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Is it really necessary to fully drain the Ni-MH batteries before recharging them?
As a rule, I always want my speedlite batteries to be fully charged. At the end of each day when I use my flashgun, I put the batteries charging, regardless whether I fired 10 or 100 times. Now I am wondering if that is the right way to do it...
NimH battery care and feeding has occupied too much of my life in recent years. :-)
It is significantly better for NimH batteries NOT to discharge them fully before recharging them.
NimH life can be enhanced substantially by never discharging them fully on any occasion. Even when using multiple sets during a day's shooting, if you can manage to leave the last 10% or so of capacity in the battery you will get substantially longer life from them. Similarly, if you do not charge them quite to capacity it will also extend lifetime. The latter is harder to do but, if you are able to monitor your charger and if you use a "fast charger" (around 1 hour for a full charge) then you will find that the batteries are very hot at the end of charge. If you stop charging when the battery temperature first starts to increase rapidly they will thank you for it longer term.
If you discharge to only say 50% ever you get more than 2X life cycles so a net whole of life gain in capacity per dollar. eg IF you get 500 cycles deep discharge (and it varies) you may get 1200-1500 cycles of 50% discharge.
500 x 100% = 500 cycles.
1200 - 1500 x 50% = 600 - 750 full cycles equivalent.
NO memory effect in Nimh batteries - One problem with NiCd batteries was that repeated partial discharge caused "memory effect" and the batteries lost capacity. Much has been written on this over many years. Modern NimH batteries do NOT have memory effect to an extent that you will ever notice. If you repeatedly carefully discharge them to exactly the same point multiple times you may find that there was a very very small reduction in available capacity. But this is removed as soon as you discharge them to any other point and then recharge them. It is not an issue of any concern at all. So effectively this means that you NEVER need to discharge your NimH cells, and you should try to avoid doing so.
Note that modern NimH cells MUST NOT BE TRICKLE CHARGED.
Trickle charging modern NimH cells at even VERY modest rates will very greatly shorten their lifetime. Older cells could be trickle charged safely at C/20 or even higher in some cases. Modern ones cannot. (C/20 = mAh/20. Si C/20 for a 2000 mAh cell = 2000/20 = 100 mA).
Older NimH AA cells (up to about 1500 to 1800 mAh capacity) had inbuilt arrangements for recombining generated gases during overcharging. The drive to achieve maximum capacity has resulted in this mechanism being removed from modern cells. ANY overcharging will result in gas generation. This will cause pressure relief valves to operate and the gas and so the water content will be lost. The battery will rapidly dry out and die. As a "bonus" the venting gas will carry some corrosive liquid with it which may damage associated equipment.
Charging NimH:
For AA and AAA cells I recommend the Powerex MH-C9000 WizardOne Charger-Analyzer.
Despite the silly name this is a superb device. Shop around - price varies quite widely.
The C9000 will charge, discharge, analyse, cycle and "recondition" NimH AA and AAA cells. It provides charge rates up to 2A/cell and discharge rates up to 1A/cell. This device is not perfect but is far better than all except a few chargers in this class that I am aware of. They are Taiwanese made. I use one personally and thought enough of them to buy one to take to China to give to a factory to assist them in battery testing.
The following advice, quoted from the Batteries for less page which Eshwar cited, is incorrect. Not his fault of course. :-) But, it will not do too much harm.
NiMH Batteries need to be deep cycled once every 3 months or every 40 charge cycles to prevent the 'memory effect'. This is recommended for all Ni-MH batteries.
The initial conditioning cycles that they recommend also do little harm but are of arguable use. A NimH cell will be of low capacity for the first 1 to 3 cycles. (I have seen many cells assume very close to full capacity by the 2nd cycle. )
Added - May 2014:
Here are a few more references. Note that the Wikipedia ref is to a 2002 paper.
Here is a Battery University article. BU is usually an excellent source of battery information. Here they somewhat contradict my original assertions. Some of what they say goes counter to the majority of modern opinion elsewhere. So I'll reference what they say and also a few other views.
Here is Candlepower forum user comment - CPF is usually (not always) an excellent source of practical information. Well worth reading.
Wikipedia - mentions NimH in the first few sentences then never again. Cites 2002 reference.
Green batteries
Leans towards voltage depression needing managing.
Battery stuff say: Older generation and batteries with other chemical make-up were subject to a memory effect. This is when a battery must be fully drained before recharge or their capacity is reduced. The New Generation of NIMH batteries do not develop a memory effect and can be recharged at anytime during usage cycle. When uncertain about battery charge level or condition, recharge it.
Dan's Quick Guide to Memory Effect, You Idiots.
DOES note voltage depression which is NOT memory effect but can cause issues. Also points out that just discharging a whole pack is often a very bad idea - and suggests alternatives.
From what I see, not on a regular basis. Once in a while the batteries must be cycled completely, but otherwise no.
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:
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.
