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I'll apologize in advance as I'm sure this answer is already somewhere on the site. I don't have enough of an electrical background to know the proper terms to search for so I came up empty. I'm looking to install a diesel heater that draws 15A at 12V for about 2 minutes before dropping to 2A. The power pack I was hoping to use can supply 10A at 12V. I was wondering if a big audio capacitor could handle the startup but I have no idea how to calculate what I'd need or how to go about wiring it. I'm guessing the capacitor would need to be stupid big but now that I'm thinking about it I'd really like to know how stupid big it would need to be. Thanks in advance.

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  • \$\begingroup\$ you're right, it would need to be stupid big. and you would need other active parts to make it work, likely a boost+buck DC-DC converter. Capacitors lose voltage as they lose charge, so it will only hold 12.0v for an instant. If you could convert the whole of the charge to 12v with a magical perfect converter, you're looking at a 300 farad capacitor at 12v (21,600 joules). Get a bigger power pack, or get another power pack and connect it in parallel. \$\endgroup\$
    – dandavis
    Dec 5, 2023 at 6:17

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No.

  1. Capacitors aren't directly suited to this kind of application, because they don't hold a steady voltage (or anything even close to it) as they charge/discharge. To put a capacitor in parallel with a constant-voltage source for a meaningful "current boost" you would need a bunch of regulation circuitry that would be more complicated than the thing you already have. If you wired it up without that, it would basically not help at all (and would create some new problems).

  2. Even being generous with the numbers, yes, you need "stupid big". 5 A * 12 V * 120 s = 7200 J (aka 2 watt-hours), just to serve the "over 10 amps" portion of your load. The capacitors we work with most of the time are a millionth of that. A "reasonable" car audio capacitor is, say, 1 to 5 Farad @ 16V, which works out to between 256 and 1280 J (if you had a way to charge and discharge it with nonexistent 100%-efficient converters). There are a few products that claim to be in the 30F range, which would theoretically do the job, but their specs are pretty likely to be lies.

Delivering power over long periods of time is a job for a battery, not a capacitor, and what you really need is a battery box that's rated for your load current in the first place. 15A isn't asking that much.

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  • \$\begingroup\$ I appreciate the response and the numbers behind it. That's pretty much what I was expecting the answer to be and I've learned something along the way. \$\endgroup\$
    – Jack Weems
    Dec 5, 2023 at 6:38

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