Is it really necessary to fully drain the Ni-MH batteries before recharging them?
Depends on your charger.
If you have a charger that detects the end of the charge from negative delta-V (a slight drop in voltage as a function of time) or temperature, it's actually beneficial not not drain the batteries fully.
The most optimal state of charge for a battery to work is its middle region. For example, in Toyota hybrids, the nickel metal hydride battery pack experiences a cycle in approximately two kilometers. Thus, in 300 000 km city driving, the battery pack has experienced 150 000 cycles. There are Toyota hybrid taxis operating in densely populated cities that never see any rural driving. In 150 000 cycles, the batteries are working just fine.
The secret? Toyota hybrid batteries operate only in their middle state of charge region. They are never fully charged and never fully discharged. They probably barely use 50% of the state of charge of the battery.
The chargers that have advanced end of charge detection methods are typically fast. Two reasons for this are that (1) for slow charging (10-20 hours) the negative delta V and increasing temperature effects are so slow an unreliable you can't reliably stop a charge using these methods, and (2) for fast charging these methods are necessary of else the battery would have an unacceptable lifetime.
Sadly, today slow chargers with timed end of charge are still sold. In some cases, buying a name brand battery such as Sanyo Eneloop in a battery + charger set can have a slow timed charger. So by buying a charger from the maker of very best NiMH batteries is not a guarantee for getting a decent charger.
If you have such a slow timed charger, you should be aware of two problems:
- If you experience power blackouts often, the timer resets so the battery overcharges often.
- If you recharge a battery that is not completely flat often, the battery overcharges often.
The reason this is not a huge problem is that you maybe get 100 maybe 200 cycles even when torturing a battery in this manner. Some people might be satisfied with 100-200 cycles.
For such slow timed chargers, the optimal way to charge is to first completely drain the battery and only then recharge.
For any decent charger (2-4 hours) having a negative delta-V + temperature end of change detection, there's no need to fully drain. In fact, fully draining limits the cycle life of the battery.
For lithium ion, it is impossible to do timed slow charge (the battery would probably overheat and explode), so lithium ion batteries do not have the problem of some chargers being very poor. All officially sold Li-Ion chargers work just fine.
My recommendation: buy only 2-4 hours chargers from name brand battery makers like Sanyo. They are guaranteed to have end of charge detection.
About the memory effect: the memory effect in Ni-Cad batteries is only observed if the battery is cycle all the time to exactly the same state of charge, day after day after day. Happens only in satellites. Practically never happens for terrestrial battery use.
When battery users saw their Ni-Cad batteries that were not fully discharged prior to recharging have an unacceptable cycle life, they attributed this to the memory effect. This is false! This is not a memory effect. This is the effect of overcharging happening in a slow timed charger.
A charger having proper end of charge detection would not damage batteries in this manner. NiMH does not have a memory effect, but it still suffers from overcharging exactly like Ni-Cad. Do not use these poor timed slow chargers! Not for NiMH, not for Ni-Cad.