We are making a robotic crab. There are four legs and 2 servos per leg so 8 servos. Gthey are connected to a V4 sensor shield.
We have the arduino connected to a 9v battery but because of the voltage regulator we rerouted the 5V and ground pins to another 9 volt battery. Still 9 volts is not enough to make the robot walk. We're are thinking of connecting another battery in series. Will 18 volts blow the sensor shield?
9 volt battery
If you are talking about the small "smoke detector" 9v battery, you need to get real and get much larger batteries to power servos.
This sensor shield ?
http://www.seeedstudio.com/depot/Electronic-Brick-Shield-V4-p-458.html
You can not use the Arduino 5V output to power a servo. The Arduino can not supply enough current.
If you have a 6V battery, you can use that to power the servo motors.
If you have a battery pack of 9V or 12V, you can use a DC-DC converter to make 5V for the servo motors.
A small 9V battery can supply maybe 100mA.
A servo motor that starts rotating might require 500mA, or even 1A.
With 8 servo motors, you need 4A.
Before we waste any more time on this how about a schematic . Draw one and take a photo and upload it . From what I am reading there is a better than 50% chance you are doing things without knowing what is the correct method.
Re: Sensor Shield-
These are plug-on pc boards that break the arduino pins out to easy-to-connect pins.
BeWare! There is NO agreement on what a "Version 4" Sensor Shield is. I have seen 4 different ones, and two Version 5's..
UPDATE: There are UNO/Duemilanove size Sensor Shields that have a separate power connection for the Power Pin (of the 3-pin connectors GND-VOLTAGE-SIGNAL) . That is what you need for lots of Servos.
For more about Sensor Shields, see the ArduinoInfo.Info WIKI HERE:
UPDATE: I found a regular Arduino size Sensor Shield that has the separable 5V power option. It's from a regular supplier I have and I already added it to my shop. I am sure you can also find it elsewhere. It looks like this:
You can see details of the shield here: http://yourduino.com/sunshop2/index.php?l=product_detail&p=407
This tells about the separated +5V for Servos etc. There will soon be a chart of all the connections up there also.
DISCLAIMER: Mentioned stuff from my own shop...
Hi, I think you guys are thinking that more volts = more power, well it does, but not when the load is only rated at a certain voltage.
In this case you need MORE CURRENT CAPACITY, so instead of putting another 9V in series, put it in PARALLEL. Same voltage, keep servos happy, but twice the current is now available.
Simple BASIC ELECTRONICS.
And yes, forget about 9V smokey batteries, they are for transistor radios, DMMs and stuff that draws mA, not Amps.
Tom...... 
instead of putting another 9V in series, put it in PARALLEL. Same voltage, keep servos happy, but twice the current is now available.
The Problem is the 8 servos need a lot of current (several amps perhaps) at 5 to 6 volts. The Arduino onboard 5V supply can NOT supply this, no matter what current is available into the Arduino external power connector. There must be a separate power supply for the servos. At least 4 or 5 AA NiMH rechargeables would be good. that would be 4.8 or 6 volts. If you apply 6 volts to the sensor shield for the servos, don't use the Sensor Shield pins for other devices needing 5 volts. Easier to stick with 4.8 to 5 volts, probably fine for a robot.
A 2S LiPo pack (7.4V nominal) would be very able to handle the current requirements of
lots of servos, but would need the voltage dropping a little (some high-power rated diode(s)
perhaps). Servos are usually rated for 6V or 7.2V, not 5V, and a 5V supply that's dipping
won't be reliable.
8 servos will draw on peak anywhere from 4 to 20A depending onhow big/powerful
they are, that's a lot of current. Arranging that they are not all active together will
help a lot to reduce max current draw. Average current draw when idle is a lot lower.
See the ArduinoInfo.Info WIKI HERE: for servo information.
Down the page a ways is a table that shows how servos are usually rated for speed and torque at two voltages: 4.8 and 6.0 volts and can be used anywhere between those voltages. 5.0 V is widely used. (or 5.7V - See the ZOOM solution below).
Easier to stick with 4.8 to 5 volts, probably fine for a robot.
At 4.8v the servo will develop significantly less torque than at 6v. If there is no load on the servo, then no problem. If the servo needs to develop max torque, the 6v is the way to go. The below setup ups the 7805 output from 5v to 5.7v, and improved servo performance can be observed from that simple change.