What scr4 said. Having an appliance with an odd number of cells is a pain, as most standard chargers only charge in pairs. A company I’ve consulted for had a widget that took 5 AA cells, and customers were constantly complaining about the awkward arrangement.
If a number of cells are being discharged in series, then the weakest cell will be flattened first, and if over-discharged it will have its polarity reversed by the remaining partly charged cells. This will ruin an NiMH cell, but if the cells are well matched (i.e. out of the same packet and with the same charge/discharge history) then discharging down to 1 volt per cell is perfectly safe. At 1 V an NiMH cell is nearly fully flat anyway.
Likewise, charging unequal cells in series will mean that one will reach a full charge before the others, and any further charging could cause this to damaged through overheating.
I’ve designed NiMH chargers, and I can report that they’re not all created equal. NiCds were easier to charge as they had a detectable voltage hump as they approached a full charge, and though this hump is sometimes present in NiMH cells it is quite small and difficult to detect, and in some types absent altogether. The best way to charge an NiMH cell is to monitor the temperature, and use this to drive a bunch of algorithms that initiate various charging sequences. Crucially, the temperature monitoring is used to terminate a fast charge current, which will normally cease when the cell temperature reaches about 50 deg. C, or shows a rate of rise of about 1 deg. C per minute. Sadly temperature monitoring is expensive (and inaccurate if done badly, which it normally is), so cheap NiMH chargers will often have no thermal feedback at all and often just rely on timers to set the charge duration. The compromises inherent in these cheap chargers mean that they will reduce the lifetime of the cells.
SeanArenas - I’m not familiar with that charger model, but I can tell you what to look for in a charger. The cell manufacturers recommend a 3 or 4 stage charge. If the cell is very flat, an initial small recuperative charge is applied until the cell voltage reaches about 1 V. Then a fast charge is applied (a current of about 0.5 C works well here I find, where C is the cells rated capacity, e.g. an 1800 mAh cell would get 900 mA) until the temperature rise criteria is met, and then the charging current is dropped to a top-up charge (about 0.2 C, say) for a predetermined period (couple of hours maybe), and after that the cell is subjected to an indefinite small maintenance charge (maybe 0.05 C). This is often in the form of a trickle charge, and though the charger may apply this indefinitely it is recommended that this is terminated after a day or two. A better maintenance charge regime is to apply brief bursts of a higher current periodically - this will keep an NiMH cell happily topped up until ready to use.
This is a bit of a simplification (it gets as complicated as you like), but under normal conditions that’s the essence of it.
As NiMH cells are so particular in their charging requirements, to obtain best results each cell should be charged independently. This would mean independent thermal monitoring circuits and current sources for each cell, which makes decent chargers quite expensive.
Classic NiMH cells have an unfortunate self-discharge characteristic, and the final low current charging step is mainly to counter this effect. However, a new breed of NiMH cells has been around for a couple of years now that have very low self-discharge rates, to the extent that they can now be used in previously verboten items like remote controls, doorbells and smoke alarms. GP ReCykos are good, but the best of the bunch by far are Sanyo eneloops. Get some of these and a decent charger and they’ll last a very long time.
