 0°C: the low voltage level changes from 2.8V to 2.4Vat -20°C: the low voltage level changes from 2.8V to 2.0V

3) Battery operating temperature

We do not recommend to leave the battery unattended to become frozen to low temperatures. In cold climates, we suggest to make sure the temperature of the battery pack will stay at least at +5*C (or more).

We do not recommend operating the battery in bellow zero temperatures.4) Some tips to operate the battery in cold climate

a) Make proper insulation of the pack so that the pack does not lose temperature to outside cold environment

b) Arrange for the pack heating (during charge). Properly insulated pack may be preheated to quite high temperature (even to 30*C) to absorb the thermal energy. This way the properly insulated pack of large capacity may stay warm for several days.

c) Arrange for the pack “defrosting”. Properly insulated pack may use some part of its energy to warm itself before or at the beginning of the operation in cold climate. Usually about 5% to 8% od energy may be used to increase the temperature of the properly insulated battery pack.

FAQ: Low temperature operation: What is the capacity of the LiFePO4 battery at different temperatures? (at 25°C, at 0°C, at -20°C ?)

1) Battery capacity

For 10*C to 40*C the battery nominal capacity is 100%. For temperatures bellow 10*C the internal resistance of the battery increases and as a result the total capacity decreases. The information is published here at GWL/Blog.

at 0°C: we estimate some 90% of available capacity.
at -20°C: we estimate some 60% of available capacity.


2) Low voltage disconnect (under load)

he discharge voltage level changes (from suggested 2.8V) with the change of temperature.

The information is published here at GWL/Blog:

at 0°C: the low voltage level changes from 2.8V to 2.4V
at -20°C: the low voltage level changes from 2.8V to 2.0V


3) Battery operating temperature

We do not recommend to leave the battery unattended to become frozen to low temperatures. In cold climates, we suggest to make sure the temperature of the battery pack will stay at least at +5*C (or more).

We do not recommend operating the battery in bellow zero temperatures.

4) Some tips to operate the battery in cold climate

a) Make proper insulation of the pack so that the pack does not lose temperature to outside cold environment

b) Arrange for the pack heating (during charge). Properly insulated pack may be preheated to quite high temperature (even to 30*C) to absorb the thermal energy. This way the properly insulated pack of large capacity may stay warm for several days.

c) Arrange for the pack “defrosting”. Properly insulated pack may use some part of its energy to warm itself before or at the beginning of the operation in cold climate. Usually about 5% to 8% od energy may be used to increase the temperature of the properly insulated battery pack.

FAQ: The difference between the LiCoO2 and LiFePO4 battery technology

Learn about the differences of these two battery technologies: 1. General information, 2. Faster charging and safer performance, 3. Large overcharge tolerance and safer performance, 4.  Longer cycle life, 5.  High temperature performance.

Check and download the GWL Support document

FAQ: The difference between the LiCoO2 and LiFePO4 battery technology

Learn about the differences of these two battery technologies: 1. General information, 2. Faster charging and safer performance, 3. Large overcharge tolerance and safer performance, 4. Longer cycle life, 5. High temperature performance.

Check and download the GWL Support document

FAQ: The discharge level of the 12V batteries with PCM

** Question: I keep discharging the 12V battery with PCM. But when the voltage gets to some 11V, the voltage suddenly drops to 0V. Is the battery defective?

No, the battery is not defective. The PCM board in side this type of battery will disconnect the output when the voltage level of the cells inside the battery is getting too low. That is why the voltage will change to 0V – the battery protection will disconnect the battery.

Note: we strongly recommend not using the PCM protection as a last resort to stop the equipment from draining the battery. Follow the suggestions at this article.


** Question: When I discharge the 12V PCM battery, the voltage will drop to 0V. After that the voltage stays at 0V. I need to disconnect the battery from my equipment to get the battery to work again. This is annoying. What should I do?

The LiFePO4 battery has the protective circuit – called the PCM. The PCM will physically disconnect the battery from the load to prevent the discharging. In order to release the PCM back to ordinary function, the load must be disconnected completely. (Note: in some cases the release may take some time, until the voltage of the cells increases to safe level again.)

If you keep draining the battery, the voltage will stay at 0V (disconnected) until the equipment is turned off or disconnected. This is a proper function of the PCM battery. If you have some equipment that keep draining the battery, we suggest: A) install a mechanical switch to disconnect the battery from this equipment B) start charging the battery BEFORE the battery is fully depleted.

** Question: I discharge the 12V battery with PCM. Some of the batteries will discharge till 10.5V some to 11.2V. The label on the battery says 10V. What is wrong?

As long as the battery has the nominal capacity, nothing is wrong. The battery consists of 4 cells inside the package. Each of the cells is monitored by the PCM. The nominal voltage of the low voltage disconnect is at 2.5V per cell. This means: in case all of the cells will be discharged identically, the disconnect voltage will be at 4x 2.5V = 10V. However the cells are seldom discharged at the same level. One of the cells will always be discharged earlier. In such a situation the voltage level of the total pack will be higher. For example: 2.5V + 2.8V + 2.8V+ 2.9V = 11V. However the difference may be even bigger: 2.5V + 3.0V + 3.0V+ 3.1V = 11.6V. This is not a fault of the battery; this is the normal function of the PCM board.

In addition to the various voltage levels of the cell, the tolerance of the PCM board is about 5%. This means the low voltage-disconnect level may be around 2.5V to 2.62 V per cell. This again can cause some difference of the results from different units of the battery with PCM.

Note: in earlier deliveries of the cells the low voltage was set at 2.0V per cell, making the disconnection level as low as 8V. Based on our long term tests the setting was changed to 2.5V per cell, making the level at 10V (with tolerance between 10V to 11.6V)

Proper position of the LiFePO4 cells

The LiFePO4 should be always mounted with the terminals facing upwards. Installations with the terminals on the side is not supported. The cell operation in such a possition may result into a hazardous situation, such as the internal short circuit, buring or fire.

Avoid storing and using the cells in this kind of possition. Always store and use the cells in the regular position.

FAQ: The shipping costs of the delivery of repaired products

Question: I have a product that needs a repair. I want to send the product for repair. However I see that the customer needs to pay for the delivery on both ways. I think this is a quite bad service to pay for delivery on both ways.

Answer: First we are sorry to hear that your product needs a repair (RMA). We will do our best to provide a repair or replacement (based on the instruction from the manufacturer). For the shipping of the defective products to get the repair, the customer needs to care for both delivery (to our company) and for the return way (from our company). This is a standard situation in any business.  

See an example:  you buy a nice watch in a watch shop that is across your city. But for some reason the watch is not working fine and needs a repair from the watchmaker specialist. What do you need to do? First you need to travel to the watch shop to place the watch for repair. When the watch is repaired after some time, you need to travel to the watch shop again to take the watch from the repair.  You need to travel both ways. You always need to do this – to travel two ways – and it is a normal situation. We have never heard that a watchmaker specialist would make the delivery of the watch back to you home – you always need to go to the watch shop again.

In the same way, as a standard situation, the customer needs to take care (to pay) of the costs for the delivery to and return from our company. 

As a special benefit to returning customers, we offer FREE shipping in this way:

The customer can make a new order (after the RMA is processed and solved) and the RMA product will be shipped together with the goods form this new order without any additional shipping fee. 

Be sure to check with our team of technicians if you wish to make a new order to cover the costs of the return of the repaired (RMA) products.

FAQ: The shipping costs of the delivery of repaired products

Question: I have a product that needs a repair. I want to send the product for repair. However I see that the customer needs to pay for the delivery on both ways. I think this is a quite bad service to pay for delivery on both ways.

Answer: First we are sorry to hear that your product needs a repair (RMA). We will do our best to provide a repair or replacement (based on the instruction from the manufacturer). For the shipping of the defective products to get the repair, the customer needs to care for both delivery (to our company) and for the return way (from our company). This is a standard situation in any business.

See an example: you buy a nice watch in a watch shop that is across your city. But for some reason the watch is not working fine and needs a repair from the watchmaker specialist. What do you need to do? First you need to travel to the watch shop to place the watch for repair. When the watch is repaired after some time, you need to travel to the watch shop again to take the watch from the repair. You need to travel both ways. You always need to do this – to travel two ways – and it is a normal situation. We have never heard that a watchmaker specialist would make the delivery of the watch back to you home – you always need to go to the watch shop again.

In the same way, as a standard situation, the customer needs to take care (to pay) of the costs for the delivery to and return from our company.

As a special benefit to returning customers, we offer FREE shipping in this way:

The customer can make a new order (after the RMA is processed and solved) and the RMA product will be shipped together with the goods form this new order without any additional shipping fee.

Be sure to check with our team of technicians if you wish to make a new order to cover the costs of the return of the repaired (RMA) products.

The operation of the battery pack and its capacity
The battery should never be discharged too low. It should also never bee overcharged. You should always stay within the safe limit of the operation. See the suggestion of the capacity indication by the BMS123 software settings.

The operation of the battery pack and its capacity

The battery should never be discharged too low. It should also never bee overcharged. You should always stay within the safe limit of the operation. See the suggestion of the capacity indication by the BMS123 software settings.

BMS123 - Frequently Asked Questions
What cells can be used? Most block-type LiFeYPO4 cells, say from 60 Ah and up. As the boards are mounted on the positive pole, different lenghts of thick copper-wire  to the negative pole adjusts for different cell-dimensions. Can I use other cells then LiFeYPO4? As all voltage-levels are user programmable (between 2 and 5 Volts) other battery types can be used too. Off course, battery technology is changing rapidly, and we are always prepared to help you solve particular problems.   What charger should I specify? As the BMS-controller forms a happy marriage with any TC-charger…specify a model with the required voltage- and current- level….no need to order the CAN-bus-option, the standard three-wire interface will do!
Do the BMS-boards increase self-discharge? Yes, off course, although the BMS-boards use « 100 uA, which is roughly 1 / 50-th (!) of the self-discharge-current for a 90 Ah LiFeYPO4-cell.  The controller is also using very little current: less then 15 mA from a 12 Volt battery. Low enough to stay alive for months (!) on an average car-battery. Can I split my battery-pack? Yes, we thought about that too! Off course, we see people hide one pack in the back, and another pack in the front of the car, for instance. It is important to treat each pack separately…each pack will have to be fitted with its own “IN”- and an “OUT”-board. What if a cell gets too empty? If the cell-voltage gets way too low, that particular board would block communication, and THUS the charging. By selecting ‘Emergency charge’ the charging process can be started, even if there is no BMS-communication.
How does the balancing work? Each BMS-board is equipped with a so-called ‘programmable current-source’. Bypass-current is set to 1 Amp, irrespective of the bypass-voltage chosen.  Depending on the TC-charger used, the “minimum charging-current” should be set to ~ 1 Ampere. (For a 20 Amp. charger this would be 5 %) When balanced-charging is selected, the controller will slowly reduce the charging-current, until all cells have reached the bypass-voltage set. In quick-charging mode however, the entire charging process is carried out with maximum charging current, until one of the cells reaches the maximum-voltage set. How many cells can I use? A maximum of 255 cells can be used, albeit that the time needed to get a message through that many cells slows down the response-time of the electronic PC-dashboard notably.

BMS123 - Frequently Asked Questions

What cells can be used?
 
Most block-type LiFeYPO4 cells, say from 60 Ah and up. As the boards are mounted on the positive pole, different lenghts of thick copper-wire  to the negative pole adjusts for different cell-dimensions.
 
Can I use other cells then LiFeYPO4?
 
As all voltage-levels are user programmable (between 2 and 5 Volts) other battery types can be used too. Off course, battery technology is changing rapidly, and we are always prepared to help you solve particular problems. 
 
What charger should I specify?
 
As the BMS-controller forms a happy marriage with any TC-charger…specify a model with the required voltage- and current- level….no need to order the CAN-bus-option, the standard three-wire interface will do!

Do the BMS-boards increase self-discharge?
 
Yes, off course, although the BMS-boards use « 100 uA, which is roughly 1 / 50-th (!) of the self-discharge-current for a 90 Ah LiFeYPO4-cell.
 
The controller is also using very little current: less then 15 mA from a 12 Volt battery. Low enough to stay alive for months (!) on an average car-battery.
 
Can I split my battery-pack?
 
Yes, we thought about that too! Off course, we see people hide one pack in the back, and another pack in the front of the car, for instance. It is important to treat each pack separately…each pack will have to be fitted with its own “IN”- and an “OUT”-board.
 
What if a cell gets too empty?
 
If the cell-voltage gets way too low, that particular board would block communication, and THUS the charging. By selecting ‘Emergency charge’ the charging process can be started, even if there is no BMS-communication.

How does the balancing work?
 
Each BMS-board is equipped with a so-called ‘programmable current-source’. Bypass-current is set to 1 Amp, irrespective of the bypass-voltage chosen.
 
Depending on the TC-charger used, the “minimum charging-current” should be set to ~ 1 Ampere. (For a 20 Amp. charger this would be 5 %)
 
When balanced-charging is selected, the controller will slowly reduce the charging-current, until all cells have reached the bypass-voltage set.
 
In quick-charging mode however, the entire charging process is carried out with maximum charging current, until one of the cells reaches the maximum-voltage set.
 
How many cells can I use?
 
A maximum of 255 cells can be used, albeit that the time needed to get a message through that many cells slows down the response-time of the electronic PC-dashboard notably.

BMS123 operating outputs Ry2 and Ry3
Question: What is the function of the relay contact for the BMS123?  There is only one error indicating contact (Ry3) – How to recognize the overvoltage or deep-voltage error?
Answer: To indicate an error-condition, two relay-outputs are available: one output (Ry2) is active to indicate that the charger is still connected to the mains, and the other output (Ry3) tells you that the communication between the cell-boards is lost and/or that one cell exceeds maximum temperature and/or the cell-voltage is outside the specified range.
For the Ry3 output – it is a multiple conditions output, the voltage level conditions may be recognized in the following way: 
If there is an error state of the Ry3 without any current flow, there is a permanent problem with the battery pack. The condition of the battery pack must be inspected and the system cannot be operated until the problem is handled and solved.
If there is an error state of the Ry3 during the discharge of the batter pack (current flows from the battery), the output will most likely indicate the low voltage status. Following the error warning the flow of the current (discharge) should be stopped.
If there is an error state of the Ry3 during the charge of the batter pack (current flows to the battery), the output will most likely indicate the high voltage (overvoltage) status. Following the error warning the flow of the current (charge) should be stopped.
Conclusion: Depending on the current flow direction it is possible to distinguish the low voltage or the high voltage condition. Other conditions can read from the BMS123 status screen on the PC display.

BMS123 operating outputs Ry2 and Ry3

Question: What is the function of the relay contact for the BMS123?  There is only one error indicating contact (Ry3) – How to recognize the overvoltage or deep-voltage error?

Answer: To indicate an error-condition, two relay-outputs are available: one output (Ry2) is active to indicate that the charger is still connected to the mains, and the other output (Ry3) tells you that the communication between the cell-boards is lost and/or that one cell exceeds maximum temperature and/or the cell-voltage is outside the specified range.

For the Ry3 output – it is a multiple conditions output, the voltage level conditions may be recognized in the following way: 

If there is an error state of the Ry3 without any current flow, there is a permanent problem with the battery pack. The condition of the battery pack must be inspected and the system cannot be operated until the problem is handled and solved.

If there is an error state of the Ry3 during the discharge of the batter pack (current flows from the battery), the output will most likely indicate the low voltage status. Following the error warning the flow of the current (discharge) should be stopped.

If there is an error state of the Ry3 during the charge of the batter pack (current flows to the battery), the output will most likely indicate the high voltage (overvoltage) status. Following the error warning the flow of the current (charge) should be stopped.

Conclusion: Depending on the current flow direction it is possible to distinguish the low voltage or the high voltage condition. Other conditions can read from the BMS123 status screen on the PC display.

BMS versus Circuit Breaker – understanding the BMS operation
The Battery Management System (BMS) has a function similar to the circuit breaker in the 230V AC power network.  The circuit breaker is the last instance protection hardware to avoid problem in case there is some irregular situation on the 230V power network.
The circuit breaker is not designed to be used as a main switch, or to be used as a some regular protection device. In regular operation the circuit breaker is „not used at all“. 
The same is the operation of the BMS. The BMS protective function should be used as the last step to avoid some hazardous situations that would lead to battery malfunction and damage.
The primary protective function for the battery pack should be designed by means of the regular operation limits. For example there is a battery pack in some device and the regular operation of the battery pack will be kept within the 20% to 80% of the SOC [state of charge] of the pack. This limit can be made by some time counters (timers to charge and to discharge only for a certain time). It can be made by voltage level monitoring (e.g. N x 3.00V low voltage, N x 3.60V high voltage [N is the number of cells]), or it can be made by some capacity meters (charge and discharge energy counters). In such a situation, the BMS protective function will never become active, because the charge-discharge cycle will be always within the safe operation limits.
The secondary protective level should be done by the charging and discharging equipment. This means the equipment should have settings to stop working when there is an irregular situation. For example: the charger will charge to a maximal voltage level and it should stop when the voltage level is over-reached (typically at N x 3.65V as full charge voltage).  Or the discharging apparatus will stop at certain low voltage level – e.g. the DC/AC inverter should stop at Nx 2.80V low voltage level).  This means that in the case the primary (operational) protective level should fail, there will be another level of protection by the equipment itself. Again, in such a situation the operation of the battery pack will stay within the safe limits, without the need for the BMS to act as a final protective device.
In some installations there may be some tertiary [3rd stage] protective measures that may be applied to protect the battery pack. These may involve temperature monitoring of the battery pack, current monitoring, and voltage monitoring (for example using the CellLog devices or similar products). The tertiary protective measure will again work in case there is some irregular situation that was not handled on the primary (operational) and secondary (protective) level.
As a must !!! - Every battery pack should be equipped with DC power high current fuse to disconnect the battery pack in case of hard core short circuit. We also strongy recommend to install fire, smoke, heat alarm warning equipment to limit the risks of smoke, overheating and fire.
Finally: as the last protective measure, there is the BMS that should interact with the operation of the battery pack only in the extreme case of some irregular operation that was not handled by the above mentioned protective levels. The BMS system is a low level monitoring system that monitors the status of each cell. It should really be used exclusively as a last resort protective measure, especially in the case there is some irregular operation (e.g. dis-balancing) on the lowest level – on the individual cell voltage level.
Our recommendation is that the BMS should be always used in connection with the other protective levels.  Remember the example of the circuit breaker – in the all the normal conditions, you should always avoid the situations of needing to use the circuit breaker to stop your equipment. The same is with the BMS: it should interact only in case of rare and extreme situations.
Additional notes:
The success of the operation of the battery pack is related to the proper initial charging of the cells to the same voltage level. This means: if all of the cells in the pack are properly balanced to the same voltage level, the pack will operate very coherently (identically) and all cells will stave within the same voltage level.  Check our tips for the charging of the cells.
Additional success of the operation of the battery pack is to stay within the safe voltage levels. Simply said: there is not much benefit of discharging the battery pack to very low levels, and charging the pack to very high levels. It is the best to stay within the save limits where all the cells behave identically.

BMS versus Circuit Breaker – understanding the BMS operation

The Battery Management System (BMS) has a function similar to the circuit breaker in the 230V AC power network.  The circuit breaker is the last instance protection hardware to avoid problem in case there is some irregular situation on the 230V power network.

The circuit breaker is not designed to be used as a main switch, or to be used as a some regular protection device. In regular operation the circuit breaker is „not used at all“. 

The same is the operation of the BMS. The BMS protective function should be used as the last step to avoid some hazardous situations that would lead to battery malfunction and damage.

The primary protective function for the battery pack should be designed by means of the regular operation limits. For example there is a battery pack in some device and the regular operation of the battery pack will be kept within the 20% to 80% of the SOC [state of charge] of the pack. This limit can be made by some time counters (timers to charge and to discharge only for a certain time). It can be made by voltage level monitoring (e.g. N x 3.00V low voltage, N x 3.60V high voltage [N is the number of cells]), or it can be made by some capacity meters (charge and discharge energy counters). In such a situation, the BMS protective function will never become active, because the charge-discharge cycle will be always within the safe operation limits.

The secondary protective level should be done by the charging and discharging equipment. This means the equipment should have settings to stop working when there is an irregular situation. For example: the charger will charge to a maximal voltage level and it should stop when the voltage level is over-reached (typically at N x 3.65V as full charge voltage).  Or the discharging apparatus will stop at certain low voltage level – e.g. the DC/AC inverter should stop at Nx 2.80V low voltage level).  This means that in the case the primary (operational) protective level should fail, there will be another level of protection by the equipment itself. Again, in such a situation the operation of the battery pack will stay within the safe limits, without the need for the BMS to act as a final protective device.

In some installations there may be some tertiary [3rd stage] protective measures that may be applied to protect the battery pack. These may involve temperature monitoring of the battery pack, current monitoring, and voltage monitoring (for example using the CellLog devices or similar products). The tertiary protective measure will again work in case there is some irregular situation that was not handled on the primary (operational) and secondary (protective) level.

As a must !!! - Every battery pack should be equipped with DC power high current fuse to disconnect the battery pack in case of hard core short circuit. We also strongy recommend to install fire, smoke, heat alarm warning equipment to limit the risks of smoke, overheating and fire.

Finally: as the last protective measure, there is the BMS that should interact with the operation of the battery pack only in the extreme case of some irregular operation that was not handled by the above mentioned protective levels. The BMS system is a low level monitoring system that monitors the status of each cell. It should really be used exclusively as a last resort protective measure, especially in the case there is some irregular operation (e.g. dis-balancing) on the lowest level – on the individual cell voltage level.

Our recommendation is that the BMS should be always used in connection with the other protective levels.  Remember the example of the circuit breaker – in the all the normal conditions, you should always avoid the situations of needing to use the circuit breaker to stop your equipment. The same is with the BMS: it should interact only in case of rare and extreme situations.

Additional notes:

The success of the operation of the battery pack is related to the proper initial charging of the cells to the same voltage level. This means: if all of the cells in the pack are properly balanced to the same voltage level, the pack will operate very coherently (identically) and all cells will stave within the same voltage level.  Check our tips for the charging of the cells.

Additional success of the operation of the battery pack is to stay within the safe voltage levels. Simply said: there is not much benefit of discharging the battery pack to very low levels, and charging the pack to very high levels. It is the best to stay within the save limits where all the cells behave identically.

The alarm output for CellLog
One of the Cell Logger’s functions is the alarm output. The Alarm output can be triggered according to several settings of the CellLog. It uses the open collector design - this kind of output can be found on many integrated circuits or electronic modules.
How to connect a DC relay to this alarm output: The output forms either an open circuit or a connection to ground. This gives you the advantage of switching loads up to 50V and 500mA. In other words it can be possible to control 12/24V relay easily. In most cases the power to trigger such a relay will be provided from a separate power supply (e.g. isolated DC-DC). In case you connect the relay to the battery directly, please note that ground pin of the ALARM is not isolated! It is connected to the GND of the battery pack.
See an example connection of 24V relay to 8 cell LiFePO4battery pack. This way you can turn off your charger or disconnect the main contactor in case alarm is triggered. Do not forget to connect the protective diode parallel to the relay-coil otherwise you will most probably break the output!

The alarm output for CellLog

One of the Cell Logger’s functions is the alarm output. The Alarm output can be triggered according to several settings of the CellLog. It uses the open collector design - this kind of output can be found on many integrated circuits or electronic modules.

How to connect a DC relay to this alarm output: The output forms either an open circuit or a connection to ground. This gives you the advantage of switching loads up to 50V and 500mA. In other words it can be possible to control 12/24V relay easily. In most cases the power to trigger such a relay will be provided from a separate power supply (e.g. isolated DC-DC). In case you connect the relay to the battery directly, please note that ground pin of the ALARM is not isolated! It is connected to the GND of the battery pack.

See an example connection of 24V relay to 8 cell LiFePO4battery pack. This way you can turn off your charger or disconnect the main contactor in case alarm is triggered. Do not forget to connect the protective diode parallel to the relay-coil otherwise you will most probably break the output!

FAQ: What is the quality inspection for solar panels?

During production the solar panels are inspected several times and electrical parameters are tested repeatedly to make sure the panels are 100% quality.

The cells received from the cell factory come with an inspection report giving test results for each cell. In the module factory, the cells are once more inspected and sorted to several subcategories based on their nominal specifications from the manufacturer. The goal is to select groups of cells that match perfectly.

Each group of cells is once more tested by light exposition to verify that the specifications of the cells within the group are really identical.

After the cells are soldered together they make a section (also called a branch). Again, each section is inspected and tested electrically to make sure the parameters of the section are identical after the soldering is done.

The sections are placed together on the back sheet of the solar module. In this moment the whole module is inspected by infra-red camera to see any discrepancies or problems. The infra red camera will discover any kinds of possible quality problems.

The goal is to be 100% sure about the quality before the lamination of the solar module. The reason is simple: after the module is laminated to the front glass, there is no more way to repair or change anything on the module.

After the model is laminated, the alum frame is added and the junction box is installed.  The final product is tested again on the illumination testing machine. The machine will report the total specification of the solar panel including the MPPT values.

Due to the strict inspection, it is possible to make sure that all panels are within the guaranteed parameters.  It is mostly unlikely there would be a solar panel that would have a manufacturing defect that would escape the strict inspection.