MadSci Network: Engineering
Query:

Re: why does a parallel battery connection have a greater capacity series?

Date: Thu Aug 26 16:28:33 1999
Posted By: Steve Czarnecki, senior technical staff member, Lockheed Martin
Area of science: Engineering
ID: 935195507.Eg
Message:

Irene,

You have reached the correct conclusions concerning voltage and current (as 
we call it here) in series and parallel connections of batteries.

Here's an analogy to help you understand why this is so.  Think of 
electricity as water, wires as pipes, and batteries as water pumps.  Then 
"water pressure" is analogous to "voltage" and "flow rate" is analogous to 
"current".

Two measurements describe pumping ability:  the first is the pressure 
rating of the pump, and the second is the maximum flow.  For exampe, I 
recently bought an irrigation pump for my farm rated at 75 psi (pounds per 
square inch) at 60 gpm (gallons per minute).   This means I can take in 
pond water at 0 psi, and discharge water at 60 gpm flow rate into a pipe at 
75 psi pressure.  

If someday I need a larger flow rate, I can buy a second pump, 
and have it work in parallel with the first (i.e., both pumps take in pond 
water and both discharge into a single pipe), for a total combined 
flow rate of 120 gpm at 75 psi.  This is similar to the case where you've 
connected your batteries in parallel: the current capacities (flow rates) 
add.

On the other hand, if I need higher pressure, I could connect the inlet of 
a second pump to the outlet of the first, giving me 60 gpm at 150 psi. That 
is, the first pump takes in pond water at 0 psi, discharging it at 75 psi. 
The second pump takes in this water and raises the pressure by an 
additional 75 psi before discharging it, giving a net pressure rise of 150 
psi.  The flow rate stays at 60 gpm; the second pump can only discharge as 
much water as the first pump gives it -- it can't create more! 

Similarly, connecting batteries in series adds the voltages (pressures).

In principle, I could go on connecting pumps in series this way to reach 
any high pressure I desire.  The only thing that stops me from adding pumps 
this way are practical issues such as the ability of the shaft seals, pump 
casings, and pipes to withstand the high pressures (much as electrical 
circuits must have insulation strong enough to withstand the very high 
voltages involved).   

If we look a little closer at what's going on, we see that a pump works by 
adding energy to the molecules of water.  Bernoulli's Law tells us this 
energy is divided between the potential energy of the water (proportional 
to its pressure and height above the pumping point) and the kinetic energy 
of the water(proportional to the square of the flow rate).  

Similarly, a battery works by adding energy to the electrons flowing 
through an electrical circuit.  This increase in voltage instigates the 
flow of current through the circuit, much as a pump's increase in water 
pressure causes the water to flow from my pond to other useful places. 
Thus, combining batteries in series allows them to sequentially raise the 
electrons to higher and higher energy (voltage), while combining batteries 
in parallel allows more and more electrons to flow, with each battery 
contributing its current capacity to the total flow.  

By the way, the unit of current flow, the ampere, is defined in terms of 
the number of electrons per second moving past the measurement point;  
sounds like a flow rate, doesn't it?  The unit of voltage, the volt, is 
defined in terms of force per unit charge, which reminds me of pressure, 
which is force per unit area.

I hope this is of some help; good luck with your project!

Steve

P.S. A fine point for sticklers:

The direction in which electrons actually move is opposite to the direction 
in which we say current flows.  Thank Benjamin Franklin for this, who 
originated the convention of electrical current flowing from positive to 
negative; he didn't know about electrons, but the convention has stuck with 
us even after they were discovered. 

I say that this is a nuisance point of no consequence until you're ready to 
start designing solid state devices (e.g., transistors and integrated 
circuits).  For now, you'll do fine simply remembering that current flows 
from positive to negative and not worrying about what the electrons are 
actually doing.




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