This paper is intended as the first part of a two paper series that evaluate the efficiency of a battery pack with charger that has an integrated cell balancing method. For the first paper a simple passive balancing method is used while for the second an active shared transformer balancing method will be used. The paper first discusses the existing works on the subject of cell balancing efficiency. Further the results of experimental testing of cell capacity are provided. A previously designed modular passive balancing system is briefly introduced as it is used for the battery efficiency evaluation. Testing results reveal the battery pack efficiency with balancing as the difference between the energy that is required to charge the pack and the energy that can be obtained when the battery the battery is discharged. While the first efficiency test uses a laboratory power supply to charge the battery pack, the second efficiency test uses a prototype charger circuit. The same charger circuit with different secondary side is to be used for the second article. The prototype charger is required so that the efficiencies of both passive and active balancing methods can be reliably compared.