Notes and Considerations For Battery Power
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Current Configuration
- Chemistry: NiMh
- Cell Form Factor: AA
- Pack Configuration: 3 parallel packs of 5 cells in series.
- Total rating: 6V nominal, 10000mAh (at 0.2C)
NiMh chemistry was chosen for its low cost and ease of use. However due to the relatively high power demand of the LEDs, lithium cells should also be considered. They are, however, not compatible with the current gas gauge.
Design Considerations
Many points have to be considered while designing a battery pack. In terms of capacity, lithium cells win easily; but their cost is prohibitive. On the other hand NiMh have many issues to take into account such as fast self discharge and high operating voltage range. The above configuration is far from ideal. But will do fine if you're not picky. A better option would be using five or six D cells in series.
NiMh Charging and Discharging Characteristics
Nickel Metal Hybrid (NiMh) chemistry batteries usually have a slow charge rate of about 0.1C (0.1 x battery rated capacity) and a fast charge rate of 0.5C. This means a 1000mAh cell can be fast charged at a current of 500mA and will take around 2h to fully recharge. A slow charge should be done every 50 or so cycles to maintain an optimal cell capacity. In what regards discharging, it should be noted that rated capacities are usually given from anything between 0.1C to 1C. Higher drains will reduce the actual capacity of the cell. One of the biggest disadvantage of NiMh batteries is their high self discharge rate. An AA type cell can lose as much as 80% of it's capacity in 30 days at room temperature! To prevent this, the charger or system can provide a trickle charge. The trickle current should be around 0.05C. Another important issue to consider while building battery packs is balancing. Overtime the capacity of the cells in a pack will start to vary from one cell to another. As the difference between cells gets bigger, the chances of destroying a cell by over charging or discharging become much higher. Lastly, the voltage from NiMh varies over the current capacity. In AA style cells, for example, the voltage will vary from 1 volt at full dischage up to 1.5V after a full charge. This means the circuit needs to handle a wide input voltage range and that efficiency of linear regulators is effectively never optimal and varies along the curve of the batteries' discharge.
Typical Characteristics
Taken from Tenergy AA 2300mAh cell datasheet.
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Charger
A PIC based charger kit from the Silicon Chip magazine is used at the moment. The firmware will have to be modified to provide the gas gauge with a "done charging" output. This is a simple yet very effective MOSFET based charger which monitor the battery's charge through temperature rise. However due to the high number of cells, a balancing charger would be better suited in the long run. It would also be practical to have a monitored charger/discharger during development.
Useful Links
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