Solar usb charger?

Question

I'm building a solar usb charger. I have everything connected and each solar panel produces .5 volts. I have 2 connected. They are connected through a diode to 4 rechargeable AA batteries. How long will it take for them to fully charge the batteries in direct sunlight? How many can i connect to make it recharge the fastest. Each battery produces 1.2 volts if that helps.

Answer 1

Your question can not be answered without knowing the mAh of the panels, and the batteries, as well as knowing if the panels are connected in series or parallel.  First off, I do not think they will ever charge with .5v, or 1v if connected in series.  Chargers for NiMh or NiCd are typically between 1.4-1.6v output during the charging cycle.

In addition to this, if the batteries are connected in series or parallel while charging, that will effect charge time and required charge voltages.

Now to answer your question, lets say you get enough panels to bring the charging voltage to 1.5v, and that your mAh output is 50, and the batteries are 900mAh each.  Lets say you are charging two at a time, and they are in parallel.  That would make the required voltage to charge them 1.5v, and the total capacity 1,800mAh, so at 50mAh it would take 36 hours to fully charge.  Now if you were to double the panels (need to double so you can run a 1.5v series circuit in parallel with the other) you would have a 100mAh charging circuit and it would only require 18 hours to fully charge the two batteries.  You can keep adding panels, but the faster you charge a battery the less cycles it can handle.  This is not a noticeable difference until you start charging them rather quickly, I would say 8-12 hour charge, maybe even 6 would give you the most charge cycles for a relatively acceptable time.

So just to sum it up, right now your setup will never charge the batteries, you need to run panels in series to get 1.4-1.6v (assuming you are charging the batteries in parallel  To get the number of hours required to charge you add up the mAh output of the panels, and the mAh capacity of the batteries, then divide the batteries by the panels.

This was assuming that the panels are always putting out peak power, since they will not your calculations will only be estimates at best, and without more advanced circuitry you will never be able to tell when a battery is really 100% charged.

If you wish to charge batteries in series, then you need to have 1.4-1.6v for each battery connected, and the time it takes to charge is a single batteries capacity divided by the panels output, however I would recommend charging the batteries in parallel or one at a time.