Solar Charger for Cellphone

General project help for Adafruit customers

Solar Charger for Cellphone

I've thought about building a portable charger for my cellphone that's similar to the solar mintyboost. For my Droid smartphone, how do I determine how large a collector I need to charge the phone in a reasonable amount of time (say 1 hr in direct sunlight). I'm looking to build a backup for the times that I'm out for a while on a sunny day and my phone runs dead. I've seen the tiny chargers on various sites and have read a number of reviews indicating the ability to charge was poor. So, I'm not that concerned if the size is 8x8" as long as I can get a decent charge.

What do I need to look at in the specs of my phone (or other device) to determine the power consumption and choose the right panel?
d7x

Posts: 43
Joined: Tue Oct 25, 2011 9:41 pm

Re: Solar Charger for Cellphone

say 1 hr in direct sunlight

The limiting factor in any case is going to be the charge rate. The MintyBoost will charge at 500mA. Some wall-chargers will do 1000mA. But with phone batteries in the 1400 mAh range and a charge efficiency of about 80%, you are not likely to get a full charge on an empty battery in 1hr.

But a solar minty boost based on the large panel in the store should be able to keep a good-sized LiPo topped off. Then you can plug your phone in at any time for a re-charge.

Posts: 15977
Joined: Sat Feb 07, 2009 9:11 am

Re: Solar Charger for Cellphone

say 1 hr in direct sunlight

But with phone batteries in the 1400 mAh range and a charge efficiency of about 80%, you are not likely to get a full charge on an empty battery in 1hr.

But a solar minty boost based on the large panel in the store should be able to keep a good-sized LiPo topped off. Then you can plug your phone in at any time for a re-charge.

1) What is a reasonable expectation for the amount of time it would take to charge a phone battery with a large solar panel and a good-sized LiPo? I'd love to understand the formula to calculate the amount of consumption/hr vs. the amount of charge/hr with different solar panels.

2) I'm not yet up to speed on power consumption specs. Is there a good reference you would suggest for getting up to speed on topics like understanding power consumption and amps vs. volts and current vs. watts?
d7x

Posts: 43
Joined: Tue Oct 25, 2011 9:41 pm

Re: Solar Charger for Cellphone

There are a lot of variables. Assuming good direct of sun, you should be able to generate 500mA. But the charger won't just dump that into the battery straightaway. LiPo batteries need a charge cycle that starts and finishes 'gently'. The max charge rate will only be achieved for the middle of that cycle. Mor information on the charge cycle can be found in the datasheet for the charger chip.

The basic equations for power consumption are :
Watts = Volts * Amps
... and ...
mAh (milliamp-hours) = mA * hours.

The "Power" page of the Mintyboost tutorial describes the practical application of this - especially the last sections of it.

Posts: 15977
Joined: Sat Feb 07, 2009 9:11 am

Re: Solar Charger for Cellphone

Wall type USB charging can give you about 1A @ 5V, so that's around 5w per hour.

In order to do the same with Solar you would need the 5w plus about 20% for efficiency, so a 6w panel. In order to get a 6w panel you would have to go with a 12v model or make one. Prebuilts can be had for under \$30 online. But its not that simple, since a 12v panel is actually around a 16-18v panel. At there rated 6w they are actually around 16-18v, the reason is simple, to charge a 12v battery you need more voltage then the battery in order to charge it, and car batts actually sit over 13v charged. So you need a StepDown from 17v or so to 5v. A cheap but effective tool for this would be the MC34063, it can output up to 1.5A and can be used to step up or step down voltages.

http://www.eevblog.com/2010/09/10/eevblog-110-lets-design-a-dc-to-dc-switchmode-converter/ Handy video blog on it.
http://dics.voicecontrol.ro/tutorials/mc34063/ A calculator that does all the math for you.

Add some form of battery and charge controller and your set, it wont be compact at that rating thou, some of the 6w panels are 10x12 inch.
And ive seen 5w 6v(8v) panels at around 8x12inch. So they wont fit in your pocket, but are around book size.

This is actually something I was looking at but burned most of my funds building a smaller unit designed to solar charge a custom 3.6v NiMH battery with homemade charge controller and power a homemade mintyboost. Building a solar panel is far more expensive then just buying one:(
Donziboy2

Posts: 32
Joined: Sun Feb 26, 2012 12:45 pm

Re: Solar Charger for Cellphone

I recently purchased the minty boost and the solar lipoly charger. I'm having a problem with the charger working when I try to charge my cell phone off of the battery alone. I use a blackberry curve 8500. I have sen it work occasionally but the it will stop suddenly. I keep thinking the battery is being drained but when I go to charge the battery it states it is full. I'm using a 3.7v 2700mAh lithium battery connected into the solar lipoly charger. The minty boost is also connected into the lipoly charger. I didn't deviate from any of the settings specified in the tutorial. Is the battery output to my minty boost not enough to charge my phone? If so, i believe you can increase the power by soldering in a resistor. I'm not sure where to solder in the resistor. I would appreciate some help. I plan t use this on long mountaineering trips. I'll be in the wilderness for weeks at a time with this as my only means of charging. It's important that the battery charges the phone. Any help would be fantastic.
DIG_Marketing

Posts: 5
Joined: Thu Mar 29, 2012 5:21 pm

Re: Solar Charger for Cellphone

@DIG_Marketing - To avoid confusion, let's keep this discussion in one thread: http://forums.adafruit.com/viewtopic.php?f=19&t=27226

Posts: 15977
Joined: Sat Feb 07, 2009 9:11 am

Re: Solar Charger for Cellphone

i am in a similar situation, i have my incoming current from solar stopped down to 5v @1A, cause i am tricky like that, but i need to get the lipo charger to charge my battery @ more than 500mah. I know this can be changed by playing with the resistor on the charger, but i am not sure how to do so, do i just add a 2.2k risistor over the existing one, or do i remove and replace that resistor on the board?

p.s. i am charging the 6600mah batt, and i have a thermistor already attached
MrBiek

Posts: 9
Joined: Sun Jan 10, 2010 9:34 pm

Re: Solar Charger for Cellphone

do i just add a 2.2k risistor over the existing one, or do i remove and replace that resistor on the board?

You can do it either way. If you wire them in parallel, you can calculate the effective resistance this way:

Rt = (R1 * R2) / (R1 + R2)