Howto use USB to Recharge Lithium Battery (Li-Ion) and other considerations
In order to keep the recharging process safe and the battery healthy, the designer has to pay careful attention to the operating and charging specifications. These include:
- Charging input source
- Constant-current charge rate and accuracy
- Output voltage regulation accuracy
- Charge termination method
- Cell temperature monitoring
If a battery is not charged properly, the batteries’ recharge cycle life is put in jeopardy and more importantly the battery could also become unsafe and leak or even explode. A proper battery charging design will maximize the batteries life expectancy and ensure the end users’ device satisfaction.
Li-ion batteries are a commonly used rechargeable battery chemistry. Li-ion batteries should be charged in four stages:
- Trickle Charge
- Constant Current
- Constant Voltage
First the trickle charge stage. A battery that has been deeply depleted needs to be pre-charged with lower current before it can be quickly charged with a higher current. The trickle charge slowly restores the battery voltage to a threshold which allows for safe fast charging.
The second stage is the constant current charge which brings the battery voltage close to its maximum level. Charging the battery above its specified charging current should be avoided and will not actually reduce the overall charging time. When charging with a higher current the constant current stage charging time will be reduced, however the constant voltage stage charging time is increased at the same time, so overall nothing is gained and there is a risk of damaging the battery.
The next stage is the constant voltage charge which applies a constant voltage to the battery which should be within 1% accuracy.
The final stage is the charge termination. The constant voltage charging is terminated in one of two ways, after a minimum charge current is sensed or after a set time. Terminating the charging is complicated when the system is being charged and used at the same time. A system that is drawing current to the device while charging may trick the termination stage into thinking that the minimum charge current has not been reached, and therefore the battery could start overcharging. It is important not to overcharge the battery as it will decrease the battery recharge life cycles and may be dangerous and even cause an explosion. Also a system that is being used and charging at the same time can run into a risk of overloading the power supply since the charge current summed with the system current could exceeds the power rating of the supply.
There are a few options for controlling the recharging of Li-ion batteries. Some really highly integrated chips exist from companies such as Linear Technologies, Intersil and National Semiconductors. I always prefer to go with these types of solutions because they are easy to implement without spending too much design time or money, however it is possible to add your own recharging circuitry and use the embedded microcontroller from your device to control the charging cycle.
There are two main types of charging control, linear regulation and switch-mode regulation. The benefits of linear regulators are that they are typically low in cost and size however they are highly inefficient and cannot support charge currents higher than 1A. Most of the time these regulators also include dynamic power management features which switch the power between the system and its battery as well as multiple input sources like USB and wall adapters. Switch-mode regulators are far more suited for higher capacity batteries since they can support charging currents higher than 1A. These regulators provide the highest power conversion efficiency of all dc/dc regulators. They are also friendlier to work with from a thermodynamics point of view. Since they have a higher efficiency they do not heat up as much comparing to linear regulators which means that the thermal management is simpler and in some cases not necessary.
When charging from a USB source it is important to remember that the USB standard supplies no more than 500mA of current. If the charging circuitry draws more than 500mA the current limit protection circuit will shut the USB port down and charging will be stopped. There are several charging ICs that address this issue. They have a USB charge reduction control feature which automatically adjusts the constant current charge level in a linear fashion as the system demands more power. So when the USB voltage starts to drop the IC lowers the charging current in order to avoid tripping the current limit protection circuitry of the port. This allows for a maximum possible current to the battery while complying with the USB standard.
In devices that can be charged through wall adapters as well as USB ports I like to use a solution from Linear Technologies. The LTC4097 accepts a dual input one from a DC source like a wall adaptor and the other from a USB port. The IC takes care of most of the above considerations and can safely charge a Li-ion single cell battery. Moreover if you want to add device power management the more advanced LTC4098 also includes an ideal diode that will seamlessly switch between the systems supplies and deliver the power to the device and battery. These solutions greatly reduce the amount of design time needed to implement a power management system for an embedded device. Although most of these features can be implemented with a microprocessor and some external circuitry, it is far more efficient to use one of these ICs especially since they are relatively cheap.
It is important to remember that although there are many advanced battery management ICs available, which at a first glance seam to do everything that you desire it is still necessary to understand the exact process of recharging your battery. Battery companies use different manufacturing techniques that yield in slightly different battery properties, some batteries are far more suited for a specific type of recharging IC over another. Time should be spent in understanding the batteries datasheet and being able to match it to the appropriate charging IC. If a charging system is designed correctly, the end user will enjoy a problem free device with no blackouts and maximized battery life. The battery will be recharged in such a way as to take full advantage of its recharging cycles and will last a lot longer before having to be replaced.