Batteries are essential for a mobile robot. In the same way some half decent batteries are also essential. That is why we decided to visit the shop with true staple of quality... Poundland (wait what?). While it is true that Poundland haven't always had the best reputation, in recent years they have stepped up to the mark with their electronics section and have earned the trust of many - despite their low prices. For this reason, we decided to save ourselves the hassle of trying to get LiOn cells shipped and visited Poundland to purchase some power banks. We were then able to strip out the cells and salvage the protection circuits. Before we could place the cells into their batteries, we had to balance them out in order to avoid high currents flowing between the cells and causing [potentially thermonuclear] damage. We did this by simply connecting up the cell to a load until the voltages were roughly matched. In the setup below, we accidentally had the adverse effect when we tried to use a power bank as load. This then started to back-feed the cells and charge them up - whoops.
Our next job was to 3D print the housing for our new cells to go into. These would need to involve a plastic casing to fit two cells in parallel. Once this was designed, we printed it off two parts in orange filament and the last little end cap in white to finish the design. Below is the empty battery shell:
This was then populated with LiOn cells and the cap was screwed to the top. Our first cap designed looked like this.
See any problems? Well perhaps we forgot to open our eyes because with hind sight it really was a stupid idea to put such exposed connectors on unprotected LiOn cells but, we were set straight by the wonderful people of twitter and after just a few mins of designing, a better system had been instated. Here is the current battery connector:
One of its main advantages is the decreased chance of thermonuclear meltdown! This will then be pushed into the battery holders on the robot and the 8 pins will make a connection to the battery. Our robot design has a total of two batteries or 4 cells (2 in parallel, then those in series) Which gives a total capacity of 4.4Ah (or because we can, 117.2kJ of energy). This may not be the best in the world but should get us through the competition and we will make some new batteries for backup purposes. The final thing we did to tidy everything up was to create a battery charger. Its just a simple design with two battery holders mounted to a wooden board (incidently from an old IKEA unit) and then two protection modules from the afore mentioned Poundland power banks.
Lets just hope that we can squeeze enough juice out of the LiOn cells to keep our robot powering through and we should keep our fingers crossed that this power can be delivered without any 'little accidents'. I think that we should end with a celebration of this amazing battery technology and how it has revolutionised modern technology.
See you in the next post!