SuperCaps to Smooth out the Spikes
So you managed to cut down on active and standby power consumption, but your battery still dies quickly. What next? Well look closer at your power profile; are instantaneous pulses of power required? If so you can optimize further. Power spikes are strenuous on most battery chemistries. For instance alkaline batteries have a large equivalent series resistance (ESR) which causes an internal voltage drop when current is drawn. This means that the batteries ability to deliver short spikes of current is relatively poor, especially in cold conditions and as the battery gets older. The voltage drop will eventually be so significant that it may blackout the device, defiantly something that has to be avoided. If you want to avoid changing batteries often, a higher energy density power supply is needed. If you want to stick to the cheap and widely available alkaline batteries the best solution is to introduce a supercapacitor to the power supply. This is known as load sharing. The peak load on the battery can be reduced by placing a large value capacitor in parallel with the battery. For instantaneous loads the energy is supplied by the capacitor which than recharges during the inactive period. This considerably reduces the duty cycle, stress on the battery and the undesired voltage drop. Supercapacitors are perfect for the job because they have energy efficiencies that rarely fall below 90 per cent, can be charged extremely quickly and safely, can survive hundreds of thousands of charge-discharge cycles without significant degradation in performance. A test is described involving drawing 2A, 10ms pulses from an arrangement of three AAA batteries connected in series, at a rate of one pulse per second. The experimenters then measured the time taken for the battery voltage to drop below 3V. Circuits equipped with a supercapacitor exhibited up to four times the battery life of those using batteries only. This is a significant increase in battery life!!!! Adding a supercapasitor to your application should be relatively simple, without any serious design changes. However watch out for the higher current draw at power on, a simple current limiting circuit may be required. As well, a low voltage lock-out function should also be designed. Overall, with the amazing technological advances in supercapacitors it only makes sense to apply them to a pulse intensive application.