Some stuff about NiMH rechargable batteries.

This is basically going to be selected highlights of GP's technical resource on NiMH cells, put into a practical conext with my experience.

• Standard NiMHs can be recharged at a rate of 1C; i.e. a 1800mAH cell can be charged at 1800mA for a little over an hour (charging isn't 100% efficient). This is higher than I thought, but has a caveat about overcharging, below. Trickle charging is most often done at 0.1C for 12 to 14 hours - this is seen as being most friendly to the batteries (i.e. an 1800mAH cell would be trickle charged at 180mA... I find this "0.1C" notation bemusing, but useful).
• NiMHs lose their charge on the shelf at a significant rate. If you want to be able to pick them up and use them, you should leave them on a trickle charge at about 0.05 - 0.1C. If you do leave them on the shelf for a while, it may take a few charge-discharge cycles to recover the capacity.
• Over discharging is a significant problem. Particularly so when cells are arranged in series to form a battery. Additional chemical reactions take place, and one of the electrodes may become plated with oxide. Cells are constructed with this in mind, and good NiMH cells should not be damaged by being completely discharged into a resisitor. In a battery, if one cell becomes discharged before the rest of the cells, the voltage generated by the battery will continue to cause current to flow. This forces current through discharged cell, and voltage inversion occurs. Hence batteries should be constructed from cells of the same type, manufacturer, capacity, age, and charge state. This is more important than I'd realised. Moreover, when using a battery, it would be best if the voltage was monitored, and the current turned off when the battery voltage drops 1.2V below its nominal voltage (taking nominal voltage as 1.2V/cell). This ensures that current cannot be driven through any cell whose voltage has dropped to 0V.
• When you over charge a cell, it begins to produce oxygen gas. The oxygen migrates through the cell to the other electrode, and re-combines to form water or OH- ions. In a trickle charge the migration and recombination occurs at a satisfactory rate, and the increased pressure in the cell due to the oxygen gas is not excessive; the process generates a little heat, which must be monitored, as standard cells should be kept below 40deg C during charging. However, in fast charging, the migration and recombination cannot keep up with oxygen production. The pressure inside the cell rises, and may eventually trigger the safety vent. The cells have an internal void to accept the contents of venting, but electrolyte is lost when a cell vents, and hence the capaicty is reduced.
• Charging is best done with a constant current circuit. There's no reason why batteries shouldn't be trickle charged in series, but for series fast charging, similar restrictions to those of battery construction need to be imposed, and detecting the charging complete state is made more difficult. NiMHs should not be charged in parallel.
• As NiMHs rely on the presence of water for reaction, the cells have difficulty operating below 0deg C, due to ion mobility problems. Such low temperatures may also cause a hazard by freezing shut the safety vent. By contrast, temperatures above 40deg C adversely affect the cell's chemistry. Nickel Hydroxide decomposes (part of the self-discharging mechanism), and the rate of decomposition increases with temperature. Overall battery life is also adversely affected by elevated temperature, and cells should best be kept below 40deg C during operation, storage, and charging. A rough guide: if it feels warm, that's ok. If it feels hot, you're starting to reach danger point. If its too hot to hold, its too hot, stop whatever you're doing to it.
• An interesting contrast between NiMH batteries and Lithium-Ion ones is that NiMHs are best stored and used in a cool environment because of the self-discharge reaction, whereas Lithium Ion technology performs best at a slightly elevated temperature. I don't know what the optimum temperature is for Li-Ion, but I have heard that some photographers have developed a habit of keeping their spare batteries in a trouser pocket, to keep them warm. Also, some folks apparently keep their NiMHs in their fridge... this sounds a bit risky, as condensation could create a short circuit!