An assembly of batteries includes a first battery and a second battery electrically connected in parallel. The first battery is configured to deliver a battery capacity in a first power supply voltage range. The second battery is configured to deliver a battery capacity in a second voltage range. An upper limit of the second voltage range is set between upper and lower limits of the first voltage range. In an operating system, if supplied battery power falls below a threshold, the parallel connected first and second batteries are disconnected from the load.