The problem with using disposable batteries is that they present a logistical nightmare. Surplus batteries must be carried for each mission and used batteries must be collected at the end. Standard procedure is to always replace the batteries in the PRR before every mission, regardless of previous use. The military have expressed a strong dissatisfaction with lithium-ion batteries, on the grounds that they are slow to charge and their state of charge is unpredictable/unreliable.
Our brief was to research and present an alternative technology which could be retrofit into the PRR. Ideally it would have competing capacity and be of comparable size. Importantly it must overcome the failings of present Li-Ion technology. Bring on the Ultracapacitor:
The beasts we are using are Philips 50F 2.3V capacitors from Farnell. We also have some MAXIM 100F 2.5V Capacitors to play with. They are pretty chunky, and present some problems in themselves, namely:
- Interfacing the PRR (xV to 3V)
- Fast Charging circuit (T ~ 60s)
- Cell Monitoring
The last point is interesting and of most importance. The caps are damaged if taken above their working voltage of 2.3V or 2.5V. Because of their low energy density, many ultracapacitors, connected in series and parallel will be required to replace the AA Batteries. This makes it easy for unbalanced cells to exceed their working voltage during normal use. Over Voltage Protection (OVP) circuitry is required, and that forms my part of the project. I have just finished the final design of my circuit which is capable of detecting the OVP condition, isolating the capacitor and bypassing the charging current. I hope to blog more about it soon.