Hello Bolek,
Welcome to CPF.
You are on the right track, however you can improve your cycle life by limiting the depth of discharge.
When determining the state of charge for a Li-Ion cell, you need to be able to measure the resting voltage of the cell. Resting voltage is taken at around 10 minutes after the cell has been removed from the load (either charging or discharging).
Charging to 4.15 volts will have you starting out with around 95% of the capacity of your cell. Charging to 4.10 volts is roughly 90%, which is also a good starting point.
The low voltage cut off is a little difficult to define. At low discharge rates, you need to stop at a higher voltage than at higher discharge rates. You are looking for a resting voltage after discharge of 3.4 volts or higher.
You will have to do some testing to figure out what works in your situation. You can do a runtime test until the light just starts to dim. Pull the cell, let it rest, then measure the voltage.
This will give you an idea of how long you can run the light.
Next, you need to monitor your average weekly usage of the light. If you find that you can go longer than a week without charging, then charging on a weekly basis will limit the depth of discharge and will result in longer cycle life.
The final consideration is storage. If you find that you infrequently use the light, a charge may last a month - or two. In this case, you are basically storing the cell. Storing a Li-Ion cell fully charged is not a good idea, and the health of the cell can deteriorate, especially at higher temperatures.
In this case, you want the cell partially discharged. 4.0 volts is a good compromise between storage and having power available if you need the light.
I forgot to mention charging. The ideal charging rate charges the cells in around 2.5 hours.
Tom
