A Digression: Batteries


Well, ChipiKart is done(ish). It raced at the San Mateo Maker Faire, but that’s another story.

One thing I’ve been meaning to do to ChipiKart is replace the lead-acid batteries with something better, like one of the more stable lithium technologies. Those are expensive, though. But at the maker faire, I was able to pick up some pretty neat surplus.


This is a A123 module. It’s 16.4v, 2.5 Ah. But that’s not the fun bit. The fun bit (aside from the super-flat discharge curve) is the (reported) maximum discharge of 200A, which is rather a lot of amps. It will also take somewhere in the vicinity of 60A charge current, which is just silly. I can’t find a datasheet to verify those numbers, but I won’t be approaching them so I should be safe.

I was able to purchase four units, so I planned on making two quick-change packs. Making quick-change packs means I get to do one of my favorite things: make boxes. So I grabbed some sintra (expanded PVC foam), and headed off to the table saw.


This was the first box. It holds one pack.


An identical one gets stacked on top. Notice how the middle divider isn’t solid. This will allow me to get some airflow over the batteries, and maybe insert a temperature probe. I doubt I’ll need it, though.


The packs slide in like this.


I added some reinforcement, and hit it with a router to make it look presentable.


I don’t have any appropriate wire, so I made copper bus bars to connect the modules in the pack.


The whole thing goes together like this. I still need to cut a keyway in one side and key the receiver so that it’s impossible to insert the batteries backwards. Lots of amps backwards is bad.

But now I have a scary-pack! It weighs 4.4 lbs, as compared to my lead-acid pack, which weighed 18.2 lbs. Now that’s an improvement.

Coming up, a receiver, charger, and the rest of the battery system.