I've got a small 500W amp I want to install but running a new power cable from the battery seems like a hassle. Would it work if I just plugged it into a high-wattage USB-C car charger instead? I'm trying to avoid drilling through the firewall if possible.
No way, a phone charger won't cut it - even a high-wattage USB-C charger tops out at maybe 240W or so, and your 500W amp would either barely work or fry the charger instantly! You really do need to run a proper power cable from the battery because car amps pull way more current than those chargers are designed for, and that firewall drilling is honestly the safe way to do it.
Your 500W amp is going to pull way more current than any phone charger can supply, and that's where things fall apart. Even if a USB-C charger claims high wattage, it's designed to trickle power into a battery over time. An amp draws current *constantly* and in spikes, especially when the bass hits. The charger would overheat almost immediately, possibly catch fire, and your amp wouldn't get enough juice to actually amplify anything beyond a whisper.
The real issue nobody mentions much is voltage drop. Phone chargers step down to 5V or maybe 20V, while a car amp needs the full battery voltage - typically around 12V for most setups, sometimes higher. Even if you somehow jury-rigged the voltage, the charger's internal protection would shut it down the second it detected that kind of load. You'd destroy the charger and be stuck without it.
Running power directly from the battery isn't as scary as it sounds. You don't necessarily have to drill through the firewall - you can snake the cable through existing grommets where other wires already go, along the edge of the firewall, or under the rubber trim. It takes time but it's the only way to do this safely.
The power delivery math just doesn't work here. Even if you found a 240W USB-C charger, your 500W amp would pull way more current than that cable and connector are rated for - you'd risk melting the charger's internals or the car's USB port itself. Running a proper power cable from the battery is genuinely the only safe way; the firewall drilling part sounds worse than it actually is once you do it.
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