The original idea for the lighted magnifier was for inspection and reading small print on tools, which are generally done in a fixed location. The intended use was for continuous periods of work that made battery operation an issue. Mainly, the initial thinking was to turn it on and leave it on for the duration of a project. So, with it in hand and in use, I found in a short time it was being used for much more than its original intent. The magnifier lens in its unlit state is excellent in capturing ambient light, so I had it in mind that one lighted unit for the very tight and difficult work was great, with an unlit version for all other tasks. Problem is, the lighted unit provides over 1,780 FC on the target, transmitting over 700 FC to the eye at 4″. The unlit version produces no more than ambient levels, and if your head shades the ambient light, that is cut considerably. So, when comparing the two in actual use, the lighted version simply knocks the stuffing out of the unlit one. This meant I needed to cut the umbilical and create a battery powered version.
This is not a low power light unit, it is a 2W light source, running at 24V / 80mA, so is a bit of a power hog compared to the typical cheap-o LED junk running off two AA batteries that fills the shelves of Wal Marts and other outlets hawking the greatest technology to enter lighting at low-low prices (and performance to match). That means selecting a battery strategy is not simple, especially since I wanted the product to perform the same on the battery as it does plugged into the wall.
The following are two approaches I considered – with a warning: I have oversimplified the calculation of battery service life here to avoid going on and on about Puekert’s law/constant, discharge curve characteristics, and other such fiddly science. I do this to keep us from napping. The results of the more scientific method of estimating will vary to some degree, which is accepted. However, the relationship between the two battery approaches remains essentially the same, thus the conclusion as to which would best be applied would likely remain unchanged.
The AA Approach:
The ever popular AA battery is viable, at 1.5V it would require a rack of 16 to get the voltage up to 24V necessary. If a different LED circuit were employed, (3) AA batteries in series with a current limiting resistor could be used to get the job done, operating the LEDs in parallel rather than series. However, with a 4Wh capacity (50mA load), assuming that at about 50% of that the voltage drop will not support the LED threshold Vf demand, these will run for about 1 hour before needing a recharge or replacement. If we were to use rechargeable LiIon batteries, the Wh drops to 2.7, producing a run time of about 45 minutes before needing recharge. Even if the battery pack were stored on a charger, the run time is a bit on the short side.
The 9V Approach:
The review of the AA approach led to looking at (3) 9V Lithium batteries in series to get the 27V that is necessary for the existing circuit. Since that circuit is current regulated, with a minimum required Vf of roughly 15 before failing to remain lighted, we can run the (v pack a little deeper into their discharge cycle. 9V Lithium batteries also have a 10.8Wh capacity (50mAh load). Assuming we can get 65% of that before running afoul of any voltage drop, the result is around 3.5 hours of run time. Seemed like the way to go for now. If I used Li-Ion 9V batteries, the Wh drops to 4.5, so the run time between charges drops to 1.5 hours. Not great, but, if left on the charger when not in use, this could be workable in most situations.
In a commercialized version of this product, a rechargeable pack could be made up, tuned to an LED circuit optimized to work from that configuration, that would result in improving operating time to some degree. Further, the cost of the (3) 9V Lithium batteries at $24 (total) is troubling, leading to the conclusion that a rechargeable pack is most definitely a desirable approach. While that can also be accommodated using standard rechargeable 9V batteries and a 27V charger to keep them alive, an integrated battery pack with integral charge state regulation, etc… would be a far neater end product. Further to this, this charger could be easily connected to a small solar panel stuck in a window somewhere, using free energy to keep them fresh and ready for use.