My Robot Project--Power supply

[moebious]

I am in the design phase of a Robot project and am researcing the power supply aspect of the project. I am planning to use a PIC16F877 for the brain of a very simple robot that will have two DC Motors (5v Max) and a couple of distance sensors to detect the edge of a table. So I am thinking that I would ave a couple of 9V batteries in series and run that thru a 5v regulator. So the question is first Will the batteries last longer if i wire them in series like that or would it be better to wire them in parallel? I am very new to electronics so if anyone could explain why one way would be better than the other that would be great.

Thanks
David M.

[SupeR-NovA]

hi moebious

if you will wire them in series you will get 18V, and then regulate it down to 5v you will only create more power loss
if you will wire them in parallel, you could decrease the batteries internal resistance thus making them last a bit longer...
I wpuld suggest you to wire only one battery, and if you need more Ah get some stronger battery (a 12v car battery)

HTH

[audioguru]

Hi David,
A little 9V alkaline battery might not have enough current ability to power motors for much time. Two in parallel would double their current capacity. If they were in series then the current capacity would be the same as a single one, and most of the extra voltage would be wasted to heat the regulator.

A 9V alkaline battery's voltage quickly drops to about only 7.2V then slowly drops to 6V over its life. An ordinary 5V regulator needs 7.5V or more input voltage to function properly.
You need to use a regulator that is made for battery powered equipment called a "low-dropout" regulator. I use National Semi's LM2931A-5.0 which is in a small transistor case and has 100mA max output. Others made for higher output currents are available. 

[moebious]

hi moebious

if you will wire them in series you will get 18V, and then regulate it down to 5v you will only create more power loss
if you will wire them in parallel, you could decrease the batteries internal resistance thus making them last a bit longer...
I wpuld suggest you to wire only one battery, and if you need more Ah get some stronger battery (a 12v car battery)

HTH

Thanks for the input. A 12V car battery would be WAY to big for the robot that I am building. Do you have any other sugestions?

[moebious]

Hi David,
A little 9V alkaline battery might not have enough current ability to power motors for much time. Two in parallel would double their current capacity. If they were in series then the current capacity would be the same as a single one, and most of the extra voltage would be wasted to heat the regulator.

A 9V alkaline battery's voltage quickly drops to about only 7.2V then slowly drops to 6V over its life. An ordinary 5V regulator needs 7.5V or more input voltage to function properly.
You need to use a regulator that is made for battery powered equipment called a "low-dropout" regulator. I use National Semi's LM2931A-5.0 which is in a small transistor case and has 100mA max output. Others made for higher output currents are available.

[audioguru]

The choice of a battery depends on how much current for what length of time.
Six AA alkaline cells also produce 9V when new but last 5 to 6 times longer than a little 9V alkaline battery.
Six AA lithium cells last about 6 to 7 times longer than alkaline and have less weight.