11.4 Capacitor Banks and Ultracapacitors
Beginner Level: What Capacitors Do
Capacitors discharge faster than batteries during very short transients (1-50ms), supplying instant current for bass hits.
Energy stored:
E = ½CV²
1 Farad at 12V:
E = ½ × 1 × 144 = 72 joules
At 1000W: t = 72/1000 = 0.072 seconds
Helps with one transient. Battery takes over for sustained loads.
"1 Farad per 1000W" rule: Rough guideline for adequate buffering.
What capacitors DON'T do: Replace battery capacity, fix undersized alternators, provide continuous power.
Installer Level: Installation
Location: Within 12" of amplifiers optimal, 36" acceptable.
Wiring: Fuse at positive connection (1.5-2× amp's fuse rating). Polarity critical - reversed destroys capacitor.
Multiple caps in parallel:
C_total = C₁ + C₂ + C₃ + ...
Discharge before working: 10Ω resistor across terminals for 30 seconds.
Engineer Level: Ultracapacitors
3 parallel strings of 5 series 350F cells:
C_series = 350F / 5 = 70F per string
C_total = 3 × 70F = 210F at 12V
Energy = ½ × 210 × 144 = 15,120 joules
At 2000W: 7.6 seconds full power
Cost: 15 cells × $30 = $450 vs $300 AGM with same energy. Ultracaps excel at power density (fast discharge), not energy density.
When They Make Sense
Capacitors and ultracaps are best treated as support hardware, not miracle fixes. They help most when the charging system is already fundamentally healthy and the problem is short transient demand rather than chronic power deficit.
If the vehicle voltage is low all the time, the right fix is usually wire, battery, or alternator capacity, not a shiny capacitor added after the fact.
Quick Rule
Use capacitors for short transient support and use batteries or alternator upgrades for sustained deficit. Mixing up those jobs is where most capacitor disappointment comes from.