After building the Solar iPhone charger, I’ve been wanting to build a Solar MacBook charger. It’s almost summer, and once I get back to Alaska it will be time for more solar projects. It will be great to get my notebook off the grid.
This is a collection of theoretical data and calculations for the charge times and feasibility of using a custom 40W solar panel to charge Apple products.
To make the most efficient charger, I want to use specific solar cells in my panel. Each device has a different charging voltage, and for efficiency sake I want to get as close to that voltage as possible with my solar cells. This is because, when I regulate my panel to the voltage I need for charging, the excess voltage is being given off as heat. If I’m regulating down a lot, I have to be concerned about heat sinks, and my panel is even bigger than I need it to be. Stupid, right?
For example, my MacBook Pro’s AC Adapter puts out 16.5Volts at 3.65Amps. If I make a 40Watt solar cell array that puts out 20Volts @ 2Amps, and then I regulate it down to 16.5Volts, I’ll be loosing (assuming my DC regulator works at 90% efficiency) – (90% * 2Amps = 1.8Amps) .2 amps for every volt that I’m outputting. This means that instead of having a 40Watt panel, I’ll actually have a 1.8*16.5 = 29.7Watt panel when everything is said and done. And that’s just from the efficiency of the regulator!
When you factor in the 3.5Volts that are being regulated away, you have (3.5*1.8 = 6.3) 6.3Watts that are being given off as heat. That’s hot and unnecessary.
So, lets say that I get my panel to put out 17Volts at 2Amps. That would be a 34W panel. I would only be loosing (.5*1.8) .9Watts to heat, and I’ll have a smaller panel in the long run. After using a 90% efficient regulator, you pull 10% off that 34W (3.4W) and you have a panel that puts out 30.6W in full sunlight. Then, shave off the .9Watts that is lost to heat and you have 29.7W again. That would take (73/29.7 = 2.46) a bit less than 2.5 hours to fully charge my MacBook Pro from a completely dead state. Not too shabby.
You can see that the best way to increase efficiency is by finding the closest voltage possible and then bringing up the amperage. That’s why it’s important that you choose the best wiring circuit for your panel! (Series or Parallel)
Here are some stats for the batteries in the newest Apple products. You can see how this hypothetical 34W panel would run them.
| MacBook Pro | MacBook | iPad | iPhone 3GS | |
|---|---|---|---|---|
| Battery Capacity: | 73 Watt-Hour Lithium-Polymer | 60 Watt-Hour Lithium-Polymer | 25 Watt-Hour Lithium-Polymer | 4.51 Watt-Hour Lithium-Ion |
| Charger Specs: | 60W or 85W MagSafe | 60W MagSafe | 10W USB Power Adapter - 30-pin Dock | 2.5W USB Power Adapter - 30-pin Dock |
| Time to charge- Adapter | 73 Minutes- 60W 52 Minutes- 85W | 60 Minutes | 2.5 Hours | 108 Minutes |
| Time to charge- 34W Panel | 147.5 Minutes | 121.5 Minutes | See Below | See Below |
For the iPad and iPhone, the input voltages are completely different.
I think you can safely charge an iPhone with 5Volts at a 1Amp maximum. 5V is the USB standard, and our computers put out .5Amps at that 5V. Not wanting to go too much higher than that for the iPhone, you could make a 4.5W panel at 6V and .75Amps – regulated down to 5V @ .75A that would charge the phone fully in 72 minutes.
As for the iPad, we’ll have to see what the input voltage is when it comes out!
[...] more info I found on the net. Solar MacBook Charger: A Theoretical Analysis. A "theoretical Analysis" but he seems to think 40W should be [...]