Performance

 

The unique and patented Nilar Hydride® battery utilizes a bi-polar design; the cells are stacked horizontally to gain maximum space efficiency. This contributes to making the battery a reliable source of power for more than 20 years. In addition, the Nilar Hydride® battery can handle high charge and discharge rates and can perform under harsh weather conditions.

Important Battery Terms

When talking about batteries, there are some terms and abbreviations that are important to know. These terms are not set in stone and can differ among people, but this is the way we define them.

  • State-of-Charge (SOC) is the energy remaining in the battery in percent. For example, if a battery has 50 % SOC, it has half of its rated energy remaining.
  • The Operating Window is the range in which the battery is used in terms of SOC. For example, if an operating window is from 20% to 80%, the battery is cycled between 20% SOC and 80% SOC in operation.
  • The Depth-of-Discharge (DOD) is the size of the SOC operating window. A SOC operating window of 20 % SOC to 80 % SOC will give a DOD of 60 %.
  • A battery’s lifetime is often expressed in cycles, where one cycle is defined as charging from one SOC to another and back. For example, charging from 20 % SOC to 80 % SOC and then discharging back to 20 % SOC is one cycle with a DOD of 60 %. Charging from 0 % SOC to 100 % SOC and back is also a cycle, sometimes denoted as a ‘full cycle’.
  • The C-rate is the rate of charge or discharge for a battery, with a unit of 1/h. A discharge C-rate of 2 for example will discharge a battery from 100 % SOC to 0 % SOC in half an hour, while a C-rate of 1/2 will discharge it in 2 hours.
  • Capacity is a meassure of how much charge can be stored in a battery with a unit of Ampere-hours (Ah). It can be likened the size of a fuel tank as it shows how much can be stored in the battery. For example, a battery with a capacity of 10 Ah can store and later discharge 10 A for one hour. The capacity of a battery can also be given in percent of its rated capacity, to show how much charge can still be stored in the battery.

Discharge performance

After a moderate initial voltage drop the discharge voltage is stable over more than 80% of the discharge and ends with a distinct knee at end of the useful capacity. Discharge voltage is dependent on discharge rate, temperature and the state-of-charge. The discharge voltage decreases with increased discharge rates and decreasing temperature. The discharge performance is affected moderately at elevated temperatures, but at low temperatures the effect of temperature on discharge performance is more pronounced due to the increase in resistance. The discharge capacity, or the run-time to end of discharge, is mainly affected by discharge rate and temperature. Delivered capacity depends on the selected discharge cut-off voltage and decreases with increasing discharge cut-off voltage.

Test currents are expressed as multiples of the rated capacity value detoned by “C” (e.g. a discharge current of 2C for a Nilar battery pack with a rated capacity of 10 Ah equals 20 A.)

Voltage drop-off depending on discharged capacity & temperature

Figure 1.
Constant current discharge
with 0.2C at various temperatures. The battery was fully charged at +20°C, acclimatized for 12 h at test temperature and then discharged.

Voltage drop-off depending on discharged capacity & C-rate

Figure 2.
Typical voltage profiles for constant current discharges with various discharge rates. Charging and discharging made at +20°C.

Discharge capacity over temperature range

Figure 3.
Constant current discharge capacity at various temperatures. The battery was fully charged at +20°C, acclimatized at test temperature for 12 h and then discharged with 0,2C to 1 V per cell at test temperature.

Charge Performance

The recommended charge procedure is constant current charge with charge termination based on rate of temperature increase (dT/dt), together with a maximum allowed pressure and pack temperature. Nilar battery packs can be charged with power ranging from zero up to fast charging with 3C. As a standard charge, the recommended charge rate is 0.3C with a rise in temperature, dT/dt, corresponding to 0.3°C in 2 minutes. The charge procedure can be used for charging battery packs with battery pack temperature in the range of -20°C to +50°C. Within this temperature range, a fully discharged battery is recharged within 3.5h. Charging shall be terminated when either the maximum allowed battery pack temperature or pressure is reached, which are 58°C and 5.5 bar, respectively.

An inherent feature of the NiMH electrochemical system at charging is the build-up of pressure and temperature at the end of the charge. The unique battery pack pressure sensor, integrated in Nilar battery packs, together with measured battery pack temperature, are efficent means to secure charge termination over the whole temperature and power range. At low temperatures the charge rate can be limited by an increased voltage. At elevated temperatures the maximum charge rate is limited by the rise in temperature and pressure at end of charge.

 

Charge characteristics

Figure 4.
Typical charge characteristics at +20°C. Constant current charging with 0.2C.

 

Cell Balancing

All batteries in a system are not identical, they often have small differences in energy capacity. When each string is connected, the batteries are matched to make sure that each string contains batteries that are the utmost alike. As the battery string is cycled, the SOC of the individual batteries will become uneven, and the energy capacity is lowered.

One way to restore and even out the energy capacity of the batteries is to perform a cell balancing. This procedure consists of charging the batteries to 100% SOC, followed by a short wait, and then charging the batteries again. This is performed to push the batteries with lower capacity back to their original maximum capacity.

A cell balancing function is built into the Nilar BMS and can be triggered by the owner of the system. Although cell balancing is necessary from time to time, it is not recommended to do this too often, as this is a process with higher losses.

Self discharge

The state of charge of a Nilar battery pack during storage slowly decreases with time due to self discharge. The self discharge is caused by internal electrochemical reactions that slowly discharge the battery. The self discharge rate is high over the first days of storage, but then levels out to a few percent per month depending on temperature. The rate of self discharge is increased at elevated temperatures and decreases at low temperatures. A fully charged Nilar battery pack stored at +20°C will lose about 6% capacity after one day and 13% capacity after 28 days. Parasitic loads on the battery from charger, load and electronic systems will increase the rate of capacity loss during storage. This is common with all NiMH chemistries.

Self discharge over various temperatures

Figure 5.
Self discharge when charged to 75% SoC. Typical charge retention at +20, +40 and 0°C at various storage periods.

Cycle life as a function of SoC-window

Figure 6.
Cycle life as a function of SoC-window.

Cycle life

Cycle life is the number of charges and discharges a battery can achieve before the discharge capacity drops to a predetermined capacity. A number of circumstances have to be considered when estimating cycle life. The largest impact on the cycle life comes from the battery pack temperature, depth of discharge and the charge procedure. The more shallow a battery is cycled, the higher the number of cycles until the battery is unable to sustain the required service.

One of the superior features of the Nilar Hydride® battery is the very stable performance throughout its lifetime. Typically, the impedance of a battery increases when the battery is used. This results in reduced run time and finally, depending on how end of life is defined, the battery is not able to perform as required. The stable and well defined performance over life experienced with Nilar batteries is a consequence of the intrinsic features of the NiMH technology together with the high manufacturing quality gained by the Nilar patented bi-polar design. The main aging mechanism is dry-out, causing a slow increase in impedance over cycles. Capacity is not detoriated during cycling. NiMH batteries can be stored for many years without loss of performance. There is no decomposition of the electroyte at full charge nor solid electrolyte interface consuming charge carriers with detrimental effect on capacity and impedance. Cell impedance in a NiMH cell is determined by the amount of electrolyte in the separator. Over time, the electrolyte in the separator decreases (dryout) with a slow decrease in conductivity. Finally, depending on the load, the run time of the battery is down to a level where the battery is considered as spent. End of life is often defined as 80% of initial capacity but can be based on other application specific constraints or capacity levels.