Using Lithium-ion And Lithium-Polymer Batteries

Lithium-ion (Li-ion) and Lithium-Polymer (LiPo) batteries are a natural choice for small Internet-connected devices. The small, flexible form-factor and ability to recharge these batteries offers a great opportunity to improve industrial and mechanical design as well as end-user experience.

Li-ion and LiPo batteries do require special care when developing hardware, however. Special considerations need to be made in order to ensure safety and performance. Care needs to be taken with all parts of the design, from battery charging, to over-discharge protection, to the overall power story of the device.

Three different lithium battery solutions (including charger, battery protection and system power supply) are presented below. These solutions are mix-and-match; any of the battery charger solutions can be used with any of the system power supplies. The battery protection solution is identical in all three examples.

Battery Charging

Several aspects of the recharging process present technical challenges:

• Over-charging or over-discharging Li-ion or LiPo batteries will result in physical damage to the battery, permanently decreasing capacity and potential causing a fire hazard.
• A lithium battery must be charged in stages.
  • A deeply discharged battery must be 'trickle charged' with very limited current before entering the 'fast-charge' stage.
  • The maximum current provided during the fast-charge stage should not exceed one third of the overall battery capacity. For example, a 1500mAh battery should not be fast-charged with current in excess of 500mA.
  • Cell temperature must be monitored during recharge to protect against over-heating, which will permanently damage the cell.
  • At the end of the charge cycle ('top-off'), maximum current and charging voltage must be carefully monitored to prevent over-charging.
• Continuous top-off can wastefully reduce the overall life of the battery. This situation arises when the device is left on the charger past charge completion; the charge cycle ends, and the device begins to run off the battery. When the battery voltages dips below the 'full' threshold, the charger turns back on, topping the battery off. The cycle then repeats. Some charger ICs, including the one recommended below, include features to avoid this state.

Fortunately, there are a great many ICs that manage the charging process. Here are two recommended options:

Part Number	Manufacturer	Pros	Cons	See Design
BQ24073	Texas Instruments	Power-Path continuous top-off prevention, cell temperature monitoring, programmable fast-charge current up to 1.5A, programmable system input current, separate limiting of charge current and system current	Higher cost than non-Power-Path solutions, higher component count	A, B
MCP73831	Microchip	Lower cost, low component count, integrated temperature monitoring, programmable fast-charge current	No continuous top-off prevention, no external cell temperature monitoring, max fast-charge current 500 mA	C

Battery Protection

There are three primary 'threats' that the battery must be protected from:

• Overcharging
• Over-discharging
• Over-current

Overcharging protection is provided by the battery charging IC, but is additionally included on most battery protection ICs. Designs A, B, and C all use a single IC to protect the battery from over-discharge and over-current events.

Part Number	Manufacturer	Pros	Cons
S-8241	Ablic	Provides protection against overcharging, over-discharging, and over-current; small physical size; low cost	Requires two external NFETs

Boot Looping, Under-Voltage Lockout And Hysteresis

The most important purpose the battery protection IC serves is to lock out the system if the battery voltage becomes too low to support the system. This occurs when there is no longer sufficient energy in the battery to power the system. At the end of battery life, the battery voltage will drop substantially when the battery is put under load. Because the battery voltage will ‘recover’ most of the way when the load is released, the battery protection IC must have sufficient hysteresis to keep the system locked out when this occurs.

If the system does not have sufficient hysteresis, a boot loop will occur:

1. The imp wakes up and turns on WiFi (high instantaneous load).
2. Battery voltage decreases sharply due to sudden load.
3. The imp browns out due to low supply voltage and resets (load removed).
4. Battery voltage 'recovers' due to load removal.
5. The imp wakes up and attempts to boot again, and so on.
6. This process repeats until there is insufficient energy to start the boot cycle, at which point the battery may be permanently damaged.

With a protection IC with sufficient hysteresis:

1. The imp wakes up and turns on WiFi.
2. Battery voltage decreases below the protection IC's turn-off threshold voltage (2.5V).
3. The protection IC cuts power to the system (the system is 'locked out').
4. The system remains locked out until the protection IC detects that the battery voltage is above the turn-on threshold (2.9V).

System Power Supply

Selecting the power supply for the system is the most important piece of the design with regard to maximizing battery life. There are several parameters to consider:

• System supply voltage and headroom: A single Li-ion or LiPo cell runs from 4.2V (fully charged) to around 3.2V (fully discharged); while the nominal voltage is quoted as 3.7V, the actual cell voltage will pass through this full range almost linearly during each discharge cycle. This makes providing a 3.3V supply rail tricky.
  • There is insufficient headroom to run the system at 3.3V from a simple buck regulator or LDO; the regulator will no longer be able to maintain regulation while the battery still has some usable energy left in it, which wastes battery life. Many buck power supplies go into a low-efficiency PWM mode when headroom is at or below 400 mV, which can bring the buck's quiescent current up to several mA.
  • If the system is run from a 3.0V supply rail, a simple buck regulator can be used. Some LDOs can also maintain regulation with 200mV of dropout and supply enough current for the imp module.
  • The imp can run and operate Wi-Fi down to 2.6V. Below this point, the imp can still run code locally, but will be unable to bring up Wi-Fi and connect to the Internet. To maximize headroom and efficiency, it is possible to run the system at 2.7V to 3.0V.
  • If a 3.3V rail is absolutely required (some components are unable to function across the same voltage range as the imp), a buck/boost regulator can be used.
• Power Supply Current: A switching power supply will be able to provide more instantaneous power than a low-Iq LDO. For any application requiring more than 200mA at any one time, a switching power supply is strongly recommended.
• Efficiency: A switching power supply will provide better efficiency and overall battery life than an LDO for the majority of applications. While the quiescent current (Iq) of a switching power supply might be higher than that of a low-Iq LDO, the efficiency under load will be much better. An LDO regulating from 4V to 3V has an efficiency of around 75 per cent, while a switching power supply will have a efficiency of around 94 per cent.

Each of the three designs provided demonstrates a different power supply solution (3.0V buck regulator, 3.3V buck/boost regulator, 3.0V LDO).

Power Supply Type	Part Number	Manufacturer	Pros	Cons	See Design
3.0V buck	TPS62233	Texas Instruments	High Efficiency (94%), small size, up to 500mA output current, low Iq (22μA), less expensive than buck/boost	Lower output current than buck/boost	A
3.3V buck/boost	LM3668	Texas Instruments	High output current (1A), provides 3.3V supply	Expensive, high quiescent current (45μA)	B
3.0V LDO	MCP1700	Microchip	Inexpensive, low component count, low Iq (1.6μA), low dropout voltage maximizes headroom	Low output current (250mA max), inefficient under load	C

Reference Designs

Solution	Description	Schematic	BoM
A	Features BQ24073 Power-Path enabled battery charge controller, over-discharge protection and TPS62233 3.0V buck power supply	Link	Link
B	Features BQ24073 Power-Path enabled battery charge controller, over-discharge protection and LM3668 3.3V buck/boost power supply	Link	Link
C	Features MCP72831 battery charge controller, over-discharge protection and MCP1700 3.0V LDO	Link	Link