Battery capacity meter

picnut

New Member
Ni-cad cells can be discharged to zero and it is a common technique for balancing the cells of batteries. Cells can get out of balance very quickly with high charge / discharge applications and can even result in the reverse charging of low capacity cells in a battery and the possibility of an explosion. For this reason in critical applications (aircraft etc), batteries are capacity tested every three months, which also reveals out of balance cells. The capacity test is normally based on the constant discharge current and the time taken for the voltage of any cell to fall to 1.0 volts.

If it is decided to 'deep cycle' the battery, the discharge is continued until the voltage of each cell drops to 0.5 volts at which time that cell is shorted with a low value resistor. This stops low capacity cells being reverse charged during the continuing discharge cycle. When all cells are shorted, they can be left in this condition for years. When required for use, the shorts are removed from the cells and it will be found that the battery terminal voltage will rise to about 1 volt per cell and we are now ready to recharge the battery .... BUT ....

If a cell has a marked capacity of 1000 mAH, it means the 'useable' amount of charge. It is obvious that when discharging a cell there is still some charge in the cell when it's terminal voltage has dropped to 1 volt. The 'unuseable' charge is normally about 40% of the rated figure. So when charging a cell from zero we have to charge it at 140% rate.

The situation can get a lot more complicated than the above, because we have to take into consideration a load of other factors, such as temperature etc.

A more modern approach to cap testing and rapid chaarge is the 'Reflex' system introduced by Christie. Just GOOGLE "Christie reflex charger". There is something magic about being able to charge a large battery from zero volts in one hour and the temperature of the cell remaining at ambient temperature. The Christie system is a series of charge and discharge pulses at a rate of 60 per second, which has the same effect as thousands of deep cycles and can increase the capacity of a failing cell back to above the rated value. On one test 100 failed cells were connected up as batteries and serviced using the Christie charger and 98 of those expensive failed cells passed the capacity test at 100 %.

The above ONLY applies to nickle cadmium cells.

No .... I have no connection with Christies.
 
Top