Electronic – Measuring Discharge Rate of LiPo battery

Unless you are short circuiting the battery discharge rate is dictated by the load connected rather than the battery cells themselves.

When a cells capacity has reduced the full and empty voltages remain the same, what changes is the maximum amount of charge stored within the cells. They will take less charge to reach the maximum charging voltage and so have less charge in them to discharge.

Also keep in mind that for Li-ions voltage is significantly impacted by temperature.

I want to preface this with the following: I'm by no means an expert in the field of batteries and/or their chemistry. This is just what I know/have picked up over the years.

Depending what kind of autonomy you need, you need to decide what chemistry provides the best performance.

If it is truly a very long-life product, you pretty much rule out nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH). They have quite high self-discharge, and it is possible the self-discharge may deplete more of your capacity than your actual circuit does. In recent years, this has gotten a lot better with techologies like the Sanyo (now Panasonic) Eneloop type batteries. However, they are still outperformed by lithium-ion technology in most cases.

You have to also consider if you truly need rechargeable batteries. I'm not very familiar with the cost of modern (reliable) lithium-based technologies, but unless things have changed dramatically, it is very possible that it will be more expensive to use a rechargeable battery instead of replacing the battery every few months/years.

Regarding discharge rates: This depends on the technology, but also the design. However, as lithium batteries tend to have lower ESR, they (generally) perform better in this regard.