FAQ

Posted by elite

1. What accessories come with the batteries ?
2. Why are the cells strapped as 4-cell packs ?
3. How should Li-ion Batteries be stored ?
4. What can potentially damage batteries ?
5. What are the best practices to maximize the lifetime of Li-ion batteries ?
6. How much current can I draw from my battery pack ?
7. How many cells do I need to have for my battery pack ?
8. What cell size is most suitable for my application ?
9. What is the recommended charging process ?
10. Can I adjust the output voltage of my charger ?
11. What are the common failures of the Energy Management System ?
12. What do I have to do for my EMS system if my battery configuration changes ?
13. How much torque should I put in the battery screws ?

1. What accessories come with the batteries ?

Our standard configuration is 4-cell pack. The batteries come with strapping hardware installed. Screws, washers and jumpers are provided with battery purchases with no additional cost.

When large format prismatic cells are under high current load, over-charged or over-discharged, they tend to swell or deform. Swelling will result in severe degradation of capacity. In the worst case scenario, swelling will result in internal shorts. Strapping is to prevent the cells from swelling. We use aluminum plates and steel bands for strapping. Our standard configuration is 4-cell pack. Our factory can make customized strapping configurations such as 6-cell pack or 8-cell pack, if there is significant volume. The steel bands are made to specific lengths to achieve the best results of preventing swelling (it is not based on tension of the steel bands).

Our batteries have characteristics of very low self-leakage rate, ~ 3% per month, which allows batteries to be stored over extended period of time. Batteries are to be charged to 40-60% full and disconnect all loads before long term storage. Batteries must be stored in cool temperature environment. Avoid batteries being exposed to summer heat. Users are recommended to check cell voltages periodically, e.g. every 2 weeks, to ensure proper voltage level. If cell voltage drops close to 3.0V or under 3.0V, battery needs to be recharged.

If sense boards are left installed during storage the green led will cause a slight drain which roughly doubles the self-discharge rate which will necessitate checking cell voltages more frequently. A simple way to disconnect the sense boards is to remove the two screws from one side of the sense board and slide a piece of plastic or other non-conductive material between the battery terminal and sense board to disconnect it from the battery.

Over-discharging, over-charging, operating outside of the current specification limits and operating outside of the temperature specification limits are the most common ways of damaging the batteries.

During charging, not using a proper charger and no EMS monitoring and protection against over-voltage can lead to an over-charging condition.
During discharging, no EMS monitoring of cell voltages can result in cell voltages dropping below low voltage limit, which lead to over-discharge condition.
Drawing extreme high current (>3C) over extended period of time (>20seconds) can result in damages to the batteries.
Proper operating temperature ranges are -20 to 65C for discharging and 0 to 65C for charging. Not operating and storing batteries within these temperature ranges can result in damages to the batteries.
Mixing batteries with different characteristics, voltage ranges, capacities, internal resistance, ages, which may lead to uneven loading across batteries, can potentially result in over-charging and over-discharging conditions.
Putting batteries in parallel without EMS monitoring and control can lead to unbalanced charging and discharging conditions.

Cell voltages, depth of discharge, operating temperature and current draws are the key factors determining the lifetime of the batteries. Li-ion battery cells must be operating within the specified voltage range at all times. Over-charging and over-discharging will result in reduced battery lifetime or even kill the battery. Equipping the battery with an EPS Energy Management System (EMS) is the best way to prolong the lifetime of the battery, by preventing over-charging and over-discharging conditions. The EMS needs to be configured to automatically shut down the charger when the battery is full and automatically shut down and eliminate all current consumptions (such as motors, DC/DC, lights, and even parasitic current draws) when the battery is empty. Cell balancing feature of the EMS system is also very helpful to maximize the usable battery capacity while minimize the chances of over-charging and over-discharging.

Our batteries have no memory effect. Batteries do not need to be completely discharged before charging. In fact, reducing the depth of discharge (%DOD) can significantly increase the number of cycles. Therefore we recommend charging the batteries as often as possible instead of waiting for the batteries to be completed drained.

Operating the batteries in extreme weather conditions (extreme heat and extreme cold weathers) will shorten the battery lifetime. Having a climate control system to maintain the battery temperature around room temperature will help to prolong the battery lifetime.

High current draw during discharging and fast charging current will also reduce cycle life. Charging and discharging batteries at 0.5C will produce the best result of cycle life.

Cell voltage drops with increasing current draw. In other words, cell voltages sag with high current draw. Voltage sag may become more significant when current draw is over 2C. We recommend impulse current draw to be limited to 3C and continuous current draw to be limited to 2C. Impulse current draw must be limited within 20 seconds. Extended period of high current draw (over 3C) will shorten the lifetime of batteries, and may even damage the batteries.

Each cell has nominal voltage of 3.2V (2.5V min and 4.25V max). Based on your electric motor voltage demand, you can decide how many cells you need. If you have a 144V system, the number of cells in your battery pack will be 144V / 3.2V = 45 cells.

After you decide how many cells your battery pack requires, you need to decide what cell type or capacity for your application. This depends on power consumption of our application and the desired run time. For electric vehicle applications, your driving habit strongly affects energy consumption rate (kWh/mile). Range drops with increasing speed. Under Technical Reference session of this website you can find range vs. speed curves (EV Application Calculation). A typical small sedan consumes about 0.25 kWh of energy per mile. Based on the desired conditions for your application (vehicle weight, average speed, range, etc.), you can decide how much energy you need in your battery pack (kWh), which will then decide the capacity of your cell choice. For example, if you need 28.8kWh capacity and you have decided on a 45 cell pack based on the voltage spec of your motor, 28800 kWh / 45 cells / 3.2V per cell = 200Ah. You need cells with 200Ah capacity.

All EPS chargers use a 3-step charging program: constant current, constant voltage and re-charging steps. At the final re-charging step, you will notice that the charger is on for a minute and off a minute and repeating, which is normal. The main purpose of the re-charging step is to balance the cells. When the charging process is complete, all the green LEDs on the charger will be lit.

Our chargers have fixed output voltage for best charging result. Each charger is designed for a certain pack size, such as 8 cell, 12 cell, 24 cell packs. The output voltage is not adjustable.

Electrical shorts are the most common killers for the EMS system. Most common locations shorts can occur are:

Between 12V and GND pins of the power supply inputs to the CPU
Between OV, UV and GND alarm outputs of the CPU
Between shunt connection points on the CPU.
Shorts introduced on the sense board strings (typically by dropped screws or screw drivers touching connectors)

Actions to prevent shorts:

Install sense board strings before the CPU is powered
Install battery cover after each sense board string is installed on the battery
Use insulated connectors for 12V,GND, OV, UV, and shunt connecting pins on the CPU

The CPU of our Energy Management System (EMS) is programmed for a specific battery pack configuration: cell size, number of cells and pack configuration (number of cells in parallel and series). If any of the conditions changes, the CPU of the EMS system needs to be sent back to us (or to one of our authorized distributors) for re-programming.

There have been several types of screws used on our batteries and the torque varies with each. It is important to ensure that all fasteners are securely torqued to ensure a good connection. The unique terminal design does not require an excessively torqued fastener to obtain a good connection:

Philips head M4 screws – 12 in-lb
Hex head 2.5mm M4 screws – 12 in-lb
Torx head T20 M4 screws – 30 in-lb
Hex head 4mm M6 screws (200Ah cells only)– 40 in-lb