Solar Window Blinds + Charging Small Electronics

[holyguacamole]

Hello all,

My group and I are students who are looking to create a combination system for a project we're working on. A little background:

Our group's idea is to create a new way to gather energy from the sun and translate it to have the ability to power small electronic devices.

The idea is to use solar paneling and attaching them to window blinds which would then gather energy from the sun, which would be stored in a storage unit (battery), which would in turn have several USB outputs for the charging of small low voltage devices (ipod, phones, etc)

All this is not a terribly hard task but where we want to take our project a step up is to include a "priority charging" type method, where if we plug in 3 devices at once, our control circuity will be able to determine which device has the lowest voltage and begin to charge that device first until it reaches some sort of threshold level (lets say 4.0 volts for a 4.2 volt device) at that point, the control unit would cut power off to that device and begin charging on the second lowest charged device attached and so on and so forth.

I've read earlier posts about "measuring cell phone battery voltage" and seen a lot of good things said from people using this site so I was wondering if someone could put forth any ideas as to how we could best implement this. I've also read about Maxim IC 1551 which work as a dual input USB/AC adapter, which sounds like it'd be great to implement for our project. Even if we were to not implement a system which necessarily detects the voltage of each device, if we were to have three devices attached and tell our circuit to simply charge the first until a threshold is reached then shut off and begin charging the next one in the sequence, that would also be acceptable.

Thanks in advance for your feedback.

 

[cowana]

It's unlikely you'd be able to read the voltage of a device over USB (without actually talking to the device via USB protocol) - USB interfaces are designed to not directly connect the battery to the connector - the device will at least have a diode stopping reverse current and this measurement from being read. It's possible you could slowly ramp up the voltage and watch the current to see where it suddenly increases, but if the device has a charge controlling IC this will make this method non-functional.

Andrew

 

[holyguacamole]

I guess my biggest concern is what to do in order to create a system that would dedicate itself to fully charging one component before moving on to another. Because we're dealing with solar power stored in the battery, I want the battery to dedicate itself to charging one item completely before it transitions into charging a different device. Abstractly, I think of it as a circuit breaker, where once the desired threshold voltage for a device is reached, a control shuts off power to that device and the battery starts charging the next.

 

[ronv]

Do they all use the same battery chemistry?

 

[holyguacamole]

The battery chemistry? Do you mean same type of batteries or Li-Ion types? Some of the batteries will come from cell phone at a rated voltage between 3.7-4.2 and we would like to have one be attached to be an Ipod (not sure what the battery composition is there). In any case what would be the main difference?

 

[ronv]

If they are all different kinds and different voltages it will be more difficult because they require a different charger.

 

[holyguacamole]

Lets assume that we standardize the battery type and set them to be the same voltage? Whats a way to implement a cutoff control when the batteries reach a threshold voltage? Zener diodes, control transistors like those on Maxim, or something else?

 

[ronv]

Take a look at this site. It implies you can terminate on voltage if the charge rate is low. I've never tried it, but it might work for you. We can work on how to switch between batteries if you like it.

How to charge Lithium Batteries.

Charging Lithium ion batteries at slow rates

When the charge rate during the constant current phase is low, the charger process will spend less time during the constant voltage tail. If you charge below about 0.18 C, the cell is virtually full when the 4.2 volts is reached. This can be used as an alternative charge algorithm. Just charge below 0.18C constant current and terminate the charge when the voltage reaches 4.2 volts per cell.

 

[holyguacamole]

Thanks for the link, my group and I will look over this tomorrow and see if we've got any additional questions, we've got a snowstorm something out of the day after tomorrow going on here.