batteries - LiPoly Battery - When to stop draining?

These batteries are rated from 4.2V to 2.7V. A typical discharge graph is here.

My question is when to stop draining? What is the best practice? Should I stop at 3.2V or drive it down to 2.8V? It seems to be very little charge is left after 3.3V but I want to ask to experts about the best practice.

Answer

You asked:

My question is when to stop draining? What is the best practice? Should I stop at 3.2V or drive it down to 2.8V? It seems to be very little charge is left after 3.3V but I want to ask to experts about the best practice.

The curves you supplied cut off at 3.0V - so suggesting 2.8V is below what the sourve of that graph thought was wise.

3.0V is a sensible lower limit BUT as you graph shows, the end point voltage depends critically on load. The higher the voltage the longer the cells will last, all else being equal. If you discharge to 90% to 95% of available capacity at a given load you lose little energy and improve battery lifetime. Importantly in some cases, having a reserve means that if there are stray loads on the battery it is less likely to be discharged to a dangerously low point that it can not recover from.

You didn't ask, but at the top end you can also make capacity-lifetime tradeoffs. The chart below is from battery university. This information is not usually presented and can be immensely useful. The "capacity at cutoff voltage is the capacity achieved when you swap from constant current to constant voltage charging at the point shown.

Again, using less than full capacity extends battery cycle life.