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I saw some DIY projects about boosting car lead acid batteries. A supercapacitor bank is connected in parallel with a lead acid battery to stabilize the supply.

However, in the market, I can only find 16 Volt capacitor banks. And my lead acid battery is 12 volts. (fully charged voltage is about 13volts)

I'm pretty sure the starter motor is not going to like that extra voltage. Regardless, What will happen when a fully charged 16-Volt supercapacitor bank and a fully charged 12V lead acid battery are in parallel when the engine is stopped?

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  • \$\begingroup\$ btw, a motor can virtually always take and extra 20-30% voltage above baseline for a short time, like a starter. Heck most can run that extra full time, provided there's enough cooling. \$\endgroup\$
    – dandavis
    Commented Jun 2, 2023 at 19:53

3 Answers 3

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Capacitor voltage rating is not how much volts it WILL have when charged. It's how much volts it CAN take before it explodes. If you charge a 16V capacitor from a 12V battery it will charge up to 12V. If you charge a 16V capacitor from a 24V battery it will explode.

Basically you want you capacitor to have higher rating than your expected operating voltage. How much higher depends on how safe you want to be. Normally voltage in a car doesn't exceed 14V so 16V should be good.

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  • \$\begingroup\$ I get it now. thanks. this Q got 3 similar answers telling the same. I think i should pick correct answer by choosing with the least amount of points \$\endgroup\$ Commented Jun 1, 2023 at 23:09
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The 16 volt rating on the capacitor is the maximum safe voltage you can apply to it. You do not need to charge them to that voltage.

If you do charge the capacitor to 16 volts, then connect it to a 12 volt battery, the capacitor will discharge into the battery, charging it slightly, and losing voltage in the process.

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  • \$\begingroup\$ I get it now. thanks. this Q got 3 similar answers telling the same. I think i should pick correct answer by choosing with the least amount of points \$\endgroup\$ Commented Jun 1, 2023 at 23:09
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The capacitor voltage is not the voltage the will have when charged, it’s their maximum voltage rating. If you put one in parallel with a 12 V battery they will charge to the same voltage as the battery. The reason you see so many rated at 16 V is because that lets you use them in a 12 V system and have a safety margin on the voltage.

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  • \$\begingroup\$ I get it now. thanks. this Q got 3 similar answers telling the same. I think i should pick correct answer by choosing with the least amount of points \$\endgroup\$ Commented Jun 1, 2023 at 23:09
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    \$\begingroup\$ @user2102266 Good idea, was going to suggest that but you were way ahead of me. \$\endgroup\$
    – GodJihyo
    Commented Jun 1, 2023 at 23:17

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