I was wondering if there is a project out there that allows to use near-empty batteries, to charge a liion-battery.
The idea is that you often come across batteries that have not enough charge left to be used in the appropriate devices like a flashlight or mini-mixer. Now throwing them away would be a waste of energy, so is there some project out there that can harvest the leftover energy?
The idea would be (optimally) an input voltage from 0V to 9V and an output that is capable of charging a liion-battery (4.2V ?). And then from the liion to a USB-port to charge any of the many devices that can be charged by it.
And if there are no leftover batterys maybe a solar panel or something like that.
You might experiment with one of the small solar powered LED outside walkway lights (~$5 at walmart). It has a charge pump to keep the battery charged and a solar cell to boot!
Wouldn't you get less than 10% of the total battery capacity below 1.1v for example?
This might be useful during the Apocalypse!
Otherwise I don't see how this is useful from old batteries...
Sounds like a GREAT "science fair" project! But, it probably not economically-practical, because there are already good alternatives where it's not economical to use (and throw-away) a mostly-discharged battery.
A solar battery charger is probably more practical. But again, the economics are usually not there... With the cost of the solar panel, the "free" energy is probably going to cost more than plugging into a wall (assuming you have wall-power available). For "trickle-charging", solar can work very-well, especially if you don't have convenient access to "traditional" energy. But, if you need to charge-up something like a laptop battery in a reasonable amount of time, that's going to take a fairly large solar panel. It would also take a shipload of "dead" flashlight batteries.
Now throwing them away would be a waste of energy,
Nobody likes to waste energy, but like it or not, EVERYBODY wastes energy. Almost nothing is 100% efficient, an it often comes-down to economics... Would you pay twice as much for a car that gets 1% better gas mileage? That would be stupid, unless you have so much money that cost doesn't matter. On the other hand, it would be stupid to not to pay 1% more for an identical car that gets twice the mileage.
If you use a switch mode power supply it ought to be able to convert any input voltage up/down and flatten every battery you feed it with. But unless you have a large supply of 'flat' batteries, I doubt that you will recover enough energy to justify the investment in your device or the effort of using it. Connecting and disconnecting batteries just to get a few watt-seconds of energy from each one would be quite tedious, I would imagine.
Better just to get quality rechargeable AA and AAA cells like Eneloops that will last a really long time. One Eneloop AA cell down to 80% usable capacity per full charge approximately equals 1000 name brand alkaline AA cells in cumulative lifetime capacity potential. (This is assuming you're not recharging your alkaline cells, which is possible to an extent, but not a good idea.) A couple other nice things about Eneloops is that they don't self-discharge over time at a noticeable rate like some other rechargeable cells do, being able to hold up to about 75% of a full charge after 3 years, and they have a higher usable capacity per charge than alkalies do at higher drain rates.
Solar charging is an option, but ideally you need a charge controller of some sort that charges a battery when it isn't full and when the sun is out, and then that battery is used to charge other batteries, or function as a 5V USB power source for charging cell phones, or similar.
So anyway, what I'm saying is that the best way to not waste alkaline cells is to not get them in the first place.
