Failure Prediction of NiMH Batteries

Abstract: This master thesis has been carried out to investigate the possibility to predict battery failure of Nickel-Metal Hydride (NiMH) batteries. Assa Abloy use these batteries as a back-up in their door solution systems in case there is a power loss. The main function is to be able to open doors that are part of an emergency exit route. Thus, approximately 200 battery packs were used to collect data regarding mainly their internal resistance, capacity and age counted from production date. These parameters were then mapped to how many consecutive door openings they could perform at different temperatures before failure. The study showed that the internal resistance of the battery has a great impact on its performance. The temperature also has a big effect because the internal resistance varies with temperature. At a late stage of the master thesis, it was seen that the open circuit voltage during charge was affected by the temperature. However, more investigation regarding this needs to be done. Furthermore, the capacity does not seem to have a notable impact and neither does the age. The key to predict battery failure is to have knowledge regarding how the internal resistance changes over time as well as how it changes with temperature. Some insight regarding this has been discovered during the study but further investigation needs to be done.