Battery Charging

How do you recharge the battery of a microprocessor controlled knee when you’re half way up a mountain?

The question above was one of the first that sprung to mind when contemplating getting to the top of Kilimanjaro.  I had been using my rheo-knee based prosthetic leg for some time when Kili became a reality and I obviously wanted to use the best tools available to me to tackle this pretty extreme challenge. Some investigation would be required and fortunately (or sadly depending on your point of view) this was just the sort of thing I enjoy!

I decided that the first thing I should establish was exactly what my power requirements would be. At home I simply plug the rheo in to the mains using the supplied adaptor. As a man of the world I knew that I was unlikely to find a mains plug on the mountain and that mains power (240V AC) was not what the rheo actually used. Peering at the small print on the mains adaptor I discovered that its output was 24 volts DC regulated to a maximum current of 1.05 amps. Knowing that the recharge time was approximately 3 hours, this information gave me the voltage of the charging system I required and would allow me to calculate the energy capacity needed for one recharge. We would be on the mountain for nine days, the battery in the knee would normally provide power for two days of use, meaning I would need a minimum of four charges. I would therefore be looking for a 24 volt power source with at least a battery capacity of 12.6 amp-hours (Ah). In reality I would need more than this as, assuming I would be using a battery of some description, battery capacity figures are usually optimistically calculated under ideal conditions, not whilst shivering halfway up a mountain!

Armed with some idea of my minimum requirements I set out to find my power source. Unfortunately the fact that the rheo required 24 volts complicated things somewhat. For various reasons, perhaps largely due to the choice of power used in cars, 12 volt charging systems incorporating batteries with sufficient capacity were relatively easy to come by but this was not the case for 24 volt systems. I could however connect two commonly available12 volt batteries of sufficient capacity in an appropriate fashion to provide my requirements and came to the conclusion that this would probably be the approach I would take.

My next instinct was then to contact the challenge organisers in the UK. As Kili was being climbed in order to raise funds for the Limbless Association, and it was hoped that a large proportion of the group would be amputees, I was confident that my battery charging request would not be the only unusual enquiry they would receive.  After gaining reassurances that a pair of batteries that would meet my specification could be provided in Tanzania I felt that my battery charging issue was all but solved.  I eagerly awaited confirmation that the batteries had been sourced and got on with training for the trek.

More will be revealed soon…

The most visible part of my charging solution in use...

The most visible part of my charging solution in use…