Lithium-ion to primary battery power design conversion strategies: Page 7 of 9

August 19, 2014 //By Terry Cleveland, John Haroian & Adam Jakubiak
Lithium-ion to primary battery power design conversion strategies
Terry Cleveland, John Haroian & Adam Jakubiak  consider some lithium-ion to primary battery power design conversion strategies.
provide longer runtimes in applications with high-current demands.
 
To overcome the voltage drop, especially if alkaline batteries are used, an energy reservoir needs to provide additional current when needed. A capacitor is the natural element to use, and a wide range of sizes and capacitances is available. With values up to and above 1F available in the lithium-ion voltage range, there remain two additional challenges in implementing a solution. First, the capacitor will have very low internal resistance - particularly when empty - so a current limit needs to be placed between the primary cells and the capacitor.  Second, introducing a current limit assumes an intentional resistance that needs to be controlled and managed to keep the efficiency high. A microcontroller with integrated analog that controls an external MOSFET solves both of these issues. The remaining design challenge centers on sizing the capacitor and MOSFET.
 
Figure 9 illustrates the blocks needed to control and protect the capacitor, and which items are integrated into a microcontroller. A suitable microcontroller will contain an internal oscillator, A/D, comparators and op amps, so that the external components needed are limited to just the MOSFET.
     

Figure 9: Block diagram of microcontroller, capacitor and battery
     

Figure 10: Flow chart of software algorithm
 
To design the solution, two elements need sizing:

●       Capacitor value

●       Current limit
 
For a design example, the assumed current peak will be 1A for a maximum of 500 mS, with a minimum repeat rate of 5 seconds between peaks.
 
The capacitor needs to keep its voltage value between 3.0V and 4.2V, during the 1A discharge of 500 mS, and recharge in 5 seconds.

Based on the capacitor discharge equation and the voltage assumptions above, the following equation sizes the capacitor:

C = t*Ipeak
 
A 0.5F capacitor provides the necessary reserve to deliver 1A for 500 mS.
  
To recharge the capacitor for

Design category: 

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