|Battery Health or The Care and Feeding of Batteries||To to the HahnTronix Bike Lights Web Page|
This page was last updated on: 10/28/10.
Most of my lights are designed to operate from any voltage source producing between 8 and 24 volts that is capable of delivering enough current (0.5 to 1 amps). Below 8 volts, the lights will still operate, but will begin to dim. My lights us a buck converter that allows the light to produce about the same brightness from any voltage source that meets the above requirements. For practical purposes the lowest voltage battery I usually recommend is a 9.6 volt NiMh battery.
Batteries, beside being rated to provide a particular voltage, are rated to provide a certain amount of current, often stated in mAh (an abbreviation for milli ampere hours, 1000 mAh are equal to 1 Ah). A battery rated at 2200 mAh can deliver a current of 1100 mA for about 2 hours.
A quick approximation for figuring the run time of a battery with a particular light is to multiply the battery voltage by the current it can produce to obtain the watts the battery can deliver (a 1 volt battery delivering 1 amp of current for 1 hour, has produced 1 watt-hour of power). This is the number of watts your battery can provide under ideal conditions. Divide the number of watts the battery can deliver by the number of watts your light uses and you arrive at the number of hours your light should run. I usually fudge the run time a battery can provide by multiplying the calculated time by 80% to allow for batteries not being rated correctly by the manufacturer, batteries not being fully charged, LED driver efficiency, etc.
My lights can also run from a variety of Li-Ion (lithium ion) batteries. Some people prefer Li-Ion batteries because they weigh a bit less than NiMh, and should, in theory, hold a charge for longer. They do cost more, typically 2 to 3 times as much as NiMh batteries. They need a special charger. Never, never attempt to use a NiMh charger with a Li-Ion battery. Li-Ion batteries can be a safety hazard. They have been know to spontaneously burst into flames (typically while being over-charged or physically abused).
The 2 most common types of Li-Ion batteries available to consumers are Li-Ion 18650 cells, and Lithium Polymer (LiPo) batteries. 18650 cells typically provide 3.7 volts at around 2200 mAh, and are wired in series and parallel to provide the voltage and current you desire. Lithium Polymer batteries come in a variety of sizes, voltages, and current capacities. 18650 cells are often used in laptop computer batteries. LiPo batteries are often used in remote control (RC) aircraft and cars. Smaller LiPo batteries are often used in cell phones and MP3 music players.
Which battery type is better?
You will find some 18650 Li-Ion batteries available for quite cheap prices from a few Chinese importers. In my experience, you should treat these cheap batteries with caution. I've had a couple just suddenly stop working. I don't know if this is typical, but they failed in an open state. One time a cell in a pack of 4 wired in series suddenly died and the whole pack stopped working. This would have sucked if I had been hurtling downhill at the time.
More expensive Li-Ion batteries will have some kind of protection circuit built in. This protection circuit will prevent the battery from being over charged or over discharged. It also will typically act as a fuse to prevent problems if a battery is accidentally shorted. You should always use batteries that have a protection circuit built in, either in each cell, or in the whole pack. One possible downside to this, is the protection circuit could shut off the battery while you are using it. This is one of the reasons my lights come with a low battery warning feature.
Lithium Polymer (LiPo) batteries typically come with no protection circuit board. LiPo batteries are mostly used in radio controlled aircraft, where you want a light high powered battery, that won't suddenly stop working. Since these usually have NO safety circuit built in, use them with extreme care. LiPo batteries are also more easy to physically damage than 18650 cell based batteries. If you decide to use LiPo batteries, you should consider putting them in some kind of hard case You should never overcharge a LiPo or discharge it below about 3 volts per cell.
NiMh batteries have their problems as well. They don't do well if discharged too deeply. A good rule of thumb is never run a 1.2 volt NiMh cell below about 0.8 volts. For an 8 cell pack, never run the pack below 6.4 volts. A more conservative rule, would be never run the pack below 8 volts.
NiMh batteries don't like to be overcharged. Don't cheap out on the type of charger you use with your NiMh batteries. A "smart" charger will charge the batteries till it sees a slight bump in the battery voltage. Most cheap chargers will just start a timer when a battery is plugged into them. This can overcharge the battery if it was not fully drained beforehand. One sign of a cheap charger is if a nearly fully charged battery is plugged into it, it will heat up the pack (it will become very warm to the touch, a bit of warmth is OK), and not shut off. A smart charger, will try to charge a fully charged battery for a few minutes (an hour at most), and then shut off.
NiMh batteries exhibit a problem known as self-discharge. Just left sitting on the shelf, normal NiMh batteries will lose some of their power, typically about 1% a day. You can count on them needing recharging about every 2 to 3 months. In fact, you should plan on doing this to NiMh batteries that sit around all summer, when you don't ride as much at night. NiMh batteries are often available from stores that specialize in RC cars. On a dollar per watt basis they are usually the least expensive type of battery.
There is a new type of NiMh battery that has been on the market for only a couple of years. They are usually called Low Self Discharge, or Ultra Low Self Discharge (the manufacturers seem to shy away from the LSD acronym for some odd reason). They hold about 85% of their charge after a year. These batteries will typically have a lower current rating, 1.8 to 2 amps as opposed to the 2.5 to 2.7 amp ratings on non LSD NiMh AA cells. Sanyo makes AA cells under the Eneloop brand name that are pretty highly rated. Rayovac makes some under the Hybrid name that I have been very happy with. Rayovac also has a new version of LSD batteries called Rayovac 4.0. I've only used them a couple of months but they seem to hold their charge about as well as the Hybrid ones did. Tenergy also makes some that I have limited experience with, but the results look promising. You can often find these LSD batteries on sale for less than the non-LSD type. I haven't seen LSD cells available in pre-assembled NiMh battery packs. The most commonly available size is AA, though C and D cells are becoming available. The AA Low Self Discharge batteries make an excellent backup battery for the BFL-2010. Eight of them in an inexpensive 8 cell battery holder will give you nearly an hour of run-time on the brightest level.
More to come!