Update: September 2nd

We set up the arm rig to stress test some batteries. Great news! we may be able to rely on some pretty light and small batteries!

With the goal of keeping the prosthetic robotic arm as light as possible, we intend to use the lightest possible battery that can give us reasonable usage. For reasonable usage, we noted how many times on average Lorelei pulls her arm up and down in a school environment. Given she is a righty and her left arm is the paralyzed one, it's about once every 2 minutes. We also discussed and suggested that changing a battery once at school would be fine, this means we could split the weight of the battery between two and make sure the swapping of batteries is easy. We intend to keep the battery and Arduino off of the arm and on her belt or in her pocket, this means that changing it would be simple. The school day is about 6.5 hours, which means the battery would have to be able to power the actuator at full load for at least 98 cycles (expand/contract) (6.5 / 2 * 30). The prosthetic arm brace and actuator we intend to keep to below 150g, which should be light enough for my daughter's shoulder to support (We may get a shoulder brace for additional support). 150g is a 10x reduction in weight compared to what's readily available. My daughter's forearm weighs about 300g. So with all of those measurements, we set up a test. We set up the arm rig to pick up and drop down a 500g weight (300g for arm + 150g for prosthetic + 50g that we loose due to the joint setup and possible friction). This was the results

Test 1: A 6-volt battery, 3300 mAh, weighing 270g - completed 250 cycles, and through a crude measuring approach I estimate it was still about 70% capacity! This means it could easily last the whole day.

Test 2: A 6-volt battery, 2000 mAh, weighing 143g - completed 250 cycles, and through a crude measuring approach I estimate it was still about 40% capacity! This means it could also last the whole day

What's going to happen next: I'll be getting an actuator with more torque from Actuonix Motion Devices Inc. it will require a little more juice and so I will re-do these tests then. I also intend to get an additional battery, a 7.4 volt, and depending on how the tests go I may get a 2000 mAh or less.

See project page here: Project Page