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.

Charging

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. The charge procedure can be used for charging battery packs with battery pack temperature in the range of -10°C to +40°C. Within this temperature range, a fully discharged battery is recharged within 3,5 hours.

An inherent feature of the Nilar Hydride® 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 efficient 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.

Self discharge

The state-of-charge (SOC) 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 Hydride (NiMH) chemistries.

Self discharge over various temperatures

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

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. Among the most important are temperature, charge method, charge and discharge rates, depth-of-discharge (DOD) and environmental aspects. The largest impact on the cycle life comes from the battery pack temperature, the charge procedure, and the SOC operating window. 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 Nilar batteries is the very stable performance over life. 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 Nilar Hydride® technology together with the high manufacturing quality gained by the Nilar patented bi-polar design. The main ageing mechanism is dry-out, causing a slow increase in impedance over cycles.

Capacity is not deteriorated during cycling. Nilar Hydride® 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 Hydride (NiMH) cell is determined by the amount of electrolyte in the separator. Over time, the electrolyte in the separator decreases (dry-out) 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.

Illustration of cell balancing.

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.