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I want to create a hybrid battery module to start a car engine. I intend to use a supercapacitor bank in parallel with a normal car battery.

The bank is composed of 6 capacitors mounted on a balancing module (bought from Aliexpress) and has 16.2V (6x2.7V; 350 farads/capacitor). The car battery is a normal 12V lead acid battery.

I am concearned that by connecting the two sources in parallel these will discharge one into another. What safety precautions must I take? Would placing a diode between the two sources prevent mutual discharging? Can someone offer me some advices on how to safely proceed with this? Did anybody make something similar?

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  • \$\begingroup\$ 16.2v is not a lot of headroom (30% is norm) from 14.x, i would be uncomfortable with a supercap getting over-voltage, judging from what the "little" one do when that happens. \$\endgroup\$ – dandavis May 31 '17 at 19:51
  • \$\begingroup\$ @dandavis Can you give some advice? \$\endgroup\$ – Cristian M Jun 1 '17 at 8:36
  • \$\begingroup\$ Is your goal merely to reduce the "peak" possible instantaneous current running through the battery while starting? If so, you'll probably need a pretty hefty capacitor bank (high capacitance) if your whole design is just "car battery + capacitors in parallel". This is because a standard car battery will tend to maintain its voltage a lot better than a small bank of capacitors for the same loss of charge. So a flatter area of the battery discharge curve may force it to supply quite the lion's share of current for that instant. \$\endgroup\$ – ManRow 2 days ago
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If you connect a car battery to an uncharged supercap bank, then a very high and damaging current could flow. The trick is to bring them to same voltage before connecting them. Use a resistor to limit the current flow, a headlamp bulb would be ideal, as it's rated for the voltage and dissipates power easily.

Once they are at the same voltage (or very nearly so), connect them directly in parallel. Then you can use the parallel combination as you would a more powerful battery.

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  • \$\begingroup\$ A lamp is very inefficient. Why not use a switchmode supercap charging controller? eg: bq24640 \$\endgroup\$ – Jeroen3 May 31 '17 at 18:40
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    \$\begingroup\$ Because you only have to do it once, so efficiency doesn't matter. \$\endgroup\$ – gbarry May 31 '17 at 19:07

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