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Old 20th August 2008, 06:05 AM   #1
Clipped is offline Clipped  Thailand
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Default can a capacitor put charge into a battery?

if you have a very large bank of capacitors, fully charged at around 24 volts...

and connect it to a battery which is discharged to around 11 volts
can the charge from the caps bring the batteries charge back up?
to the nominal 12.6-12.8 volts?

this question doesnt have anything to do with caps in a stereo system...or the charging system in a car.

just a simpe transfer of energy question.

thanx!
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Old 20th August 2008, 06:12 AM   #2
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Yes of course, there will be a current from the caps into the battery. But the total amount of energy transferred will not be very great. Very quickly, the capacitors will discharge to the same voltage as the battery and then the current stops. So, the battery does receive some charge but it is only a fraction of what batteries normally need, so as a battery charger this is not useable.

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Old 25th August 2008, 03:10 AM   #3
Clipped is offline Clipped  Thailand
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thanks Jan, sorry for my slow reply...about how much current is stored ina 1 farad capacitor?

and if there was a 100 farad bank of capacitors, about how much current could be transferred to a standard SLA battery?

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Old 25th August 2008, 03:43 AM   #4
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An electronic circuit is required for efficient transfer of energy from a capacitor to a battery. Direct connection results in a theoretical maximum 50% efficiency and destructive peak currents.

The problem has to be analyzed in terms of energy, not in terms of current.

A 1F capacitor charged to 24V stores:

E = .5*C*V^2 = 288 Joules

The energy delivered to a battery while charging is:

E = I*V*T
288 = I*13.8*T
I*T = 288/13.8 = 20.87 amperes second

Or 2.09 A during 10 seconds, which translates into:

2.09*10/3600 = 0.0058 amperes hour (Ah)

Note that not all the energy delivered into the battery is actually stored, there are losses in the battery and in the transfer circuit (that must be a switching converter to be efficient).

Thus a 100F capacitor bank charged to 24V can theoretically deliver 0.58Ah to a 12V battery (more likely 0.52Ah after 10% losses are accounted for).

In general, the advantage of capacitors over batteries is very low AC impedance at the expense of low energy storage density, while the advantage of batteries over capacitors is high energy storage density at the expense of high and non linear (hysteretic) AC impedance.

In other words, a battery with an electronic transfer circuit becomes a much more convenient way to transfer energy to another battery.
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