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# Super capacitor in series with battery.

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#### myxer

##### Newbie
I have connected a supercapacitor(Maxwell 2.7V 350F) in series with a 40V battery. I charge them both separately but connect them n series when charged, hence I get 42.7V. I use this on my scooter and it runs fine.

I thought that the supercapacitor would decrease its voltage to 0 and the voltage of the combination would be the voltage of the battery (as the super capacitor has minimal resistance and can operate at 0 volts).

But I have found that the super capacitor actually decreases to a negative voltage over time! Can anyone explain why this is happening?

I guess i need some sort of simple circuit to isolate the super cap at zero voltage.

#### crutschow

The battery will keep current flowing through the capacitor in series, even after the cap voltage reaches zero, which then starts to charge it in the negative direction.
You could put a Schottky diode across the capacitor to prevent a reverse voltage of no more than about 0.5V.

How much current does the scooter motor take?

#### KlausST

##### Super Moderator
Staff member
Hi,

The mathematical formula for a capacitor_voltage over current is: V = C x integral(I) over time.
Unless you externally protect the voltage from going negative ... it will go negative. And it does not matter if it hurts/kills the capacitor.

Klaus

#### myxer

##### Newbie
The battery will keep current flowing through the capacitor in series, even after the cap voltage reaches zero, which then starts to charge it in the negative direction.
You could put a Schottky diode across the capacitor to prevent a reverse voltage of no more than about 0.5V.

How much current does the scooter motor take?
Scooter motor will take upto 30AMPS peak....if i put the schottky diode across the cap, then the minimum it will goto is 0.5v i guess but that should work, thanks! Their might be some sort of smart circuit i could use but the simple is often the best.

#### D.A.(Tony)Stewart

350 F is tiny compared to a battery C = Ic dt/dV so it will deplete >20 faster. It should be used as a temporary boost 5% of the time and must be reverse protected.

#### wwfeldman

I have connected a supercapacitor(Maxwell 2.7V 350F) in series with a 40V battery.

what is the purpose of the supercapacitor?
how does your scooter work without it?

#### Easy peasy

Super cap is no different to a much smaller battery in series - if you continue to pull current out of any cell or battery after you have drained it to "zero" then you will end up reversing the volts on the battery and of course your super cap too - basic physics ...

#### betwixt

##### Super Moderator
Staff member
Perhaps the easiest way to visualize the situation is to think of the motor as a short circuit. You then get the battery and capacitor connected directly across each other and the problem, especially the polarity, becomes more obvious.

Brian.

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