$50 robot questions

[anti]

Ive been going thru the build a robot for 50 dollars tutorial, i havent been able to start building yet (im in europe and waiting for the parts to come in) but im trying to get my batteries strait.  in the tutorial a 6v 100mah charger is rigged to charge the 6v 2000mah battery.  then only one ive been able to scrap is a 6vDC 250mAh, will this work or should it be exactly 6v 100mah.  also i have several other batteries around from various rc aircraft, some already with chargers. 8.4v/750mAh,   7.2v 900mAh,  6v/300mah,  and a 1000mah (not sure of the v, but its 7cell) would any of these be sufficient to run the robot?  Also, what sort of connection should be used(not sure if it matters) on the 6v 2000mah battery, because when i go to buy from onlybatterypacks.com im asked to choose a connection.
                             any assistance would be much appreciated.

[jsmoker]

What kind of battery you use strictly depends on the power you use.  If you're running a 5V board then you won't waste a lot of energy on the voltage regulator, but then you have to have a low dropout voltage regulator which isn't expensive, but it's not something you find at Radioshack.  If your controlling motors with variable resistors, you'll be losing energy thourgh the resistors as opposed to a pwm system to control a motor.  Actually there are even regulators that use pwm to reduce wasted energy, but they're bulkier and more expensive.  It really depends on what you're doing.  In one application, I'm using a small 600mah battery which would last me for 5 hours, but that's only cause I'm drawing only .1something amps.

ie .6 Amp hours / .1something Amps = ~5 hours

Also, mind the max amp draw of the batteries, especially if it's a LiPo.  Too much draw will light you fire.

A constantly moving robot will draw much more energy than one that moves briefly, does a bunch of calculations, then moves again.

[anti]

  thanks for the help, but honestly most of that sounded like greek to me(low drop out voltage regulator, variable resistor, pwm.).i do realize that it depends on what power i use but im not actually clear on what power i will be using, if that makes sense.  im a beginner in every since of the word.....i have some experience with servos, and plugging what wires go where, but other than that i have no background in electronics, circutry, motors or anything else.  im using the tutorial as an experimental crash course introduction to robotics, (which is something ive wanted to do ever since i can remember) so forgive my naivety.

[ed1380]

http://www.kpsec.freeuk.com/study.htm That is a great site. read everything on there.

[anti]

thanks for the link...im going through it right now and it really seems to be more of my speed at this point.

[jsmoker]

np, I should have been more specific.  I know what you mean from experience in forums for other fields I have little experience in.  But the quick and dirty answer's at the bottom.

Low drop out voltage means that a voltage regulator will regulate the specified voltage while the supply voltage is some high maximum number to a voltage that is very close to the regulated voltage.  i.e.   if you use a radio shack 5V voltage regulator, it'll give you that 5V regulated as long as your supply voltage is like between 7.5-20V, if you supply less, the 5V will start dropping.  An LDO (low drop out) regulator, will let you go down to like 5.3V before the output voltage starts to stray from the regulated 5V. 

A variable resistor aka potentiometer is something that you can physically change the resistance of, often by turning.  You can change an output(desired) voltage with something called a "voltage divider"  which includes your supply voltage and two resistors.  For example if you connect two resistors in series, then connect your supply voltage (often denoted by Vcc or Vdd on schematics or Vin in some cases)  and ground (Vee or Vss).  The voltage between the two resistors (ie the output voltage (Vout))will be Vcc*R2/(R1+R2) where R2 is the resistor closer to the ground.  If you make one of these resistors a potentiometer, then you can change Vout physically. 

PWM is Pulse Width Modulation.  I don't know if you've done any electronics at school, but a normal squarewave usually has equal parts high and low.  For example you could have like 2ms at 5V then 2ms at 0V and then back to 2ms at 5V, but the over all frequency would be 4ms period or 1/.004s = 250Hz.  Now, one can change the frequency, ie change the 2ms at 5V and 2ms at 0V to 1ms each to give you 500Hz OR  you can chance the pulse width.  In this case for example, you could change the 2ms at 5V to 3ms and the 2ms at 0V to 1ms.  The overall frequency is the same...250Hz, but the PWM has changed.  Now if you attach this to a resistor and capacitor, you can "average" out the pulses roughly to result in a output voltage less than the Vin (google RC circuits to see what I mean by "average roughly").  This means doesn't use as much power than the aforemention voltage divider cause you dont' have any resistors to waste energy due to heat.

As to the batter for your actual project.  I hadn't looked at the $50 Robot project until just now, but I see where it says 6V 2000mah.  I'd say any of the batteries that you said you had would work, but something like that 6V 300mah wouldn't bring you a lot of fun time. Also, I'd go for either the 8.4V or 7.2V batteries cause the 5V regulator recommended battery on the site has a drop out volage of 7V which means for a 6V battery it'd be more like 4.5V output and would change as the battery drains.

the short and simple:
regular voltage regulator:
Anything >7V is fine the more mah the more playtime

LDO voltage regulator:
Anything >5.5V is fine the more mah the more playtime

FYI: rechargable batteries USUALLY come in increments of 1.2V for NiCad or NiMH or 3.7V for LiPo

I hope you're not intimidated by all this info.  A lot of it I admit is more than you need to know for this project, but, knowledge is power.

[ed1380]

How would I find the amp/h rating on an old 9.6v battery pack I have.
I would like to measure it somehow since it's old and the amp/h rating has probably decreased.

I was thinking about taking a load(motor, light,etc) and seeing how long the battery lasted, but how do I know what the load is in amps if it isn't written on it?

[anti]

thanks alot for the long version,  it was immensely helpful and i appreciate you taking the time to break it down for me.  i have to admit alot of this is intimidating, and i have a lot to learn, but its exactly as you say "knowledge is power".  Thanks alot for the help.

[Admin]

yea what makes robot building so hard is that it has a steep learning curve. you need to know a lot just to do a little . . .

ok about the battery voltages . . . if you give a servo more than 6V it will fry. there are some servos now that can handle 7.2V, but those are currently expensive . . .

As for mA hours, for a two servo robot anything below 1000mAh just doesnt give enough 'play time' for me
I typically calculate expected/estimated power draw before I buy batteries so I can have an expected runtime.

in the tutorial a 6v 100mah charger is rigged to charge the 6v 2000mah battery.  then only one ive been able to scrap is a 6vDC 250mAh, will this work or should it be exactly 6v 100mah.

Some NiMH batteries can charge at 250mAh now (I have one that does). But most cant (check the battery datasheet for maximum charge rates). If you charge too fast, the battery overheats and becomes damaged (and perhaps starts a fire). It doesnt have to be exactly 100mA, but thats the recommend dosage

How would I find the amp/h rating on an old 9.6v battery pack I have.
I would like to measure it somehow since it's old and the amp/h rating has probably decreased.

I was thinking about taking a load(motor, light,etc) and seeing how long the battery lasted, but how do I know what the load is in amps if it isn't written on it?

there really isnt any easy way because current output changes with voltage. if you make a control circuit that guarentees a constant current (complicated), then you can measure the time it takes for the battery to fail. it would be better to just put it on your robot and see how long it lasts 'in the field' in a realistic setting.

[anti]

ahhh ok. all of you have been a huge help.  if only i had found S.O.R. sooner......
   thanks!  however now that i know that this point of insight and help is here, be prepared for the bombardment of questions.