These automotive battery boosters are thrown away after they lose their capacity to start a car (car starter circuits have very high current demands), so you can usually get them for free from friends or family after they have outlived their usefulness. Sometimes there is enough capacity left in the battery on these throwaways for lower current uses and can be still be useful for your project for powering small motors or electronic controllers (check the power requirements of your motor or system). If the battery is completely dead, you can buy a fresh one, if it is replaceable. Just buy a new battery with the same voltage and specifications as was in it before.
However a brand new battery booster might not cheapest solution for your project, I would compare the cost of other options first.
These boosters come in a variety of shapes and sizes and have everything you need to charge up the battery and power a 12V DC motor for your project or 5V DC if it comes equipped with it. If you have pneumatic tires on your robot, the compressor in some of these units could come in handy. You can dissemble the booster and use the parts separately but my advice is to keep it all together as one unit, so you can pull it out as a modular unit any time you want, after all it has a carry handle. Just cut the clamps off the thick gauge wires and replace them with a quick disconnect power plug that can handle the current.
- Here is what you get inside or with one of these units:
- 120/240V AC to 12/15V DC power brick or internal power supply
- Battery charging circuit, some with a battery level indicator
- A sealed Lead acid or Lithium 12V battery, usually high capacity
- Thick gauge wires
- Screws, various sizes and usually self-tapping
- Large spring clamps
- Some units have several different power adapters
- Some units have a USB outlet to power 5V systems or charge mobile devices
- Some units have a bright incandescent or LED light
- Some units have a 12V DC to 120/240V AC inverter
- Some units have an automotive DC cigarette lighter plug or adapter
- Some units have a compact air compressor that can go up to 100 psi