Solar Battery charger

Dgswaner

I'm trying to find a simple and cheap way to charger 4 AA batteries. the solar cell I'm using produces about 1.7v I want the output power of the charger to be 5v which will be regulated by a LM7805 circuit.

does anyone see anything wrong my idea. have a 2 position switch for Charge and Run mode. charge mode would wire the batteries in parallel (making them 1.5v), and connect the solar cell. Run Mode would wire the batteries in series (6v), and connect the lm7085 circuit. anything drastically wrong with this idea.

I realize that it won't have over charge protection, can't charge when it's on and no guarantee that it's charging with 1.7 v. but I can't see spending $50-70 making one of these.

thanks

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
DGSwaner

---

"When in doubt, use C4" - Jamie Hyneman, Myth Buster

Mike Green

1) You're probably using NiMH batteries and these normally run about 1.2V

2) How much current can the solar cell supply at 1.5v? Remember that rechargable batteries are limited in how much current they can accept. Usually 1/10C will not damage the battery no matter how long you leave it to charge.

3) The weakest (lowest voltage) cell will draw the highest charge current. Make sure you don't exceed 1/10C.

4) When charging batteries in parallel, the most charged cell will attempt to charge the weaker cells.

5) You're better off leaving the batteries in series for charging and using either several solar cells in series or a switching boost regulator that will step up the voltage to maybe 6V.

Dgswaner

I guess I should have mentioned, I'm trying to fit this in an Altoids tin. or I would just use multiple solar cells, I think I'll use your suggestion, and use a boost regulator. I found this one, http://www.pololu.com/catalog/product/791 and it's still a lot cheaper than other options.

Thanks Mike.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
DGSwaner

---

"When in doubt, use C4" - Jamie Hyneman, Myth Buster

sam_sam_sam

Why not use a LM317 in current mode set· to· 1/10C· and run your project off the batteries
That would easiest way to do this

If you have 1200 mill-amp hr· battery then you charge rate would be 120 mill-amp but you have look at you max voltage that your· solar panel
because if your voltage is more that your battery pack then may still over over charge the battery

I just had to redo a battery charger for a power tool because the charging board when bad

Now the battery pack is 12 volt @ 1800 mill-amp hr

I used a LM317 set to 110 mill-amp charge rate which is a 10 ohm resister from (-) to ADJ
I also used a 24 volt transformer with a bridge rectifier and a 900 uf cap

Now charger work real good except for one thing after 24 hour of charging the battery
The battery get real warm but·not HOT which can still be problem

I hope this help


▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·Now wanting to learn Spin· Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

Mike Green

Consider using 5 x AAA NiMH batteries.· I think you could get that to fit in an Altoids tin.· LadyAda has a boost regulator to produce 5V from a pair of AA batteries.· You might be able to modify their design to produce 7V from the solar cells.· Remember that you'll need a diode to prevent the batteries from discharging through the regulator (unless the regulator prevents that inherently).