DIY solar powered battery recharger

[sampenso08]

hi, i would like some help with a school project. I'm making a laser alarm system but i want to intergrate a rechargeable battery to run the laser. i wanted the battery to recharge its self with a small solar panel. the rechargeable battery is only 3-4 volts. i know exactly how I'm going to make the laser alarm system but the only step missing is the battery to operate the laser.

i also need to take into consideration that when there is no sun light i dont want the solar panel to drain the battery. i was thinking of using a simple switch the break the circuit connecting the solar panel and the battery.  i also plan on putting all of this into a small project box so i dont want to have to remove the battery when i dont want it to charge.

any ideas and wiring schematics would be greatly appreciated. thank you

[AndrewM]

Here's a link with schematic to get you started:

http://ffaat.pointclark.net/blog/pages/BeamMax.html

The 4700uF capacitor in the schematic would be your battery instead.  You would want to setup the solar engine portion so that it turns on at 4V and off below 3V.  You can use a 6 volt solar panel and throw a diode between the solar panel and the battery to prevent the battery from running through the panel.

The 8212 datasheet available on the Maxim-IC website has all the specs for the circuit, but a word of warning:  The threshold and hysterisis setup on the Maxim datasheet is wrong (or was at least).  The original datasheet was correct once upon a time (I have a hardcopy someplace), but the one they had posted to the website used an incorrect formula resulting in a loss of two 8212s on my part.

[Adilson Gonçalves]

sampenso08
I´ve same problem with u man, i´ve a solar painel with 17W of power to charge a 12v battery
 I´d know if you solved your problem

"i also need to take into consideration that when there is no sun light i dont want the solar panel to drain the battery. i was thinking of using a simple switch the break the circuit connecting the solar panel and the battery."

tkss a lot

[Joker94]

the diode should stop the battery from discharging oi lossing power

It may still lose unnoticable amount of power but that shouldn't be a problem.

[Canabots]

Yes, the diode does prevent the solar cell from draining power from the battery.

Also, what compound is the battery? NiMH? Li-po? The circuitry is different for each (NiMH and Ni-Cad are the simplest, requiring just a diode, a solar cell, and the battery. The disadvantage though is that with the simplest form of circuit, you don't get any voltage monitoring )

While charging, the voltage of the solar cell should always remain  atleast 1V higher than the battery. This is to ensure a proper and full charge. So if your battery that needs charging is 3-4V, then a 6V solar cell should be fine.

The time it takes to charge a battery: this depends on the current output from the solar cell and how many mAh you're battery is. So if you're battery is 1000mAh and your solar cell can output a max of 30mA then:

1000mAh/30mA = 33.333...hours = 1.4 days

the quickest charge rate would be 1.4 days for 1000mAh.

Hope this was helpful.

[Soeren]

Hi,

While charging, the voltage of the solar cell should always remain  atleast 1V higher than the battery. This is to ensure a proper and full charge. So if your battery that needs charging is 3-4V, then a 6V solar cell should be fine.

If you don't mind cooking the battery to pieces, that is 

The time it takes to charge a battery: this depends on the current output from the solar cell and how many mAh you're battery is. So if you're battery is 1000mAh and your solar cell can output a max of 30mA then:

1000mAh/30mA = 33.333...hours = 1.4 days

the quickest charge rate would be 1.4 days for 1000mAh.

No,  there's a loss in the charging process, so it would be roughly:
1000mAh/30mA*1.5 = 50h = 2 days+
Depending on the chemistry of the battery, it may even take longer, when charging with such a low current and some cells will be destroyed - don't even attempt to treat a lithium cell this way, or it will be cross on you and do bad stuff to you when you aren't looking (like *you tortured me, now it's my turn, muahahaaaa* - probably even with bad theme music).

[Canabots]

If you don't mind cooking the battery to pieces, that is 

Lol, I've only ever charged NiMH/Ni-Cad batteries. But if they're lithium batteries...I'd be worried

No,  there's a loss in the charging process, so it would be roughly:
1000mAh/30mA*1.5 = 50h = 2 days+
Depending on the chemistry of the battery, it may even take longer, when charging with such a low current and some cells will be destroyed - don't even attempt to treat a lithium cell this way, or it will be cross on you and do bad stuff to you when you aren't looking (like *you tortured me, now it's my turn, muahahaaaa* - probably even with bad theme music).

Hmmm. You've gotta good point there!  Also, there's the issue of batteries discharging at night...they need to be able to last long enough.

Lithium...you need to have the right circuitry and the correct regulations in terms of power and current. Otherwise...KABOOM! And the battery's gone...