# Does the voltage of the battery change if you use it? [closed]

If you use the battery to charge a capacitor does the voltage of the battery decrease or still the same? Does the voltage of a capacitor decrease if you use it to charge another capacitor?

## closed as off-topic by Steeven, Yashas, Bill N, Kyle Kanos, stafusaMay 11 at 16:19

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If you use the battery to charge a capacitor does the voltage of the battery decrease or still the same ?

It will initially decrease when the capacitor is first connected to the battery, due to the internal resistance of the battery.

All real batteries have internal resistance. When current is dawn from the battery, by a capacitor or any other load, there will be a voltage drop internal to the battery and therefore the voltage at the battery terminals will be less than when no current is drawn. See diagram below for the equivalent circuit of a real battery with capacitor load.

Assume when the switch shown is open, there is no voltage across the capacitor. When the switch closes an ideal capacitor initially looks like a short circuit. The current is then the internal Voltage of the battery divided by its internal resistance.

Now of course when a capacitor draws current from the battery it will charge up causing an increasing voltage across its terminals that opposes the battery voltage. This means the current drawn from the battery will decrease in time eventually becoming zero when the capacitor voltage equals the battery terminal voltage. As the current decreases the voltage drop internal to the battery decreases and the battery terminal voltage increases to its initial "no load" voltage, assuming little consumption of the battery chemical energy.

Does the voltage of a capacitor decrease if you use it to charge another capacitor ?Hope this helps.

Yes, because now the charge battery acts like a battery. Replaced the battery in the circuit below with the charge capacitor. But now, unlike the battery, the capacitor generally loses its stored energy much more quickly than a battery, so its voltage will drop until it eventually equals the voltage of the capacitor being charged, at which time current will cease.

This will respond to your following comment:

Assume a battery has voltage of 24 connected with capacitor , before the current start flow the voltage of the battery is 24 and the voltage of the capacitor is zero , After the current start flow , The current will keep flow until no voltage for example 12 volt for the battery and 12 volt for the capacitor (the voltage between the battery and the capacitor is zero), But out teacher tell us that the voltage of the capacitor in this situation equal the voltage of the battery before the flow start which is 24 ?

Your teacher is correct. Once the capacitor is charged, the battery voltage should be the same as before charging.

You didn’t say what kind of 24 v battery it is or how big the capacitor is, but the voltage of the battery after charging the capacitor should not be significantly less than before.

There are two main reasons for a battery voltage to drop.

1. Due to normal internal resistance of the battery. This is the example I gave you which was based on what I thought your question was. This drop is temporary- it only lasts as long as current flows.

2. Due to loss of stored chemical energy in the battery. This voltage drop is permanent and usually occurs after a long time of battery usage, or under very heavy loading conditions. Listen to the video link you sent me. The instructor gives an example in which that the energy stored in a battery is much, much larger than the energy stored in a capacitor. So when such a battery charges a capacitor the energy it loses is a small percentage of its stored energy, and therefore its voltage after charging should be little reduced. A possible exception is if the battery was very small and the capacitance was very large.

Hope this helps. • I Can't understand this , This means the current drawn from the battery will decrease in time eventually becoming zero when the capacitor voltage equals the battery terminal voltage ,The capacitor is charged and the battery is zero charged so there is a voltage so why the current doesn't flow back ? – Mohammad Alshareef May 8 at 21:30
• @MohammadAlshareef The current eventually becomes zero not because the battery voltage becomes zero, but because the voltage on the charged capacitor equals and opposes the terminal voltage of the battery. – Bob D May 8 at 21:46
• @MohammadAlshareef May I ask, what courses in electrical circuits have you taken? – Bob D May 8 at 21:47
• Assume a battery has voltage of 24 connected with capacitor , before the current start flow the voltage of the battery is 24 and the voltage of the capacitor is zero , After the current start flow , The current will keep flow until no voltage for example 12 volt for the battery and 12 volt for the capacitor (the voltage between the battery and the capacitor is zero), But out teacher tell us that the voltage of the capacitor in this situation equal the voltage of the battery before the flow start which is 24 ? – Mohammad Alshareef May 8 at 22:09
• In our country the education is very weak , And our book is full of mistakes , So I am really face many problem in understanding . I am not taking any course , I am just studying in the school. – Mohammad Alshareef May 8 at 22:14