Apologies this has probably been asked before – I have been looking but I think I don't know the terminology to answer my question.
I have a micro-controller based project and I want to power it with three LiPo 2000 mAh cells in parallel. I think I understand how to do that part the issue is with charging the batteries – which I know how to do separately but the design is such that the batteries cannot be removed.
Ideally, I want the batteries to charge quickish (couple hours). I thought about using USB but 500mA would be a slow charge. I would like to use a standard UK wall plug which I think is 12v which would be too powerful for the components in my projects (4.7V).
So I need to kind of turn the current off (is this called a circuit breaker?) just when the batteries are charging but want this to work automatically and then when they are charged I can remove the charger and turn the project on manually with a push button.
Another thing is I need to keep the charging components really small too like under 20 mm width and 60 mm length. I would like the wall plus to be mini USB ideally.
Also if the battery charging is kinda on a separate circuit (or part of the circuit is turned off – including the micro-controller) could I use AC rather than AC-DC
I hope that makes sense. Any help is hugely appreciated
Best Answer
If you want to deliver a full charge to LiPos, then you need something a little more sophisticated than 'switch it off when charged'.
LiPOs use a 2 stage charge.
In the first stage, the terminal voltage stays below 4.2v, and the charger limits the current to within the manufacturer's specifications for max charge rate. Depending on the quality of the cell, this phase need only take minutes, or with lower current you can drag it out as long as you like.
The first stage finishes when the cell voltage reaches 4.2v. At this stage, the cell only has around 70% full charge. Note that some commercial 'fast chargers' call 'charged' at this point.
In the second stage the charger limits the voltage to 4.2v, and the cell takes an ever decreasing current. Do not exceed 4.2v, or you will destroy your cell. If you stop the first stage at 4.1v, you will improve the life of your cell somewhat. The second stage finishes when the current drawn by the cell has dropped to 0.03C. This stage may take an hour, and cannot be rushed without damage to cell.
You can obtain LiPo charger ICs that will do the control and threshholding for you.