#chargefaq

FAQ: What is the Cycle Life Estimation of the Elerix Cells

As given in the datasheet the cycle lifer is specified by the charging and discharging speed, the depth of discharge and the operational temperature.

The typical life span is given at this: 

Cycle Life at 80% DOD ---- 3000 cycles ---- @0.3C/1C, @25°C, >80% SOH

This means that for the charging with 0.3C and discharging with 1C, at the 80% depth of discharge (80% DOD) and the monitored temperature 25°C, the cell will age to 80% of State of Heath (SOH) in 3000 cycles.

The 80% SOH is the typical “end of life value” for the LiFePO4 technology, meaning that the nominal capacity of the cell will drop from the original 100% to 80%. 

In this scenario the cycle takes following time:  discharge 80% at 1C  - about 50 minutes, rest time 1 hour, charge at 0.3C – about 3 hours, rest time 1 hour.  Total time per cycle is 5 hours.  

With some time-reduction on the rest time, it is possible to perform up to 5 cycles in a day. This means that 3000 cycles correspond to 600 days of continued cycling. This is about 1.6 years.

ESS Cycle Life at 60% DOD ---- 6000 cycles ---- @0.3C/0.3C, @25°C, >60% SOH

The ESS cycle life is given at 0.3C charging and 0.3 discharging at the 60% depth of discharge (60% DOD) and the monitored temperature 25°C.  This corresponds to a typical usage in energy storage applications (ESS).

In this scenario, two cycles per day are calculated as the typical performance. With this number of cycles, the life span of 6000 cycles corresponds to 3000 days of continued cycling. This is more than 8.2 years of the continued usage. 

The above graph show some possible scenarios of the life spans for the typical ELERIX products in various modes of operation.

Please take note that the crucial condition for the long-time operation is the contact temperature of the cells, and the limiting of the current peaks to the maximal continued values. 

What is the real self-discharge of the LFP cells?

Question: I have received a shipment of LFP cells from you company. When I received them, the voltage was about 3.25V for each cell. Now the voltage is below 3.10 for some cells and some have even just 3.0V or even less. Why is this so?

It is important to realize that cells shipped from GWL have only a partial capacity. 

The DG cargo safety regulation limits the maximal permitted charge to a value below 30% of SOC. In really it means that the cells have less than 30% of the SOC.  

RECHARGE: When you receive the cells, you should check the shipment first and charge the cells to full capacity as soon as possible. If you leave the cells unattended the cells continue to self-discharge.

The average self-discharge for LFP cells is between 3% to 4% per year. This means that fully charged LFP cells will stay with energy for many consecutive months.  However, the self-discharge rate increases in some conditions:

The self-discharge will increase for the cells that are nearly empty. These cells may self-discharge as much as several amps in during a few weeks. The empty cells self-discharge quickly.

Also, the self-discharge rate will increase when the cells are subjected to changing conditions. For example, if the cells are stored in warehouse with no significant changes of temperature, the discharge cells will not change.

However, if the cells are taken form the warehouse and shipped to customer, they will be subjected to changing conditions --- temperature changes, the change of the position during transportation, etc.  This will result into an increase of the self-discharge, especially in cases when the cells themselves are already nearly empty.

FAQ: Operation of the LFP cells at high temperatures above +50°C

Question: I saw that some LFP cells allow the operation up to 65°C or even more. Do you have any suggestions for a high temperature operation?

Answer: The GWL team strongly discourage the continued use of the LFP cells in temperatures above +50°C.  

Even though some datasheets may give the temperature above +50°C, it is in fact only the maximal peak temperature for a short time span or for a quick-time extreme.

For majority of the cells, the maximal operation temperature is +50°C. This is the limit when the operation of the cells must be stopped. It is not possible to keep using the cell if the ambient temperature or the temperature of the cell (or of the terminals) has reached this limit.

The recommended operational temperature for the cells is +25°C. This means there is a need to arrange for some ventilation or cooling system in areas where the ambient temperature would be significantly higher.

It is important to note that the for higher temperatures --- above +25°C --- the operation performance of the cells decreases.  This means that while the cell can be put in operation for 1C or 2C currents at +25°C, it is necessary to limit the charge/discharge rates for higher temperatures. It is not possible to operate the cell at the maximal limits, at the high temperatures, otherwise the cell would over heat and would be damaged.

All of these topics are fundamental design ideas and principles for the operation of the battery pack. This technical expertise may go beyond the basic information of the datasheets. If you need to get this knowledge, the GWL team will be happy to provide you with more insight by means of a paid technical support.

As a free support, the GWL team has published some design ideas related to high temperatures. Check these articles:

The issues of overheating of the cells in large packs

Thermal imbalancing of the LFP packs - air-flow cooling (1)

Thermal imbalancing of the LFP packs – direct sunshine (2)

FAQ: Can the lithium technology be used at 85°C?

FAQ: Testing the capacity of the cell or battery

Q: I wish to check the capacity of the cell in my battery pack.  How to do that? 

Answer: Originally there were not any suitable products in the market that would allow the quick and easy testing of individual cells or the packs in a battery. There were only some costly and professional battery testers.

However recently due to a growing share of the DIY (Do-it-Yourself) market, there are more and more ready to go products.  These products are called:

Digital Battery Capacity Tester Electronic Load Intelligent Adjustable Constant Current Discharge Resistance Power Tester or Adjustable Constant Current Electronic Load Battery USB Tester DC 12V 24V Lead-acid lithium Discharge Capacity Meter

https://www.aliexpress.com/item/33002861589.html https://www.aliexpress.com/item/32821877897.html

GWL does not resell and promote these products. Customers can review the specifications by themselves and make an order for these cell/battery testers directly at the traders who sell them.

FAQ:  Testing cells or packs at a customer location

Question:  I have a battery pack installed in my EV (in my boat, in my RV, in my house, etc.) I wish to check the condition of this pack to see the real capacity of the cells. Can GWL offer any solution for me? (Perhaps you can come to me for the weekend and make the test. Or you can send me some equipment to make the test. You know I am your old customer.)

ANSWER: Thank you for this question. However, at this moment GWL cannot offer any solution to your request. The GWL Team does not arrange for a remote service at customer location. This means “we cannot come to you”. Also at this moment, GWL Team does not have any device or testing equipment that could be rented to customers to make such tests. Additionally, making the testing of several (or many) cells at the same time requires quite costly equipment that GWL does not have, and thus cannot sell or lend.

The only suggestion or service that GWL can offer (at this moment) is to bring the cells to GWL: You can arrange with GWL Sales Team for a paid inspection of the cells at GWL’s facility in Prague. This means, you need to dismount the cells and deliver these to GWL. In this case, GWL Team can make the tests of individual cells and provide you with the test data for each cell.

Check details for the inspection and charging fees and also for the paid support.

EV Technical Support and Services

FAQ: Voltage level for the LTO cells

Question: Concerning the LTO cells, I can see that some information mentions the nominal voltage at 2.3V some other datasheets say 2.4V. I wonder what is the nominal voltage of the 5-cell pack: 11.5V or 12V?

Answer:  both versions of the information are correct. The reason is that the discharge voltage of the LTO cell is between 2.45V to 2.0V. The charge voltage of a single cell is between 2.15V to 2.8V. (valid for 1C rate and 25°C)

This means that the voltage of the 5-cell pack is from 12.25V to 10V. The charge voltage of the pack is 14V.

See more here: 

FAQ: Determining the number of cells for a battery pack

Cell Testing and Parameters - Additional Services for the Customers

Question: I wish to get a finely tuned battery pack, so I request that all cell delivered to me are charged to the full, finely balanced and have exactly the same capacity and internal resistance. How do we do that?

Answer: GWL Team will be happy to assist you. Please keep in mind that the cells are provided as they are, based on the specifications listed in the datasheets. Any extra services, like charging, capacity retesting or additional verifications of technical parameters is offered as paid services.

For majority business cases, it is the responsibility of the customer to verify the goods upon the receipt and bring about any claims in a case when things go wrong. Customer should verify the specification and report to the distributor. It is the responsibility of the distributor to provide the warranty services in case any discrepancy.

Also, please keep in mind that for international shipping, the cells must be discharged below 30% of capacity during the transportation. For this reason, GWL cannot offer charging and balancing of the cells that are going to be shipped to the customer by a transportation company. Only in the situation, when the customer comes personally and takes the goods by his own private method of transportation, GWL can offer the charging service.

For VIP customers and for large scale projects, it is possible to make an agreement that will include some special services at extra service fee. However, the minimal order requirement is USD 5000 per order, as listed at the GWL web page.

To be or not to be? -  Balancing cells with a single cell charger

Recently GWL/Team has received some feedback from the customers related the single cell charges for balancing the single cells individually – one by one.

It seems that these charges have some “hysteresis of their behavior” – in the way they behave when the cells are nearly fully charged. These chargers stop operation when the full voltage is achieved and after the current is reduced. However there maybe a certain tolerance (typically 3% to 5%) as to the exact levels of the capacity when the chargers stop charging.

As a result, when using these chargers to perform a very fine balancing, the state of charge for each cell may end up within a certain difference (or tolerance). Please keep this in mind. 🙂