Solar Controller

The solar controller looks after the batteries and stops them either being overcharged by the solar array or over-discharged by the devices running off the batteries.
Many solar controllers also include an LCD status screen so you can check the current battery charge and see how much power the solar array is generating.
More expensive solar controllers may have the option of a WiFi or Bluetooth link and an app to run on your mobile phone, so you can monitor the performance of your system.
Your choice of solar controller