How to select the right rechargeable batteries for your imp-enabled product
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.
Several aspects of the recharging process present technical challenges:
Fortunately, there are a great many ICs that manage the charging process. Here are two recommended options:
|Part Number||Manufacturer||Pros||Cons||Price at qty 1k (Vendor)||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||$1.12500 (Digi-Key)||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||$0.42 (Digi-Key)||C|
There are three primary 'threats' that the battery must be protected from:
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||Price at qty 1k (Vendor)|
|S-8241||Seiko||Provides protection against overcharging, over-discharging, and over-current; small physical size; low cost||Requires two external NFETs||$0.34485 (Digi-Key)|
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:
With a protection IC with sufficient hysteresis:
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:
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||Price at qty 1k (Vendor)||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||$0.675 (Mouser)||A|
|3.3V buck/boost||LM3668||Texas Instruments||High output current (1A), provides 3.3V supply||Expensive, high quiescent current (45μA)||$1.12 (Mouser)||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||$0.28 (Digi-Key)||C|