A hybrid battery system for power storage in a power network, the power network comprising a power source and a load, the hybrid battery system comprising: a battery system comprising: a first rechargeable battery unit having a first battery chemistry; and a second rechargeable battery unit having a second battery chemistry, the first and second battery chemistries being different from each other; a control system configured to selectively couple the battery units (20, 40) of the battery system to the power network a) for delivery of electrical energy from the battery system to the load during discharging, and b) for receipt of electrical energy from the power source for charging of the battery system during charging a power supply voltage sensor configured to detect an operating voltage of the power being delivered to the load from the power source, the control system comprising logic configured to switch the control system between charging and discharging of the battery system based on the operating voltage being in a charging range or a discharging range, respectively; characterised in that the control system further includes charging mode selection logic configured to select between modes of charging based on the operating voltage within the charging range, the modes including: a) charging both said battery units (20, 40) simultaneously each at a maximum charge rate when the operating voltage is above a first charge threshold, b) charging the first battery unit at the maximum charge rate thereof and the second battery unit at a decreased rate when the operating voltage is below the first charge threshold and above a second charge threshold, and c) charging only the first battery unit at a decreased rate when the operating voltage is below the second charge threshold. a battery system comprising: a first rechargeable battery unit having a first battery chemistry; and a second rechargeable battery unit having a second battery chemistry, the first and second battery chemistries being different from each other; a first rechargeable battery unit having a first battery chemistry; and a second rechargeable battery unit having a second battery chemistry, the first and second battery chemistries being different from each other; a control system configured to selectively couple the battery units (20, 40) of the battery system to the power network a) for delivery of electrical energy from the battery system to the load during discharging, and b) for receipt of electrical energy from the power source for charging of the battery system during charging a power supply voltage sensor configured to detect an operating voltage of the power being delivered to the load from the power source, the control system comprising logic configured to switch the control system between charging and discharging of the battery system based on the operating voltage being in a charging range or a discharging range, respectively; characterised in that the control system further includes charging mode selection logic configured to select between modes of charging based on the operating voltage within the charging range, the modes including: a) charging both said battery units (20, 40) simultaneously each at a maximum charge rate when the operating voltage is above a first charge threshold, b) charging the first battery unit at the maximum charge rate thereof and the second battery unit at a decreased rate when the operating voltage is below the first charge threshold and above a second charge threshold, and c) charging only the first battery unit at a decreased rate when the operating voltage is below the second charge threshold.