If the batteries or a battery box on your Mars Rover get hot, switch the rover off straight away. Once the cells have cooled down, take them out and do not use them again. Our Battery and power safety article covers what to do with a battery that has got hot, swollen or leaked.
Heat like this is alarming mid-build, but it has a straightforward first check. The Rover runs on six AA alkaline cells, and a cell put in the wrong way round is the classic cause of batteries heating up. With a fresh set of cells, check the following before powering on again:
Use fresh alkaline cells — all six the same brand and age. Rechargeable cells give 1.2 V rather than 1.5 V, which is not always enough for the rover to run properly.
Check the orientation of every single cell against the markings in the battery holders and the photos in the build guide. One reversed cell is enough.
Check each cell is seated firmly against the contacts and springs.
If a fresh, correctly oriented set still gets hot, or the rover still does nothing when you switch it on, do not keep reconnecting the batteries. The Rover is a soldered build, so the next place to look is a joint that has not quite taken, or two neighbouring joints that have flowed together around the battery boxes or the power switch. Compare that area of your build against the photos in the build guide at circuitmess.com/blogs/resources, and see My Mars Rover is not working. What should I check? for the general checking order.
If a battery holder has melted or been damaged by the heat, it can be replaced — Ordering spare parts for the Mars Rover explains how to identify the part and what to send us.
If the checks above do not settle it, email [email protected] with your order number, photos of both battery boxes and the soldered joints around them, and a short video of what happens when you switch the rover on. We look at each case individually and will help you get it running.