Lithium Polymer (LiPo) batteries are pretty much ubiquitous these days, especially in consumer electronics. As a battery technology, they have lots of advantages over other rechargeable battery chemistries including high energy density, a relatively flat voltage profile over their discharge range, low self-discharge, and high discharge currents.
They are however a bit fussy when it comes to how they must be charged and discharged, or they can be damaged or even catch fire. If you overcharge a LiPo cell above 4.2V, or at too high a current, the electrolyte can vaporize, and the battery can expand and potentially rupture. If you discharge one below 2.7V you can permanently damage the cell. If you have multiple cells in series, you may need to balance the voltage between them on a regular (or continuous basis) to maximize battery life.
If you want to design in a LiPo cell or two and associated charger, you’ll need to take all this into account. Fortunately, there are a plethora of dedicated LiPo charger ICs out there that will manage the details for you. A good example is the TI BQ24123 switched-mode LiPo charger. This supports one or two LiPo cells and charge currents up to 2A. Figure 1 shows the one-cell application schematic from the data sheet.
The typical charging profile for one of these chips is shown in Figure 2. If the battery voltage (shown in red) is lower than a VLOW threshold (3.0V or 6.0V for one or two-cell applications respectively) the charger enters a pre-charge phase where the cell is charged at a low constant current, ILOW (shown in blue) typically 10% of the normal charge current ICH. The pre-charge continues until the cell voltage reaches VLOW, when the current is increased to the full charging current, ICH. This can be as high as the “C” rating of the cell.
The C rating of a LiPo cell is a measure of how fast a cell can safely be charged or discharged. It’s used together with the cell capacity to determine the maximum charge or discharge current. For example, a 1000mAh cell rated at 1C can be charged or discharged at 1A, and one rated at 2C could be charged or discharged at 2A.
Constant current charging continues until the cell voltage reaches VEND (4.2V or 8.4V). At this point the charger switches to constant voltage mode, holding the cell voltage constant at VEND as the charge current decreases to the IEND level where charging is complete.
It is important that pre-charging or charging does not continue indefinitely in the case that the terminating condition cannot be reached. The charger should include a timer so that either stage of charging is terminated after a certain period of time. It is also desirable to monitor cell temperature and suspend or end charging if the battery temperature is too high (or too low if that is a risk in your application).
The BQ24123, along with many similar chips from a wide range of manufacturers, has all these features and should ensure your LiPo cells live a long and happy life.
I like this chip since it is relatively small and includes the switching elements and so requires only an external inductor and a handful of resistors and capacitors to produce a working charger. I have used it in several projects and can certainly recommend it for your designs.
“BQ24123 Data Sheet, Product Information and Support | TI.Com.” Accessed January 17, 2022. https://www.ti.com/product/BQ24123.
“Lithium Polymer Battery.” In Wikipedia, January 10, 2022. https://en.wikipedia.org/w/index.php?title=Lithium_polymer_battery&oldid=1064888568.