19 December, 2008

Of mice and batteries, and calculating.

Charge it!

No, that's not the plastic fantastic, which is probably just as well in the current financial situation. Instead, this is just a wee update to my post from April 6th, 2008 about batteries in my mouse, when I posted that Alkaline batteries seem to average in my mouse about seven months. Well, when I bought some NiMH rechargeable batteries a little while ago, I decided to give them a try, after previously having disappointing results with a couple of sets of NiCad rechargeable batteries (500 mA/h). I was charging the older ones up, and was lucky to get a week out of each pair. Anyhow, when I charged up the new batteries, I put them into the mouse on about the 14th of November. They ran for more than four weeks of use before changing them, and it's the 27th of December before I had to put another pair into the mouse. Frankly, I'm somewhat surprised, even though the batteries are only the "cheaper" 2000 mA/h capacity, and not the 2450/2700 mA/h batteries I really wanted. I also heard that someone wasn't too impressed with Eveready batteries, which were exactly the brand I'd bought. Never mind, I'll see how the batteries behave.

The batteries were a steal—at only $20 for four batteries, and with a normal 18 hour charger effectively thrown in for free, it was a better price than paying out $32.50 for one 18 hour charger and two batteries that I'd seen by Energizer; that wasn't including the $13.45 cost of two more batteries to make up the foursome. Other "cheap" batteries I'd seen were only $19.85 for four batteries, not including a charger. Again, that was for 2000 mA/h capacity, just a store brand for K-Mart. I'd hate to think what it would have cost for a fast charger—either a one hour charger or an eight minute one, apparently neither of which are any good for the batteries.

Hack it!

With regard to the previous sets of batteries, I at least was able to save the cost of the power for charging them by using two solar panel garden lights I'd found at the side of the road. The first one took one rechargeable AA battery, and could happily be left in the sun, to give me a charge over two days after having removed the LED. The second solar cell garden light was a bit interesting, as I had to hack it a bit to get a pair of AA batteries added beside the space for a single AAA battery. In essence, I hacked the battery cradle out of a dead TV remote, got out my soldering iron, and set to work melting plastic and rewiring the cradle, then mounting the cradle onto the back of the cell, opposite from the solar panel. In the end I did all this without having to strip the single AAA carrier out. Once that was done, I had charging for free, for three NiCad AA batteries and one AAA battery, all in the sun.

Unfortunately, given the restrictions for NiMH batteries, I can't use this same method for charging them, and instead have to use the provided charger, to make sure I don't overcharge the batteries by accident.

Hack it again!

While I was at it, I thought I'd convert a Casio FX-82TL (cheap scientific calculator used for secondary school) over to a FX-85TL/FX-300TL (effectively), though with a slight twist... instead of using the normal LR44 the FX-85 has inside, I'd retain the AA battery from the FX-82 (so I didn't have to wreck the case any more than necessary), and make it rechargeable via the solar panel instead. The wiring was rather simple, as all I needed was wire to the solar cell (which I'd already "filched" from another solar four-function calculator), and a diode to prevent the battery discharging through the solar cell again. So, first off, I soldered wire on to the contacts of the cell very carefully—copper pads on glass takes no time to heat up, even with a little soldering iron like I use. Then, I connected the solar cell in place of the battery, turned on the calculator, checked it works—yup, no trouble. Added in the diode (the right way around), checked that the calculator still works, which it does, though it now needs a bit more light due to the voltage drop across the diode. Finally, place the rechargeable battery in place, cover the solar cell so there's no light, and test the calculator one last time. Yup, all working.

So for a little bit of solder, a solar cell from a four-function calculator and a diode, I have a scientific calculator with what is effectively a new ID and a battery that'll last just about uhm, forever? I vaguely thought about the same trick on my FX-82MS too, but there isn't the same space for the solar cell, as the battery space takes up 90% of the space above the display. At least on the TL, I'd be able to cut out a slot and have space to spare, if I wished to do a tidy job, and not just a sellotape job over the top of the case.

I also wanted to try this trick with one of my other calculators, but I don't know enough people with calculators with solar cells on for sacrificing (I'd need at least three cells)... what a shame. Not only that, but two other aspects of this calculator lean me away from solarising it—first off, it uses alkaline batteries, though this isn't necessarily a bad thing. It affects the voltage I have to provide the calculator, of course. Secondly, the reason for the alkaline batteries: this calculator draws more current than the FX-82 I converted over. I'd also have to fit a special plug so I could detach the solar circuit from the calculator, which would remove its usefulness somewhat.

What I was thinking of initially would be: three little cells (or maybe even four) wired together in series to provide either six volts, or eight volts to allow for halfway usable voltage at low light conditions, connected via diode to charge the batteries. Again, being NiMH batteries, I'd have trouble making sure the batteries weren't overcharged, and I'd have to build the same sort of circuit into the charging as a normal charger has. I.E. a bit more sophisticated than two wires, one diode and a solar cell. So for the moment at least, that project will either never get off the ground (usual for me) or I'll actually have to put some cash into the project to ensure it doesn't look like an ugly hack. If I were to do justice to the job of course, I'd put the whole calculator onto a "charging" cradle like the Palm Pilots used to have, and simply solarise that instead, my only problem would be sourcing (or making) suitable cradle to take one FX-9750G+ on its end, with plug. Interesting, but probably will never be done. So, what other mean hacks have been done to the humble calculator?

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