Difference in current to load with series/parallel sources

In summary: in summary, connecting batteries in parallel increases the maximum current that can be delivered to a load, but does not necessarily increase the current delivered to the load under normal conditions.
  • #1
BretD
3
0
I feel this is a very simple question, but I can't seem to wrap my head around it.

For example, if you have two 1.5 volt batteries in series and a load of 100 ohms then the total voltage is 3 volts and the current is 30mA because I=V/R. If you have two 1.5 volt batteries in parallel then the total voltage is 1.5 volts and the current is doubled so if you have a 100 ohm load then each battery supplies 15mA and the total current is still 30mA, right? I've read a lot of posts discussing how series increases voltage and parallel increases current, but that's with an open circuit. When you connect a load doesn't it all amount to the same current being delivered in the end despite the difference in voltage? I guess I don't understand the benefit or advantage of the different set ups.
 
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  • #2
Batteries are constant voltage sources. There's usually no point to hooking them up in parallel, just like you would never hook current sources in series. It just doesn't make sense.

If you hook the two batteries up in parallel, assuming that their voltages are very closely matched, they will give 1.5V to the load, putting 15mA through it. This current will be divided between the batteries, with each battery providing 7.5mA.

Resistors are Ohmic devices. Under normal operation, their current is always proportional to their voltage (V = IR), with no exception (barring really extreme circumstances).
 
  • #3
So two 1.5 volt batteries in parallel will supply 15 mA not 30 because the current depends solely on the voltage, if I understand you correctly. So why is it that sometimes car batteries are connected in parallel to deliver larger amounts of current (this is what I've heard people say). Car batteries are genrally 12V and supply DC so why would this supply more current? shouldn't it supply less because with any load, the current is directly proportional to the voltage, so batteries in series should supply more current?
 
  • #4
BretD said:
So two 1.5 volt batteries in parallel will supply 15 mA not 30 because the current depends solely on the voltage, if I understand you correctly.
Not solely on the voltage, on the resistance too.

BretD said:
So why is it that sometimes car batteries are connected in parallel to deliver larger amounts of current (this is what I've heard people say). Car batteries are genrally 12V and supply DC so why would this supply more current? shouldn't it supply less because with any load, the current is directly proportional to the voltage, so batteries in series should supply more current?
The problem of paralleling car batteries or other voltage sources is that if the batteries have slightly different voltages or internal resistances, one battery will supply more current than the other. If you put a very heavy load on a battery so that the internal resistance is limiting the current to the load, then adding a second battery may help. Both batteries should be fairly evenly matched however.

I have designed high reliability circuits that required 100% redundancy. That meant paralleling 5V regulators. To do this I used 6V regulators and added enough series resistance to the output of the regulators to bring the voltage down to 5V. That wasn't a problem because my current drain was constant.
 
  • #5
BretD said:
So two 1.5 volt batteries in parallel will supply 15 mA not 30 because the current depends solely on the voltage, if I understand you correctly. So why is it that sometimes car batteries are connected in parallel to deliver larger amounts of current (this is what I've heard people say). Car batteries are genrally 12V and supply DC so why would this supply more current? shouldn't it supply less because with any load, the current is directly proportional to the voltage, so batteries in series should supply more current?
Batteries have a maximum amount of current that they can put out without destroying themselves. With batteries connected in parallel, the load current is evenly shared between them, so they can drive a higher load.

Connecting them in parallel doesn't make the batteries supply more current to the load, it just increases the maximum that they can put out.
skeptic2 said:
I have designed high reliability circuits that required 100% redundancy. That meant paralleling 5V regulators. To do this I used 6V regulators and added enough series resistance to the output of the regulators to bring the voltage down to 5V. That wasn't a problem because my current drain was constant.
Something's iffy about that setup, unless you left something else out.

Under normal conditions, with both regs running good, both regs would be sharing the load equally (1/2IL each). When one of the regs fails, the other reg would have to pick up all the slack. With the full IL coming through it, the series resistor would drop 2V instead of just one, dropping the power supply to 4V.

Doesn't seem like a very good redundancy.
 
  • #6
BretD said:
I feel this is a very simple question, but I can't seem to wrap my head around it.

For example, if you have two 1.5 volt batteries in series and a load of 100 ohms then the total voltage is 3 volts and the current is 30mA because I=V/R. If you have two 1.5 volt batteries in parallel then the total voltage is 1.5 volts and the current is doubled so if you have a 100 ohm load then each battery supplies 15mA and the total current is still 30mA, right? I've read a lot of posts discussing how series increases voltage and parallel increases current, but that's with an open circuit. When you connect a load doesn't it all amount to the same current being delivered in the end despite the difference in voltage? I guess I don't understand the benefit or advantage of the different set ups.

if you connect a 100ohm load to two 1.5V battery connected in parallel, you can sustain a current of 15mA twice as long as if you were to have only one 1.5V battery.
 
  • #7
Jiggy-Ninja said:
Something's iffy about that setup, unless you left something else out.

Under normal conditions, with both regs running good, both regs would be sharing the load equally (1/2IL each). When one of the regs fails, the other reg would have to pick up all the slack. With the full IL coming through it, the series resistor would drop 2V instead of just one, dropping the power supply to 4V.

Doesn't seem like a very good redundancy.

Yes there is something I left out because I thought it would only confuse the issue. Before I tried this I called the application engineer from the regulator manufacturer and I followed his instructions. He suggested putting a diode in series with the resistor at the output of the regulator in case the regulator failed as a short and to avoid the problem you mentioned. So the diode dropped 0.7V and the resistor only 0.2V. Once he said I should use a diode I knew I had to use a higher voltage regulator. If one regulator were to fail the supply voltage would drop from about 5.1V to about 4.7V.
 
  • #8
if you connect a 100ohm load to two 1.5V battery connected in parallel, you can sustain a current of 15mA twice as long as if you were to have only one 1.5V battery.

+1

Simple and to the point.
 
  • #9
skeptic2 said:
Yes there is something I left out because I thought it would only confuse the issue. Before I tried this I called the application engineer from the regulator manufacturer and I followed his instructions. He suggested putting a diode in series with the resistor at the output of the regulator in case the regulator failed as a short and to avoid the problem you mentioned. So the diode dropped 0.7V and the resistor only 0.2V. Once he said I should use a diode I knew I had to use a higher voltage regulator. If one regulator were to fail the supply voltage would drop from about 5.1V to about 4.7V.
That makes more sense.
 
  • #10
Thank you Jiggy-Ninja and everyone else who commented. It all makes a bit more sense now!
 

What is the difference between series and parallel sources?

In series sources, the positive terminal of one source is connected to the negative terminal of another source, while in parallel sources, the positive terminals and negative terminals are connected separately.

How does the current differ in series and parallel sources?

In series sources, the current through each source is the same, whereas in parallel sources, the current is divided between the sources based on their individual resistances.

Which configuration is more efficient for powering a load: series or parallel sources?

It depends on the specific load and the sources being used. Generally, if the load has a high resistance, parallel sources would be more efficient, and if the load has a low resistance, series sources would be more efficient.

What happens to the total current when sources are connected in series?

The total current remains the same as the current through each source, but the voltage is divided between the sources, resulting in a lower voltage across each individual source.

How can I calculate the total current in a series/parallel source circuit?

To calculate the total current, you would need to use Ohm's Law, which states that current (I) is equal to voltage (V) divided by resistance (R). In a series circuit, the total resistance is the sum of all individual resistances, while in a parallel circuit, the total resistance is the reciprocal of the sum of the reciprocals of each individual resistance. Once you have the total resistance, you can use Ohm's Law to calculate the total current.

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