In Part One of this article, we discussed various battery types, their differences, and how to select a battery technology for a specific application. In this part, we’ll discuss how to implement a battery charger using Li-Ion technology as the example.
Li-Ion battery chargers use constant current (CC) – constant voltage (CV) profile for charging. The charging process passes through several stages to ensure the battery is charged to its full capacity while at the same time following certain safety rules. The CC-CV profile consists of several stages:
1. Pre-charge 2. Activation 3. Constant current 4. Constant voltage
Charging begins with a pre-charge stage to check if the battery is in good condition. During this stage, a small amount of current, usually 5 percent to 15 percent of the battery capacity, is provided to the battery. If the battery voltage rises above 2.8V, then the battery is considered in good condition and the charging process transitions to the Activation stage where the same amount of current is maintained for a longer duration of time. When the battery voltage rises above 3V, a rapid charge is initiated where a constant current equal to or less than the battery capacity is provided. This state is maintained till the battery voltage rises to full charge voltage (4.2V) or till time out; whichever is earlier. When the battery voltage reaches full charge voltage, the state is moved to a constant-voltage stage where the battery voltage is maintained constant. To achieve this, charging current has to be reduced over time. This stage of the charging process takes the longest time as compared to other stages of charging. In this process, when the charging current drops below the “termination current” limit, usually 2 percent of the battery capacity, the battery is fully charged and the charging process is stopped. Note that a time limit is kept for each stage of the charging process. This is one of the important safety features.
Figure 1: Charging profile of Li-ion battery
To implement this profile, battery voltage and charging current needs to be known at every time. Besides these, the temperature of the battery also needs to be kept in check. This is required because while charging, the battery tends to get heated. If the temperature exceeds the battery’s specified limit, it can cause damage to the battery.
The user has two options when it comes to battery charger implementation: using a dedicated battery charger IC or a more general-purpose microcontroller. The first option offers a quick solution to the problem but with limited configurability and user interface options (LED indications). Alternatively, the use of a microcontroller will take longer to design but offers configurability options in addition to the potential to integrate other functions such as battery state of charge (SOC) calculation and sending such information over a communication interfaces to the host processor in the system. In addition, a microcontroller is not equipped with the power circuitry necessary for a charger and requires external BJTs or MOSFETs. However, the cost of these power components is lower as compared to the microcontrollers or dedicated charger ICs.
Unless a sophisticated charging circuit is needed, using a PSoC would be an overkill. In general standard charging ICs will need a lot less design efforts.
But some time, the user might need to know the health life of the battery, how many charging cycles took place etc. In that case this will be helpful.
Another possibility for being able to use a single-ended ADC in this system would be to simply put a small-value resistor in series with the negative terminal of the battery pack, before its ground connection. This way the microcontroller system and the power supply can share the same ground. There is a small efficiency cost, but this can be mitigated by using a very small-value resistor, (for example, a 0.1 ohm resistor at 100mA would drop 10mV, at a power cost of only 1 mW).
Now a question about simplification of the process: would a current limited constant voltage charge work as well? It would not be as fast, it may take overnight to recharge completely, but it would certainly be simpler. Most importantly, would it damage the cells to start out with a current limited constant voltage charge? Or is it a situation that must have the higher voltage at the beginning?
Sometimes a regulator with a current limiting resistor on the input side is used as a cheap coin cell charger. The precharge time is very short, under a minute, on a Li-Ion battery. I think if the resistor was on the output side, it would be hard to top off the battery. You can use a simple 2 transistor circuit to up the current when the cell voltage goes up.
I don't think they mean to use the PSoC strictly for this function, but as a part of the overall PSoC system function. The battery health can be judged by a gas gage IC. They generate pulses proportional to the current versus time of the charge. Use it with a processor and you can make decisions about battery life.
In Part One of this two-part article, Cypress describes different types of rechargeable batteries, their differences and how to select a battery technology for a specific application.
The article is available here: http://www.eetimes.com/design/smart-energy-design/4375627/Battery--The-source-of-a-device---Part-1?Ecosystem=smart-energy-design
Implementing battery charger using Li-ion.
I have used Linear Technologies battery chargers for Li-Ion and Li-Poly. They work great. Using digital pots in place of fixed resistors for the charging voltage and charging current. These of course controlled with a micro-controller. life doesn't get any better than that. The micro allows for charging algorithms to suit different battery chemistry's and provides for status flexibility. It doesn't get any simpler than that. Piece of cake.