AA battery packs are cool, but check for corrosion.
If your bike is transportation, or if you like endurance bike sports, you need bike lights. Hikers, climbers, spelunkers, and other outdoor sportsters also often need lights or other battery-powered devices. AA NiMH batteries are a cheap, effective solution. Damp weather and vibration can corrode contacts and degrade performance. Damp combined with vibration degrades performance faster.
The picture at right shows a corroded spring contact in a battery holder. This contact is at the bottom of the holder, and the spring at the top of the holder was not noticeably corroded, so moisture was the main problem here. (Leaking batteries cause corrosion too, although not in this case.)
GPS units that take AA batteries are notorious for vibration problems causing arcing, resulting a sort of electro-fretting that damages contacts on both the battery and the GPS. This is noticable because the GPS tends to reset whenever it loses power. The same problem happens with battery-powered lights, it's just hard to tell when it is happening. But the damage is serious. You can sand the corrosion off the contacts, but spring contacts are usually plated to combine long fatigue life with good electrical performance. Once the plating is corroded, the contact will never work as well again. Many GPS units now build Li-ion batteries into the device, eliminating this problem. Garmin makes a kit for their older GPS units with an extra tension spring to increase the spring force on the batteries. If you have this kit, I strongly recommend using the tension spring. It looks like an extra bit of hassle, but it is actually very useful.DiNotte make nice lights that provide several hours of operation from a pack of four NiMH AA batteries in a standard "BH343" battery holder, shown at right. The holder has a connector like a 9v battery and cost $1-$2. Good NiMH batteries like Eneloops cost $3 each and a 4-pack will hold 10-12Wh.
I tape the batteries into the holders with electrical tape and use a battery pack charger (popular with RC enthusiasts) as I explain here. Taping the batteries seems to eliminate vibration problems. (Otherwise, the batteries will sometimes slip all the way out of the holder, a serious vibration problem!) But you then have to use a pack charger, which is very convenient anyway.
A battery pack is only as good as its weakest battery, so it's even more important to use good quality NiMH batteries when pack charging. I notice that after about two winters, battery life seems to get much shorter. So I tear the battery packs apart and recondition them with a La Crosse BC-700 battery charger.
Actually, a battery pack is only as good as its weakest link. I started scrutinizing the battery holders when I noticed that they would eventually crack, which reduces spring tension, and the connector to the light would also loosen, and would need to be (carefully, without shorting!) re-crimped once in a while. Then I started noticing corrosion. Once I started looking for corrosion, I started to find a lot of it.
I think 2 Seattle winters is probably all one of these battery holders is good for. After that, it's time to break down the battery pack, and recondition the batteries. The BC-700 tells me what the reconditioned capacity of each battery is. If it's over 90% of new, I put them into a new battery holder and give them another 2 years. Otherwise they get relegated to less demanding applications and eventually recycled. Either way, I now only use the battery holders once.