Design Article
Implementing battery charger using Li-ion
Pushek Madaan and Rajiv Badiger, Cypress
6/26/2012 9:01 AM EDT
Charger architecture
From the charging profile, it is understood that a controlled current source is required for a single-cell Li-Ion battery charger. The current source output needs to be altered, depending on the battery state. Considering these requirements, the following functional blocks are required for a microcontroller-based implementation:
1. Current control circuit
2. Battery parameter (Voltage, current, temperature) measuring circuit
3. Charging algorithm (for implementing CC-CV profile)
This is shown in block diagram below:
Figure 2: Block diagram of a Li-ion battery charger
The current control circuit can be built using a voltage source and a current feedback. It works like a typical negative feedback control system. The feedback is taken by allowing the charging current to pass through a small resistor, thereby developing some voltage.
The voltage source can be created using two methods:
1. Linear Topology
2. Switching – Buck or Boost Topology
The charging current is controlled by controlling the bias to the series pass transistor Q1. The bias can be controlled using a digital to analog converter (ADC) or pulse width modulator (PWM) with an external RC low pass filter. The linear method is suitable for low charging currents <1A due to power dissipation in series pass element.
Next: Page 3
From the charging profile, it is understood that a controlled current source is required for a single-cell Li-Ion battery charger. The current source output needs to be altered, depending on the battery state. Considering these requirements, the following functional blocks are required for a microcontroller-based implementation:
1. Current control circuit
2. Battery parameter (Voltage, current, temperature) measuring circuit
3. Charging algorithm (for implementing CC-CV profile)
This is shown in block diagram below:
Figure 2: Block diagram of a Li-ion battery charger
The current control circuit can be built using a voltage source and a current feedback. It works like a typical negative feedback control system. The feedback is taken by allowing the charging current to pass through a small resistor, thereby developing some voltage.
The voltage source can be created using two methods:
1. Linear Topology
2. Switching – Buck or Boost Topology
The linear topology uses a series pass element (either BJT or MOSFET) in linear mode as shown in figure 3.
Figure 3: Linear topology
Next: Page 3
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GREAT-Terry
6/26/2012 11:58 AM EDT
Good article!
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Sanjib.Acharya
6/26/2012 12:37 PM EDT
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.
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Hughston
7/9/2012 12:09 PM EDT
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.
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Mark Moulding
6/28/2012 3:11 PM EDT
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).
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WKetel
7/4/2012 1:12 PM EDT
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?
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Hughston
7/9/2012 12:00 PM EDT
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.
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anne-francoise.pele
7/16/2012 11:25 AM EDT
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
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cgul
9/27/2012 2:39 PM EDT
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.
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