One thing many manufactures of laptops and cellphones recommend is that you wait until the battery is completely dead before you recharge it or else it will slowly loose its maximum capacity or have a reduced lifespan. To what extent is this true? Is this only true for certain rechargeable battery varieties?
This is a very complicated issue because of number of myths surrounding it and low speed of information transition from battery manufacturers to equipment manuals.
So let's start with the lithium batteries. There are several types of them (Li-Ion, Li-Po, LiFePO4) but they have many similarities when discharge is concerned. The most important feature of them is that they are dead when completely discharged. For that reason the nominal discharge voltage is set to around 2.75 V. At that time, the battery is considered to be "discharged". For example here are some datasheets which show the claim:
- Datasheet of Li-Po 75212223--Tenergy
- Product Specification: Polymer Li-ion Rechargeable Battery
- Aplus: Li-Po Rechargeable Batteries Data Sheet
Discharging battery to that voltage may cause problems and may make it unrecoverable. Some battery manufacturers recommend that the batteries shouldn't be discharged below 3.3 V/cell (for example ThunderPower) and that they should be used at lower currents for first few cycles.
Unfortunately, I was unable to find a good source for lithium battery best practices, but here are some I found:
- Li-On Battery Do's and Don'ts (download link)
- Tech Note: Battery Best Practice
There's also this interesting article.
What they basically say it that batteries shouldn't be allowed to discharge completely, shouldn't be allowed to remain in the discharged state and that they don't have the "memory" effect (an explicit claim for that can be found here and here).
On the other hand, it is considered that nickel-cadmium based batteries have memory effect (some believe that NiMH do too, but that is a separate story) and should be completely discharged to their nominal discharge voltage which is (according to this datasheet for example [link now broken]) 0.8 V. For NiMH batteries, the discharge voltage is 1.0 V, according to this datasheet [link now broken]. The base for the discharge claim comes from various best practice documents. For example, there's an article here which claims that discharging batteries to voltages lower than their nominal discharge voltage will increase amount of charge the battery can hold.
What is interesting in this whole story is that I've been unable to find a document from a battery manufacturer which explicitly explains how to properly condition batteries. It would be great if someone could dig up such documents, if they exist.