Carry out capacitor power outage work

How long does an ACC capacitor last?

ACC is switched off during cranking, so it always cuts out. It can be for several seconds so a 4,700uF capacitor isn't going to hold up that long, it depends on the load though. The car will have a second ACC power line that has power throughout cranking, but it can be hard to find on the car's electrical wiring diagram.

Should a capacitor be 0V or 3V?

If you are using a capacitor to power something, then you must treat it similarly: It doesn't matter if your capacitor is truly dead when it's 0V if whatever you're powering requires at least 3V. Elliott’s answer explains the physics, but to answer the “applications” question about how much time your circuit will run, more information is required.

Does capacitor voltage drop linear to its capacity?

Yes, the capacitor voltage will fall as current is drawn from it, so you must initially charge the capacitor to a higher voltage than you need and then draw current from it until it reaches the lowest voltage you can still use. Also is the capacitor voltage drop linear to its capacity? for example a 5v charged capacitor when at 50% is 2.5v?

How do you calculate energy available in a capacitor?

Energy available = total capacitor loss x efficiency. 0 < efficiency < 1 -> usually in the 0.7 - 0.9 range. In the following calculations efficieny is assumed as 100%. Scale answers down linearly by actual efficiency - say start with 0.8 as typical.

How do you calculate DV in a capacitor?

dV = V = drop in capacitor voltage - Volts T - Time - seconds I - current - Amps C - Capacitance - Farad Eg 10 F cap, 20 mA, allowed drop = 5V6 to 3V3, dropout = 0.2V. T = V/IC = (5.4-3.3-0.2) / (0.02 x 10) = 1.9 /0.2 = 9.5 seconds. 10,000 uF = 10 mF = 0.01F capacitor Vstart = 5.4. Vout = 3V3.