The batteries are one of the most expensive and heavy items in the car. This is really a case where the more money you spend the lighter the batteries get.
Originally I was going to use lead acid batteries. These are similar to normal car batteries but have bigger plates so they can be deep discharged. These are the cheapest but are also the heaviest and have have problems charging and discharging quickly.
I am now working on Lithium Ion batteries. These weight well over 3 times less than lead acid batteries and can be charged and discharged quickly. They are very expensive so I am currently testing recycled lithium ion laptop batteries. Laptop batteries usually have a capacity of 50-100wh and have 6-12 lithium ion cells that are about the sized of a standard AA battery.
These cells are often connected in a way that if one cell is dead the whole battery will not work. Many people throw out batteries packs that have 5 cells that are still functional. I am collecting used laptop batteries that are sold for a very low price and removing all the good cells. I can then connecting them to create very powerful batteries. I would need to have about 600 functioning cell to create my 6000wh battery for the solar thermal car. I will be testing smaller homemade Lithium ion batteries packs in the electric bicycle electric drive test unit and use one as a test battery in the solar test units. I will be making car battery sized groups of cells that I can connect together to form the large solar thermal car battery. The batteries will be connected in groups of parallel cells to increase capacity and in series to increase voltage.
The cells come in different capacities but are all around 3.7 Volts. It is important that I can connect the cell groups in a way to get the required high voltage for the the large drive motor of over 70V. I also need to think how these cells will be charged. Lithium ion cell can be badly damage by over or under charging. A charge controller is need that can measure the voltage in each cell to balance all their voltages by adjust the current to each cell. A charge controller will be purchased or I will use the Arduino if possible.
I am still looking into other battery types. I am working on battery tables that compare the cost, weight and discharge rate of different battery types. Below is an early table. Weight, discharge rate and cost values are for the entire solar thermal car large battery bank of 6000kwh.