DesignShine Input Voltage Range (part 3)
Faced with a dying battery, and armed with all this information about li-ion packs in general, what's a DS owner to do?
Before I look at specific battery replacements, let's talk about the electrical characteristics of the work horse inside the magic black box... the MaxFlex. This LED driver board is based around what's called a "boost-mode" power supply. In other words, the circuit expects to see an input voltage that is LOWER than the necessary voltage for driving the specific LED load (in this case, SIX Cree LED in series). The circuit will Boost the incoming voltage up to the necessary HIGHER voltage required to drive the LEDs with a fixed current.
Originally, I specified and supplied an 11.1V pack for use with these lights because it was the ideal compromise to allow a single battery to be used with both the headlight and taillight. BUT, as you'll see in the image below, the useable range of the 11.1V battery (the BLUE shaded region) actually starts out ABOVE the necessary voltage to drive the taillight emitters (not the case for the headlight). So for the taillight, the BOOST mode supply is left feeling dejected, since it does not get to participate in the "party" at all until the battery has discharged partially.
With a freshly charged 11.1V battery, the supply voltage is too high for the first three power levels. The only thing that the power supply can do is pass the voltage straight through to the LEDs. Essentially, this is what is called "direct drive." It's very efficient, but what if you really wanted to run the light at power level 2? Looking at the RED trace, you can see that the battery pack would need to be discharged down to around 11.1V. For most folks, this is not a problem, since level 3 is still marginally acceptable for night time use (but only on steady burn).
Ignoring the DS headlight for the moment and skipping ahead to today, what we see is that the 7.4V pack (see part 1 of this forum) has emerged as the industry standard bike light battery. Not only that, the ubiquitous 5.4mm x 2.1mm power connector, otherwise known as the Magicshine style connect is being used by a number of light manufactures. It's no coincidence that I'm using the same connector. So at the end of the day, the DS-500 taillight is capable of being run equally well from either an 11.1V (3S) pack OR a 7.4V (2S) pack. The former being slightly more efficient for the amount of weight being carried around, and the latter being more appropriate if you always want to have access to the lower power levels for night time riding. For those that may have a "custom" application, note that the chart below is showing you the acceptable input voltage range. You're not restrained to li-ion only. Any supply in the 6V to 13.3V range would be safe to use. I wouldn't connect it to a running car battery, but any 12V Sealed Lead Acid (SLA) would be fine. I've gone as low as 4V on the input, but I wouldn't recommend it unless you plan on staying at the low power levels.
So how do you decide which type of battery is best for you for the future? Check out part 4 for the answer!