RWCRay West (Consultancy)
|Lithium Ion Batteries
There are not many publications readily available about Li-ion Batteries. These few notes may prove useful
Lithium Ion rechargeable batteries were originally developed by AEA in the UK. The nominal cell voltage is considered to be 3.6V, as compared with Lead acid at 2V and Nickel Cadmium at 1.2V. When fully charged the Li-Ion cell will be at 4.2V.
If correctly charged, the battery can accept 1000 charge/discharge cycles, but going over voltage by just 2% will reduce this to less than 100 cycles. Charging to 98% voltage will only reduce capacity by about 5% for that charge. That is why Keene Electronics' Li-Ion charger tends towards but never gets to 8.4V.
Li-Ion batteries lose their capacity with every charge/discharge cycle. The slope is such that after about 100 charges the effective capacity is reduced to about 75%-85% of the original.
Li-Ion chemistry inherently does not allow for fast charge, 4 hours is the fastest it can take without being damaged.
Li-Ion batteries stored for any time irreversibly lose capacity. The clock starts from the time the cell is made, and runs whether the battery is used or not. It loses approximately 10%/year.
The main advantage of Li-Ion is that it allows a high power to weight/volume ratio, but this does not come cheap.
Many camera manufacturers have included microprocessor chips in the battery, to give an indication of the remaining usage time. This is inherently inaccurate, because it makes its predictions based on previous usage of the battery, so if you are in standby mode, it will exagerate the time available for recording. I suspect the real reason for the micropressor is to prevent the use of alternative power sources.
The issue of battery capacity is also one that needs clarification. Many cameras which utilise Li-Ion will operate over a voltage range of 8.4V to 5.3V. For a camera which consumes 1.4A at 7.2V in record mode, one could assume that a fully charged 1500mAH capacity battery would allow it to run for about an hour. In practice, since the camera switches off when the voltage drops to 5.3V, much of the charge will be unuseable, and the useful record time will be reduced.
In summary, do not be surprised if your expensive Li-Ion battery does not perform as well as the supplier may have you believe, and be pleased if it is still functions well after a year or so usage.
© Ray West 1999