# Batteries in Parallel vs Series

1. Sep 22, 2015

### Bikrypto

I seem to lack a conceptual understanding of how batteries in series and parallel affect the brightness of a bulb. I understand that current adds when batteries are in parallel and that voltage adds when batteries are in series, but why does 2 batteries in series make a bulb more bright than two batteries in parallel. If the power output of the bulb is P = I^2*R = V^2/R, and if parallel doubles the current and series doubles the voltage, why is series brighter. I think I lack an understanding of how current and voltage affect the brightness of a bulb and would like an explanation.

2. Sep 22, 2015

### Staff: Mentor

Welcome to the PF.

You need to also think in terms of Ohm's Law V = I * R

When you parallel the batteries, the output voltage stays the same, so the current does not double. The current depends on the output voltage and the resistance of the bulb.

When you add 2 batteries in series, you double the output voltage, which doubles the current through the bulb's resistance R.

Batteries have an internal resistance as well, so if you try to draw too much current out of them (using too low a resistance light bulb), then their output voltage drops some.

Does that make more sense?

3. Sep 22, 2015

### Bikrypto

That does make sense, but then what changes when batteries are in parallel? Based on your response I think I understand why the bulb is brighter, but now I'm curious as to what the advantages of parallel batteries are. I read somewhere that they will increase how long a bulb will shine? What changes in 1 battery versus 2 in parallel to cause this? Thanks so much for replying.

4. Sep 22, 2015

### Staff: Mentor

Yes, there is more energy available when you parallel 2 batteries, so the bulb will shine twice as long.

Keep in mind that it is usually a bad idea to try to parallel real-world batteries, as they will usually have slightly different states of charge, and hence slightly different output voltages. This can lead to the higher voltage battery trying to "charge" the lower battery up to its higher state of charge, which does not work well for non-rechargeable batteries. Batteries are sometimes paralleled in the real world, but with some precautions taken to ensure they are always at the same level of charge and discharge.

5. Sep 22, 2015

### Bikrypto

Thanks! I think I understand the topic a bit better now.