# Small Solar powered usb charger

Registered Users Posts: 2
Hello,
I am trying to build a solar powered usb charger..
I have a 9.0v 100mAh solar panel and the requirement of the usb powered device is 5v to 6v (so using a voltage regulator 7805 )

But the 100mAh is way too slow for charging my device
so i considered using some old rechargable batteries i have..
2 batteries of 1.2v each 700mAh (connected in series)
Pack 1
2 batteries of 1.2v each 2100mAh (connected in series)---- Pack 2

I am going to connect both the pack in parallel ..

Now the Notes and Questions :
1) Both the pack (1 and 2)are of different companies with different current but are of same chemistry (NiCd)
2) is the Solar panel suffiecient to charge the batteries?

The following is the circuit (made it in paint )

Just want your comments if i can do any thing to improve it..

• Super Moderators, Administrators Posts: 33,567 admin
Re: Small Solar powered usb charger

In general, a 9 volt array charging a 2.4 volt pack (is that correct?) is really inefficient. If the panel is 9 volt Vmp (voltage maximum power, vs Voc--voltage open circuit)--You can get 2x the wattage from your panel by charging the batteries in series (assuming they are roughly equal in discharge).

A solar panel is a constant current source... I.e., the output voltage (charging the battery) may be anywhere from zero to 9 volts, with 100 mAmp output in full sun.

But, the power is P=V*I

0.1 amps * 2.4 volts = 2.4 watts (two packs in parallel sharing current)
0.1 amps * 4.8 volts = 4.8 watts (two packs in series, sharing voltage with common current)

In the end, you probably need about 10% of rated capacity for a satisfactory charging current... A 5.6 AH pack would need ~0.56 amp charging current.

However, I am confused about your pack+hub configuration... In general, you are going to need >5volts from your battery packs to charge a 5 volt generic USB battery pack (or run a USB device). 2.4 volts will probably not cut it.

Unless you are pretty good in electronics, it might be better for you to look at a standard USB charging system of some sort... Goal Zero or Anker.

This one will take charging current on one input (the micro USB) and run/charge other USB devices on the (three) large format USB outputs.

http://www.amazon.com/Goal-Zero-11406-Silver-Battery/dp/B007M5UAJC

This is a small AA Battery charger--Both USB (and solar if you can wire it up?) charging in, and USB load power out... Seems a bit on the expensive side--But it does work.

I am not sure how either of these battery chargers are wired inside--I am hoping they are switch mode (buck?) type charge controllers--Which are much more efficient than using a 3-Terminal series type charge controller like the 7805.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 2
Re: Small Solar powered usb charger

The actual motive for making this project is to learn ..
Also i cannot incur any more expenses since i am new to this and got a soldering iron multimeter etc..

So i guess my best bet would be buying 500mAh batteries and charge them in series 2 at a time??
Or buying a solar panel with 450mAh with 9v and connect it in parallel ?

Also ,
Can i buy a 6v Rechargable battery with 1000mAh and use only one battery (10% =>100mAh)

-Mihir
• Super Moderators, Administrators Posts: 33,567 admin
Re: Small Solar powered usb charger

Yes, you can do either... You probably want to read up on rechargeable batteries. There are many chemistry types and methods of "properly" charging them.

You might want to read through this web forum:

Candle Power Forums

They have a lot of discussions on battery charging and even some sources for small battery chargers.

NiCad are pretty rugged batteries. Not terribly efficient, but you can charge them at ~10% rate of charge or less, unregulated without damage (i.e., you can "overcharge them" for hours or a few days).

The simplest method would be to find a panel with the appropriate current (or use a resistor or something else to limit current) to ~10% maximum of your NiCad batteries. Place as many in series such that the total battery string voltage is less than Vmp.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,220 ✭✭✭✭
Re: Small Solar powered usb charger

Well if you want to learn then checkout adafruits excellent tutorials. THey have a kit that does what you want, the essence of solar->battery->charge->device is a pair of buck converters. The last one is a simple step down ((or up) from the battery voltage to the usb 5v out. The first one is more involved as it has to match the available voltage and current from the pv to that of the bank. As bill outlined this is tricky. The chips used by adafruit for this do an approximation of mppt, which basically says that a given pv has a max power output when its output voltage is 78% of its open circuit voltage. It then basically draws as much current out of the pv while maintaining that voltage. These chips only run a couple of bucks.

As bill said also you can sorta do this by hand, but it involves a good understanding of the IV curve of your panel. Start with wikipedia iv curve.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar