More bike and light articles
NiMH batteries are very sensitive to overcharging. Thus if you want to keep your batteries working as long as possible, you should avoid overcharging. There are two basic ways of doing this. One is to discharge your battery to an almost empty state (which is determined by the voltage of the battery dropping to a certain predefined limit, usually about 1 volt per cell), and then charging it up using a timer. This avoids both overdischarge, which leads to cell reversal, as well as overcharging.
If you don't have the equipment to precisely discharge your battery to that almost empty state, then what you should do it just recharge it after use, using a smart charger to prevent overcharging. A smart charger charges the battery at a reasonably high rate (one amp or more). When it detects that the battery is full, it turns off. Well, it doesn't turn off, it just switches to a low trickle current to counter self-discharge. Some people argue that since NiMh are so easily damaged by overcharging, the charger should really turn off completely, but occasionally let in a burst of current to counter self-discharge. However, I only know of one smart charger that does this (the Specialized one below).
There are two features of NiMH batteries that are used to terminate smart charging. One is that as the battery approaches being full, the temperature starts going up, and the temperature continues to increase as time goes on. If the current isn't turned off, the battery will overheat and be destroyed. Another feature is that the battery voltage slowly increases, and just after the battery reaches 100% capacity, the voltage drops by a small amount.
From reading NiMH battery data books, I found out that by far the best way to determine when a battery is full is dT/dt. This requires you to keep track of the temperature of the battery. When the temperature has gone up by a certain amount over a given interval, the battery is full. I seem to recall 1.5 degrees C over 2 mins, but don't hold me to that. I don't know of a chip or off the shelf charger that does this, and it requires a thermistor built into the battery pack.
I have a Lumicycle lighting system with NiMH batteries. Lumicycle offer a smart charger with their system as an upgrade, and I naturally got this one. It works by detecting the slight voltage voltage drop in a full battery. The company that makes this charger makes a very similar one for NiCad batteries, but their NiMH charger terminates fast charging after detecting a much lower voltage drop than the NiCad ones. It seems to work. That is, it charges for less time when I've used only a small part of the battery capacity, and for longer when I've used more.
However, according to the data books, this algorithm turns off the fast charge after a small amount of overcharging has already occurred, and hence a very small amount of damage is being done to the battery with every charge. The data books that I read suggest that you'll get a 20% fewer charge cycles from your NiMH batteries doing this. However, it is a far better approach than using a slow charger, like the 16 hour charger as supplied with the standard Lumicycle system. With that kind of stupid charger, it's entirely luck as to whether you undercharge it, get it just right, or overcharge it.
If you are really into DIY, one route to getting a NiMH smart charger is to make it from a Maxim 712 chip. Maxim makes two very similar charger chips. The MAX 713 is for NiCad batteries, and the MAX 712 is for NiMH. The 713 turns off when the voltage drops after NiCad battery is fully charged. The 712 turns off when the voltage levels out rather than dropping. They felt that this was a safer approach than trying to detect the small drop that occurs. I think it makes sense, especially as it terminates charging a bit quicker, resulting in less overcharging. The MAX 712 also monitors temperature, and charging can be terminated by maximum temperature. But this requires you to make up battery packs with thermistors in them. For more info, check out Maxim's Web pages. You can get ahold of a data sheet by looking up the 712 in this chart.
By far the best charging system seems to be the one included with the soon-to-be-released Specialized Fireball Team and Comp lights. According to the developer of these lights, their battery technology is essentially the same as that used by the laptop computer industry. Basically, the battery pack includes a "gas gauge" and some very smart electronics which monitor the state of the battery based on battery voltage, elapsed time, and ambient temperature. When plugged into the charger, fast charging starts. The cutoff for fast charging is determined as a combination of time, voltage, and temperature. When the pack is fully charged it does not trickle charge, it turns off. The gas gauge then begins to monitor self-discharge. The gas gauge will then occasionally turn the charger on to top-off the pack. This eliminates the chance of over-charging which is the leading cause of damage to NiMH dells.
An alternative to a smart charger is to make a trickle charger. However, since NiMH batteries are so very sensitive to overcharging, the best approach is to use a very low current in order to minimize overcharge damage. One frequent poster to the bikecurrent mailing list uses a charger that charges at a rate something like C/60 and has had good results with it. C/60 is the capacity of the battery in amp-hours divided by 60. So if you had a battery with capacity 4 amp-hours (4000 mAh), C/60 is 67 mA. At this current rate, the battery would be charged from empty in about 84 hours, i.e. about 3 and a half days. If you left it plugged in longer than that, it wouldn't matter much because of the low charge rate. Obviously this system cannot be used when you require your battery to be charged up quickly!
You can get more info in NiMH batteries from Panasonic and GP.