Those circuits Azhar aren't suitable for lithium ion batteries, which require constant current charging up to the termination voltage (4.2v for those cells), and then constant voltage until the current drops to a termination threshold (typically 1/10th the initial charge rate). Voltage drops a bit after charging is terminated, and to avoid excess wear to the cells, the charge controller shouldn't restart charging again until the voltage drops to 4.1v or so.
You have a few problems to solve:
- Implementing the charging profile described above
- Preventing overcharging, overdischarge, excessive currents
- Obtaining optimal power capture from the solar panel
You could simplify #1 by avoiding the constant current phase at the expense of giving up some capacity (~10%).
For #2, you could probably rely on the boost converter to limit discharge current, but you'd still need something to cut off discharge when the battery drops to 3v.
I think you may be able to ignore #3 if you match the panel size and charging current properly, and make sure the panel isn't shaded.
The schematic is going to depend on what components you can get. There are chips that do most of the heavy lifting for you. The ones I'm aware of are surface mount, which makes it a little harder to build your own circuit. For the charging, there are cheap TP4057 chips from China. With any luck, you might be able to get it on a cheap board like
this one. There are also some that have
built in protection for the output.
Some odds and ends you'll want to think about:
- To deliver 1A at 5V from those laptop batteries, your battery pack will have to deliver at least (5v*1a)/3.7v = 1.35A of current because the boost converter trades current for voltage. However, with efficiency losses in the boost converter, it is probably safer to figure on 1.75A or so.
- A single one of those 18650 batteries should be able to deliver that current if it isn't too worn, but you're better off using at least two in parallel, particularly since you'll want the added capacity, because...
- To fully charge the 3000mAh phone, you'll need more than 3000mAh due to conversion and of inefficiency looses in the boost converter and the phone's battery charging circuitry.
- I'm guessing your laptop pack was manufactured at least 5 years ago. Even if the cells haven't seen any use, actual charge capacity could be lower than the 2,400 mAh rated capacity due to aging. If the pack has seen much use, cell capacity could be lower still. I've seen 2,600mAh versions of those cells from packs with (only) ~100 cycles that only have ~75% of the rated capacity.
- Assuming the cells have 1,800 capacity, and you use all 6, that will be 10,800 mAh. A reasonable charge rate for that many cells in parallel could be as high as 5.4A, but most commercial power banks of that capacity would only charge at 1-2A due to limitations of USB power supplies.
- I'd suggest figuring out the typical output of your solar panel and then picking a charging current to match
- Some people use two charger chips/modules wired to the battery-bank in parallel to obtain higher charging currents.
Good luck! I'm interested to know how your project goes. I've started a blog to document my own experiments with recycling laptop battery packs. So far I've been focused on sourcing and grading the cells, but soon I'm going to design and build some of my own power banks.
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